2022 Volume 62 Issue 2 Pages 402-404
Ultrafine elongated grain (UFEG) structure with a strong <110>//rolling direction fiber texture was created for a 0.4%C-2%Si-1%Cr-1%Mo steel (mass%) through deformation of tempered martensite using multi-pass caliber rolling at 773 K with a rolling reduction of 78% (i.e. warm tempforming). Annealing of the warm tempformed steel at 843 K enhanced delamination toughening at lower temperatures without a significant loss of tensile strength at 1.8 GPa. It was suggested that delamination toughening was controlled through the precipitation of nanoscale Mo-rich precipitates in the UFEG structure.
Ultrafine elongated grain (UFEG) structures with strong <110>//rolling direction (RD) fiber textures are created in medium-carbon low-alloy steels through deformation of tempered martensite using multi-pass caliber rolling at a warm temperature (i.e. warm tempforming).1) Excellent combinations of strength and toughness were demonstrated in the warm tempformed (TF) steels with UFEG structures.1)
Toughening in the UFEG structure steels is attributed to the occurrence of crack-arrester-type delaminations, where cracks are deflected toward the longitudinal direction (//RD) of notched bars. This phenomenon is referred to as delamination toughening.2)
The microstructural factors controlling the delamination toughening have been clarified to be the transverse grain size, grain shape, <110>//RD fiber texture, and carbide particle distribution in the UFEG structure.1) The delamination toughening was observed to be enhanced at lower temperatures as the volume fraction of cementite particles decreased in (0.2–0.6)%C-2%Si-1%Cr-1%Mo steels (mass%).3) However, in the Mo-bearing steels, it is still not clear whether nanoscale Mo-rich precipitates4) can influence the delamination toughening or not.
The present study investigated the influence of annealing on the UFEG structure, tensile and Charpy V-notch impact properties for a 0.4%C-2%Si-1%Cr-1%Mo steel processed by warm tempforming at 773 K. The delamination toughening of the TF steel was discussed in relation to the precipitation of nanoscale Mo-rich precipitates.
The 0.4%C-2%Si-1%Cr-1%Mo steel used in this study had a chemical composition of 0.43 C, 1.97 Si, 0.20 Mn, 1.02 Cr, 0.96 Mo, 0.002 P, 0.001 S, 0.021 Al, 0.0015 N, 0.0007 O and the balance Fe (all in mass%).3) The quenched bars were tempered at 773 K for 3.6 ks and subsequently warm tempformed using multi-pass caliber rolling with a rolling reduction of 78%, followed by air cooling. The details of the warm tempforming were described elsewhere.3) The TF samples were annealed at 843–973 K for 3.6 ks, and then water cooled (TFA sample).
FE-SEM/EBSP analysis was carried out at a step size of 50 nm. Development of <110>//RD fiber texture was evaluated by measuring the integrated intensity ratio of (110) XRD peaks for the samples (Im) to that for the standard sample (Is). The carbide particles extracted in carbon replicas were observed by TEM with energy-dispersive X-ray spectrometry (EDS). Tensile tests were performed at a crosshead speed of 0.85 mm/min for JIS Z 2201-14A specimens with a gauge diameter of 6 mm and gauge length of 30 mm. Charpy impact tests were performed for the full-size 2 mm V-notch specimens.
Figure 1 shows the UFEG structure of the TFA sample annealed at 843 K. Average linear intercept for high angle boundaries in the transverse direction (ILHAB_T) was measured to be 0.28 μm. The bimodal distribution of spheroidized carbide particles was similar to that observed in the TF sample;3) the intergranular carbides were larger than the transgranular carbides. Most of the spheroidized carbide particles were identified as cementite, where substitutional elements such as Cr, Mn, and Mo were dissolved. TEM observation further revealed the dispersion of nanoscale Mo-rich precipitates (<10 nm) inside the grains. The annealing at 923 K was observed to result in the formation of needle-shaped Mo2C particles and Fe3Mo3C particles, in addition to the coarsening of cementite particles.4) Figure 2 summarizes the microstructural factors for the UFEG structure as a function of annealing temperature. In response to the growth of carbide particles, the matrix grain size increased while the KAM average value, which reflected the geometrically necessary dislocation (GND) density, decreased. The integrated (110) XRD peak intensity ratio (Im/Is) indicated that there was no change in the <110>//RD fiber texture. The comparison of the microstructural factors therefore indicated that the annealing at 843 K had little influence on the grain size, grain shape, <110>//RD fiber texture, and GND density in the UFEG structure.
