Carbides in 0.04C–1.5Mn–0.2Mo in ferritic steel steels

Table 1: Chemical composition of steels investigated (mass%).

Figure 1: Schematic diagram of thermomechanical treatment in dilatometric test.

Figure 2: Scanning electron micrograph showing microstructure of Steel C (0.04%C–1.5%Mn–0.2%Mo–0.09%Ti). Scale bar: 10 µm.

Figure 3: Transmission electron micrograph showing fine carbides in Steel C; (a) bright field image and (b) EDS spectrum of fine carbides. Scale bars: 25, 100 nm.

Figure 4: Transmission electron micrographs showing chemical extracted precipitates of Steel C; (a) bright field image, (b) magnification of fine precipitates, (c) EDS spectrum of a large precipitate, (d) EDS spectrum of fine precipitates. Scale bars: 0.5 µm, 50 nm.

Figure 5: X-ray diffraction spectrum of chemically extracted precipitates in Steel C.

Table 2: Results of quantitative analysis for precipitates in Steel C.

Table 3: Chemical composition of mill trialed steel (mass%).

Figure 6: Transmission electron micrographs showing carbides in Steel X and conventional precipitation-strengthened sheet steel with EDS spectra; (a) bright field image of Steel X; (b) bright field image of conventional steel; (c) EDX spectrum of carbides in Steel X; (d) EDS spectrum of carbides in conventional steel. Scale bars: 25, 100 nm.

Carbide name: Carbides
Record No.: 1079
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: EDS, SEM, TEM, XRD
Steel name: 0.04C–1.5Mn–0.2Mo
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Ferritic steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: Table 1 shows chemical composition of steels investigated. Chemical composition of the base steel was 0.04%C–1.5%Mn–0.2%Mo. Manganese was added to prevent fine carbides from growing by lowering austenite–ferrite transformation temperature (Ar3). The small amount of molybdenum was included to retard the deposition of both pearlite and large cementites at grain boundaries. Titanium content was varied in steels from A to D to investigate the influence of titanium content on tensile strength.8) Nitrogen content was reduced because nitrogen exhausts titanium through forming large TiN. Furthermore, titanium and molybdenum contents were varied in steels from E to I to investigate the influence of Ti/Mo atomic concentration ratio on tensile strength under a condition that the sum of atomic concentrations of titanium and molybdenum was equal to that of Steel C.
These steels were vacuum-smelted and cast to 50 kg ingot. The ingots were rolled to 30 mm thick bars after soaking at 1473 K. Laboratory slabs in 120 mm length and 100 mm width were cut off from the rolled bars. The slabs reheated at 1523 K for 3.6 ks were hot-rolled to 4.5 mm thick by 7 passes. Finishing temperature was controlled at 1173 K. After finishing rolling, they were air-cooled to 893 K (approximately 10 K/s) and held at 893 K for 3.6 ks followed by furnace-cooling to room temperature for hot coiling simulation. After reducing the thickness to 2.3 mm by grinding to remove scale defects, tensile test was carried out in the longitudinal direction using JIS No.13B type specimen (GW; 12.5 mm, GL; 50 mm). Cross head speed in the tensile test was kept at mm/s. Cylindrical specimens of 12 mm height and 8 mm diameter were prepared from the rolled bar and dilatometric experiment was carried out. Figure 1 shows a schematic diagram of the dilatometric experiment. The specimens were austenized at 1523 K for 300 s to resolve carbides and then deformed three times with a strain of 0.3 and a strain rate of 20/s. Cooling rate was fixed at 10 K/s in the measurement of austenite–ferrite transformation temperature.
Note: A ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to obtain a high strength hot-rolled sheet steel having tensile strength of 780 MPa grade with excellent stretch flange formability. Manganese in a content of 1.5% and molybdenum in a content of 0.2% were added to 0.04% carbon Ti-bearing steel in order to lower austenite–ferrite transformation temperature for fine carbides and to retard generating of pearlite and large cementites, respectively. Tensile strength of hot-rolled sheet steel increased with titanium content and it was achieved to 800 MPa in a 0.09% Ti steel. Microstructure of the 0.09% Ti steel was ferrite without pearlite and large cementites. Fine carbides of 3 nm in diameter were observed in rows in the ferrite matrix of the 0.09% Ti steel with transmission electron microscope. The characteristic arrangement of the nanometer-sized carbides indicates that the carbides were formed at austenite–ferrite interfaces during transformation. By energy dispersive X-ray spectroscopy, the carbides were found to contain molybdenum in the same atomic concentration as titanium. Crystal structure of the nanometer-sized carbides was determined to be NaCl-type by X-ray diffractometry. The calculated amount of precipitationstrengthening by the carbides was approximately 300 MPa. This is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels.
Based on the results obtained in the laboratory investigation, mill trial was carried out. The developed hotrolled high strength sheet steel exhibited excellent stretch flange formability.

