Abstract
The effect of Nb addition of less than 0.05 mass% on the quenching and tempering behavior of spheroidized eutectoid steel, which has usually being applied to knitting needles, was investigated. The results obtained are as follows. 1) Hardenability upon brief heating was markedly improved by 0.01 mass% Nb addition. 2) Both quenching elongation and its standard deviation decreased with 0.01 mass% Nb addition compared with those of Nb-free steel. 3) While hardly any effect of Nb addition on the hardness was observed during low- temperature tempering, not only the impact toughness but also the fatigue durability was improved by 0.01 mass% Nb addition. 4) Atom probe tomography (APT) analyses revealed that the precipitation of carbon in solution directly resulted in the formation of ε- and/or θ-carbides with carbon contents of around 25 at% without the formation of clusters with 10–15 at% C upon the addition of a trace amount of Nb. 5) For the same P content, the average bulk concentration of P in the martensite phase markedly increased with the addition of up to 0.05 mass% Nb. 6) Regarding the optimum Nb content of 0.01 mass% for various mechanical properties upon the low-temperature tempering of martensite, it is considered that the mechanical properties are dominated by the balance between the positive effect of promoting carbide precipitation during low-temperature tempering by Nb addition and the negative effect of deteriorating the toughness with increasing bulk concentration of P in the martensitic phase upon the addition of more than 0.02 mass% Nb.
1. Introduction
Typical knitting needles, which are important parts of knitting machines, have both a hook and a movable latch on the tip of them for knitting fabrics with rapid reciprocating motion. The manufacturing process of a needle is composed of multiple steps, such as stamping (shearing to form the shape of the needle), swaging of the hook portion, grinding and U-bending of the hook, and cutting a groove on the needle edge that is used to insert and caulk the latch inside the groove. The needle in its final shape is subjected to heat treatment to adjust the hardness in the range from 660 to 700 HV to finish the product with impact toughness, fatigue durability, and abrasion resistance.1)
Hypereutectoid steel with a carbon content of 0.8 to 1.1 mass% has been globally used as a material for knitting needle, and by repeated cold-rolling and spheroidizing annealing, its microstructure is controlled to recrystallized ferrite and homogeneously dispersed spherical fine cementites.2,3) In recent years, to increase the knitting speed and meet the growing demand for fine stitches, high reliability, high durability, and high wear resistance have been required for needles. To satisfy these requirements, the combination of the use of steel with both high cleanliness and low segregation of harmful elements and the application of low-temperature tempering (hereinafter referred to as LTT) at 250°C or less after quenching to adjust the hardness of the products to higher than 700 HV is becoming mainstream when manufacturing knitting needles.
To meet the above strict requirements for knitting needles, we previously investigated the substantial effects of phosphorus (P), the amount of which was less than 0.03 mass% as an impurity level, on the LTT behavior. It was revealed that the impact toughness recovery temperature during LTT increased with increasing P content in the range of 0.005 to 0.023 mass%.4,5,6) Even in the current steelmaking process, however, not only hot-metal treatment but also ladle refining is indispensable to reduce the P content to 0.005 mass%, which increases the steelmaking cost.7)
Thus, as a method of promoting the precipitation of carbides during LTT regardless of the P content, the possibility of adding a trace amount of niobium (Nb) was studied. Regarding the effect of Nb on the mechanical properties of steel, a number of studies have been carried out with the aim of refining the microstructure of steels.8,9) The substantial effects brought about by Nb addition are summarized as follows: Nb is likely to segregate at the interface similarly to Mo,10) even a small amount of Nb addition markedly reduces the mobility of the α (ferrite phase)/γ (austenite phase) interface,11) and Nb, an element belonging to the V group in the periodic table, has an attractive interaction with C.12) Regarding the interaction between Nb and P in the α-phase, on the other hand, Nb likely to form a phosphide in α-iron as same as Mo at 800°C. However, this was observed in steel with a high P content of 0.3 to 1.0 mass%,13) and it is known that the solid solubility limit of P is markedly reduced in α-iron by adding Nb.14) It has also been found that Nb had a repulsive interaction with P, when it segregated at the grain boundary of γ-phase.15) In addition to the interaction between trace amounts of Nb and P during the quenching and tempering of high-carbon steel, if the harmuful effect of P on the precipitation of C during LTT5) can be alleviated by adding trace amount of Nb, it might be useful suggestions for improving the properties of high-carbon steel used for knitting needles, and useful hints for understanding the LTT behavior of high-carbon martensite.
Thus, in this study, focusing on the application to knitting needles, the hardenability and quenching elongation, the change in crystalline texture, and the LTT behavior are investigated to clarify the optimum content of Nb in hypereutectoid high-carbon steel with a Nb content of less than 0.05 mass% with atomic-scale changes in C, Nb, and P analyzed by atom probe tomography (hereinafter referred to as APT).