Microstructure of warm tempformed (TF) sample annealed (TFA) at 843 K: IPF map for the striking direction (SD) of impact bar (a), TEM bright-field images of extracted replicas ((b), (c)), and EDS spectra of nanoscale Mo-rich precipitate (d). The black lines represent high angle boundaries (HABs) with a misorientation angle of 15° or above in (a). The arrows in (c) indicate the nanoscale Mo-rich precipitate. (Online version in color.)
Average linear intercept (ILAV) for the HABs along the transverse (ILHAB_T) and longitudinal (ILHAB_L) directions in the UFEG structure, average KAM value for a 1st neighbor rank (KAMAV), and integrated intensity ratio (Im/Is (110)) for XRD (110) plane as a function of annealing temperature.
Figure 3 shows nominal stress–strain curves at room temperature. Except for the TFA sample annealed at 843 K, discontinuous yielding behavior was observed in the TF and TFA samples; however, the TFA samples annealed at 923 and 973 K exhibited a sharper yield point than the TF sample. The TFA sample annealed at 843 K exhibited continuous yielding. It was observed in the warm tempformed (0.2–0.6)%C-2%Si-1%Cr-1%Mo steels that the yield drop decreased with increasing the carbon content, and was less pronounced with the carbon content of 0.6 mass%. This suggested that the carbide particle distribution influenced the yielding behavior in TF steels. Since the spheroidized carbide particle distribution and the microstructural factors in the UFEG structure are almost identical between the TF sample and the TFA sample annealed at 843 K, it is suggested that the yielding behavior is influenced by the precipitation of nanoscale Mo-rich precipitates. It must be emphasized that tensile strength hardly decreased through the annealing at 843 K while yield strength (YS) slightly decreased. Figure 4 shows the variations in tensile property as a function of testing temperature and annealing temperature. In the TF and TFA samples, YS linearly increased as the testing temperature decreased from room temperature to 123 K, below which it tended to increase abruptly. On the other hand, the reduction in area (RA) decreased as the testing temperature decreased. However, the temperature dependence of RA was influenced by the annealing. Although RA for the TFA sample annealed at 973 K showed the highest average value at room temperature, it dropped to 5% at 77 K. In the TF sample and TFA sample annealed at 843 K, RA tended to decrease abruptly below 123 K. The drop in RA, however, was suppressed through the annealing at 843 K; for the TF sample the average RA at 77 K was 26%, while for the TFA sample it was 37%. The total elongation also decreased in response to the decrease in necking elongation.
Nominal stress–strain curves at room temperature. (Online version in color.)
Yield strength (YS), total elongation (TEL), and reduction in area (RA) as a function of testing temperature. (Online version in color.)
Figure 5 shows the changes in absorbed energy (vE) as a function of testing temperature and annealing temperature. Here, 1) ○ denotes the data for the sample exhibiting almost complete ductile fracture at an elevated temperature, 2) ◇ denotes the data for the sample exhibiting a crack-arrester-type delamination whose crack branching angle relative to the RD (β) was below 15°,3) and 3) △ denotes the data for the sample whose β was 15° or over. At 573 K, all the samples failed in a ductile manner, and the values of vE (i.e. upper-shelf energy, vEUS) were almost identical. In the TFA sample annealed at 973 K, the crack-arrester-type delamination hardly occurred, and vE markedly decreased at temperatures ranging from 253 to 77 K. By contrast, the TF sample and TAF sample annealed at 843 K exhibited significant delamination toughening; vE increased as the testing temperature decreased. Some of the impact bars were broken into two pieces, as indicated by +. However, it should be noted that the occurrence of delamination toughening was significantly shifted toward the lower temperature side through the annealing at 843 K. The delamination finish temperature (TDF), below which vE decreased to the level of vEUS or less,1,3) was estimated to be approximately 233 K for the TF sample, and 173 K for the TFA sample annealed at 843 K.