Figure 2 shows a scanning electron micrograph of a cross section of Steel C containing 0.09% titanium. While pearlite and large cementites were not observed, only ferrite grains can be seen. The SEM observation and saturation of tensile strength with titanium addition more than 0.09% suggest that almost all carbon in Steel C formed carbides containing titanium.

Figure 3 shows a transmission electron micrograph and an EDS spectrum of carbides in Steel C. Carbides of 3 nm in diameter can be seen in rows. Titanium, molybdenum and carbon were detected in the fine carbides except iron and manganese included in matrix. The Ti/Mo atomic concentration ratio measured by quantitative analysis with the EDS spectrum was 1.03. Steel C is found to be strengthened by the nanometer-sized carbides containing titanium bearing steel, V-bearing steel and Mo-bearing steel. Gray et al. and Davenport explained the rows of carbides in terms of precipitation at austenite–ferrite interfaces during transformation.
It has been demonstrated that distance among rows becomes shorter with lowering austenite–ferrite transformation temperature. A distance among rows in Steel C is constant at approximately 15 nm in the transmission electron micrograph shown in Fig. 3. The observation suggests that the austenite–ferrite transformation occurred at a constant temperature after rolling in the hot-rolling process. The Ar3 transformation temperature of Steel C was determined by the dilatometric experiment to be 893 K and it is the same as the coiling temperature. This indicates that the interface precipitation of the nanometer-sized carbides in Steel C takes place at the coiling temperature.

Figure 4 shows transmission electron micrographs and EDS spectra of chemically extracted precipitates of Steel C. In Fig. 4(a), fine precipitates on the micro grid can be seen with a large precipitate. The large precipitate will be TiN since only titanium was detected as shown in Fig. 4(c). Figures 4(b) and 4(d) show a magnification of Fig. 4(a) and EDS spectrum of the fine precipitates, respectively. The diameter of the fine precipitates was approximately 3 nm. Except carbon, oxygen and copper from the grid, titanium and molybdenum was detected as shown in Fig. 4(d). The fine precipitate was the nanometer-sized carbide in Fig. 3.

Figure 5 shows an X-ray diffraction spectrum of the precipitates. Several peaks that should be from the nanometersized carbides can be seen with peaks of TiN. Each peak of the nanometer-sized carbides located at lower angle side of that of TiN. This result indicates that a crystal structure of the nanometer-sized carbides is NaCl-type. Moreover, a lattice parameter of the nanometer-sized carbides was 0.433 nm which is similar to that of TiC (0.4327 nm).

Table 2 shows a result of quantitative analysis for the chemically extracted precipitates of Steel C. The amount of carbon as Fe3C was determined using the amount of Fe. All the nitrogen combined with titanium as TiN while no nitrogen formed AlN. All the carbon in Steel C probably precipitated as the nanometer-sized carbides containing titanium and molybdenum since carbon as Fe3C was not detected. Moreover, a Ti/Mo atomic concentration ratio in the nanometer-sized carbides was 1.16. The ratio is in good agreement with the ratio obtained with the EDS spectrum in Fig. 3(b). This result confirms that the nanometer-sized carbides in Steel C contain molybdenum as much as titanium in atomic concentration.
Consequently, the nanometer-sized carbides is determined to be (Ti,Mo)C with the lattice parameter of 0.433 nm. The fact that (Ti,Mo)C precipitates directly in ferritic steel have not been reported yet since (Ti,Mo)C has been observed only in a quench-tempered 0.5%Mo–Ti–B steel.
Mill Trial: Mill trial was carried out for Steel X in Table 3. A finishing temperature and coiling temperature were 1173 K and 923 K, respectively. A width and thickness of the hot-rolled sheet steel were 700 mm and 3.2 mm, respectively. After acid-pickling, tensile test was carried out in the transverse direction using JIS No. 5 specimen (GW; 25mm, GL; 50mm) with a cross head speed of 1.67 x 10 exp(-1) mm/s. Hole expanding test for evaluation of stretch flange formability obeyed JFST1001 of Japan Iron and Steel Federation Standard.

Figure 6 shows transmission electron micrographs of carbides in Steel X and a conventional 780 MPa grade precipitation- strengthened hot-rolled sheet steel (0.12%C–0.4%Si–1.8%Mn–0.08%Ti–0.04%Nb) with EDS spectra of carbides. Very fine carbides in rows were observed in Steel X. The diameter of the fine carbides was 3 nm. A Ti/Mo atomic concentration ratio of the fine carbides was approximately 1. On the other hand, large carbides over 30 nm in diameter were observed in the conventional sheet steel. The large carbides contained titanium and niobium.
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