M23(C,B)6 carbides in microalloyed constructional steel


Table 1: Chemical composition of tested steels (weight %).


Table 2: Results of the mechanical properties and hardenability of the investigated steels.


Figure 1: Fine-grained structure of the statically recrystallized austenite; finishing forging temperature 900C; isothermal holding time 12s; steel B. Scale bar: 50 µm.


Figure 2: Fine-grained structure of the statically recrystallized austenite; finishing forging temperature 900C; isothermal holding time 16s; steel C. Scale bar: 10 µm.


Figure 3: Martensitic-bainitic structure of steel B water quenched from a finishing hot working temperature of 900C after holding for 12s. Scale bar: 20 µm.


Figure 4: Lath martensite structure of steel C quenched from a finishing hot working temperature of 900C after holding for 16s. Scale bar: 3 µm.


Figure 5: Lath martensite structure with the dispersive Fe3C precipitations (steel C): a light field, b diffraction pattern to Fig. 5a [111]Fe-alpha and [012]Fe3C. Scale bar: 0.4 µm.


Figure 6: Dispersive M23(C,B)6 precipitations at the grain boundaries of the primary austenite (steel C); a light field, b diffraction pattern to Fig. 6a [111]Fe-alpha and [001]M23(C,B)6. Scale bar: 3 µm.

Carbide name: M23(C,B)6
Record No.: 1108
Carbide formula: M23(C,B)6
Carbide type: M23C6
Carbide composition in weight %: No data
Image type: LM, TEM
Steel name: Structural steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Microalloyed constructional steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: The goal of this experiment is a structure and mechanical properties of forged elements using thermo-mechanical treatment, made from C-Mn steel with micro-additions of Ti, V, B and N (table 1) melted using: after furnace treatment of metal bath, and continuous casting ingots with intersection 100..100 mm. Ingots after their solidifications were adjustly rolled for bars with a dimention about 36 mm, and after using the same conditions for bars with dimention of 17 mm. The range of adjusted rolling temperature were taken basing on calculation of solubility in the austenite microadditions added to the steel.
Note: Purpose: Effect of the thermo-mechanical treatment conditions on the structure and mechanical properties of the forged elements of constructional C-Mn steels with Ti, V, B and N microadditions.
Design/methodology/approach: Metallography, electron microscopy, tensile test, hardness measurements, hardenability calculations, Charpy-V tests have been used.
Findings: The thermo-mechanical treatment allows to obtain the fine-grained austenite structure during hot plastic deformation, and gives forged elements obtaining: yield point Rp0,2 over 690 MPa, UTS over 770 MPa, hardness 220 up to 250 HB and breaking energy KV over 180J after high tempering.
Research limitations/implications: It is predicted TEM investigations on structure of the forged elements after thermo-mechanical treatment.
Practical implications: Investigations carried out showed full usability of micro-alloyed steels for producing forged machine parts with the high strength and cracking resistance, using the energy-saving thermo-mechanical treatment method.
Originality/value: Production conditions of energy-saving thermo-mechanical treatment of forged elements of HSLA constructional steels with the diversified hardenability, were presented.

Thermo-mechanical treatment was realized thanks to open die forging of experimental segments with 17 mm diameter and 150 mm length in the temperature range of 950 to 900C and 1000 to 900 C - respectively from steel A and B type to rods with intersection 12 x 12 mm. Before quenching in water these rods were hold at a temperature of end forging for 3 and 12s. Quenched rods were tempered at a temperature of 600C for 1h. Whereas rod segments from steel type C with intersection 24 x 24 mm were forged in a range of temperature from 1150 up to 900 C to the shape of rods of intersection 12 x 12 mm. Before quenching in water the rods were hold in a temperature of end forging for 16s, and after were tempered in temperature of 500 and 600 C for 1h.
Investigations showed that examinated steels after thermomechanical treatment and after quenching have a fine-grain structure of primary austenite (fig. 1, 2) with a grain size about 10, 5 and 8 and martensitic-bainitic structure (fig. 3) and hardness 42, 44 and 49 HRC respectively for a steel A, B and C type. Hardness of the steels is decreasing after high tempering from 220 to 250 HB, which not create difficulties, during mechanical treatment of forged elements.
Examinations of the structure of thin foils made in TEM (transmission electron microscope) showed that steel C type (quenched from a temperature of end forging 900 C and hold before that in this temperature for 16s) have lath martensite structure (fig. 4). Inside of martensite laths it was approved a presence of dispersive particles of cementite (fig. 5), whereas at the boundaries of primary austenite M23(C,B)6 type dispersive particles were found (fig. 6), which have occure in a steel during self tempering process.
Links: No data
Reference: Not shown in this demo version.

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