Charpy V-notch absorbed energy (vE) as a function of testing temperature and annealing temperature for the TF3) and TFA samples. Data for the sample quenched and tempered at 773 K (QT)3) are also shown for reference. β is the angle between delamination crack and RD. Data points with + indicate the specimens that did not separate into two pieces during the impact test. (Online version in color.)
In the UFEG structure with a strong <110>//RD fiber texture, weak {100} cleavage planes and grain boundaries involving carbide particles are present parallel to the longitudinal direction (//RD) of the impact test bar. Hence, the crack-arrester-type delamination can be induced through the interaction between these weak boundaries and planes and the tensile stress (σt//SD) that is generated along the SD by the localized plastic constraint at the notch and/or the crack tip.1,3) σt scales with YS and decreasing YS can lower TDF. Since many {100} cleavage planes also exist on the 45° planes inclined at an angle of 45° to the RD, it is necessary to suppress the transverse brittle cracking along the 45° directions to enhance the delamination toughening at lower temperatures. Decreasing the transverse grain size is effective to suppress the occurrence of transverse cracking.1,3) As for the TFA sample annealed at 973 K, the transverse brittle cracking became more pronounced at lower temperatures rather than the delamination cracking along the RD, resulting in a decrease in vE. This is considered to be due to the larger and more equiaxial grain structure (ILHAB_T = 0.69 μm).5) Furthermore, the crack-arrester-type delamination was characterized by a stepwise crack propagation consisting of delamination cracks (//RD) and ductile steps (//SD).3) Once delamination cracks propagate in the RD, crack re-initiation occurs under conditions of nearly uniaxial tension, raising vE. In this case, higher toughness and ductility in the RD plane are considered to be effective to enhance the delamination toughening. The annealing of the TF sample at 843 K resulted in a slight decrease in YS and improved tensile ductility at lower temperatures (Fig. 4). This is considered to have caused a marked decrease in TDF.
Strength and toughness have been reported to be variable depending on the precipitation of nanoscale coherent precipitates in Mo-bearing steels.6,7) Thermal desorption spectroscopy analysis of the hydrogen absorption capacity of the TF and TFA samples suggested that the coherency and interfacial area between the nanoscale Mo-rich precipitate and matrix varied during annealing at temperatures between 773 and 923 K.4) More recently, Lee et al. demonstrated by three-dimensional atom probe analysis that Mo existed as either cluster or nanoscale carbides (~4 nm) in 0.6%C-2%Si-1%Cr-0.4%Mo steel quenched and tempered at 843 K.8) The content of phosphorus, which might cause temper embrittlement,6) was as low as 0.002 mass% in the present steel. Therefore, it is concluded that the nanoscale Mo-rich precipitate might be one of the microstructural factors that control the delamination toughening of Mo-bearing steel with a UFEG structure. As well as transition carbides (2–4 nm),9) Mo-rich precipitates are too fine to act as void nucleation sites. Hence, the influence of Mo-rich precipitates on ductile fracture is considered as small. The change in the precipitation state of Mo-rich precipitates through the annealing at 843 K might enhance brittle fracture stress in the UFEG matrix. However, further detailed investigation is needed to clarify it.
The influence of annealing on the microstructure and mechanical properties was investigated for a 0.4%C-2%Si-1%Cr-1%Mo steel with a UFEG structure processed by warm tempforming at 773 K. The following results were obtained.
(1) The annealing at 843 K had little influence on the spheroidized carbide particle distribution in the UFEG structure, but did affect the precipitation of nanoscale Mo-rich precipitate.
(2) Through the annealing at 843 K, YS slightly decreased while tensile strength hardly decreased. Furthermore, the tensile ductility was improved at lower temperatures.
(3) The delamination toughening was enhanced at lower temperatures through the annealing at 843 K. It is suggested that this resulted from the change in the precipitation state of nanoscale Mo-rich precipitates with annealing.
The study was partly supported by grants from the JSPS KAKENHI Grant Number 19H02468, and the Japan Science and Technology Agency (JST) under Collaborative Research Based on Industrial Demand “Heterogeneous Structure Control: Towards Innovative Development of Metallic Structural Materials.”