2. Experimental Procedure
Four types of hypereutectoid steel with different amounts of Nb, namely Nb free (Steel-A), 0.010 mass% Nb (Steel-B), 0.021 mass% Nb (Steel-C) and 0.055 mass% Nb (Steel-D), were prepared by vacuum melting followed by casting into 30 kg ingots. Their chemical compositions are shown in Table 1. After blooming the steel ingots, they were hot-rolled to a thickness of 4.0 mm with a laboratory hot-rolling mill. After hot-rolling, the hot-bands were promptly subjected to hot-coiling treatment by soaking at 680°C for 1 hour and then cooled in the furnace. Finally, cold-rolling and spheroidizing annealing at 680 to 700°C were repeated multiple times to obtain cold-rolled steel sheets with a thickness of 0.4 mm.
Table 1. Chemical composition of steels used (mass%).
| Steel No | C | Si | Mn | P | S | Cr | Mo | Nb |
|---|
| A | Nb free | 1.01 | 0.26 | 0.73 | 0.009 | 0.0032 | 0.419 | 0.021 | 0.001 |
| B | 0.01%Nb | 1.01 | 0.24 | 0.71 | 0.011 | 0.0025 | 0.409 | 0.019 | 0.010 |
| C | 0.02%Nb | 1.02 | 0.25 | 0.71 | 0.012 | 0.0030 | 0.392 | 0.019 | 0.021 |
| D | 0.05%Nb | 1.01 | 0.25 | 0.72 | 0.011 | 0.0028 | 0.391 | 0.020 | 0.055 |
Figure 1 shows the heat treatment sequence carried out in this experiment. Hardenability was evaluated by heating samples up to 780°C and 800°C in a furnace with N2 atmosphere for 2 to 16 min, which was followed by quenching in an oil-bath kept at 80°C. The soaking of hypereutectoid steel before quenching has generally been performed at a temperature within the (γ + θ) inter-critical region to minimize the remaining of retained austenite after quenching. Therefore, to match the industrial conditions, 800°C was set as the basic heating condition, and heating at 780°C was also performed to evaluate the effect of Nb on the hardenability upon heating at an insufficient temperature.
The hardness (HV) of a cross section was measured for all quenched samples using a load of 5 kgf. Furthermore, for 40 specimens of 2.5 mmW × 135 mmL prepared from Steel-A and Steel-B, the quenching elongation was evaluated by the change in length due to quenching after heating at 800°C for 12 min. To examine the crystalline texture before and after quenching, (001) pole figures were analyzed from the electron backscatter diffraction (EBSD) data obtained by field-emission scanning electron microscopy (FE-SEM) observation at the cross sections of the transverse direction (TD) to the rolling direction.
To examine the changes in various properties caused by LTT, samples were soaked at 800°C for 10 min to obtain a constant hardness for all steels after quenching. Tempering was carried out at a temperature from 150 to 350°C for 60 min. The following tests were carried out for all samples: SEM observation of the microstructure in the TD cross section, Vickers hardness test with a load of 5 kgf, and impact toughness test using a small 1 J Charpy impact tester with a rectangular test piece of 10 mmw × 60 mmL having an I-notch, whose tip radius was 0.1 mm and depth was 2.5 mm, on one side of the center in the longitudinal direction, which was processed by wire discharging (Fig. 2). For the samples tempered at 250°C and 300°C, fatigue tests were performed using specimens with a size of 2 mmw × 15 mmL and a shoulder radius of 12.5 mm under a maximum load of 984 N and a stress ratio of 0.1 by the tension-tension mode.
Furthermore, in this study, the following experiments were carried out to examine in detail the mechanism underlying the atomic-scale phenomena occurring in the LTT process. For the quenching process, to determine the effect of Nb on the change in hardenability resulting from the brief heat treatment, the Snoek damping (Q−1) was evaluated by an internal friction test to evaluate the amount of C in the solution before heating, and transmission electron microscopy (TEM) observation and energy-dispersive X-ray (EDX) analysis of precipitates in the samples were performed after quenching. To examine the LTT process, APT analysis was carried out by focusing on the spatial distribution of C, P, and Nb atoms using a LEAP 4000 XHR system (CAMECA Co., Ltd.) for needle-shaped specimens prepared using focused ion beam (FIB) equipment. For the APT analysis, the temperature was set to 50 K, and the voltage pulse mode (pulse fraction 20%) was applied.
3. Experimental Results
3.1. Effect of Trace Nb on Microstructure and Hardness after Spheroidizing Annealing
The SEM images of the microstructures and the corresponding HV values after spheroidizing annealing and the obtained size distributions of spheroidized cementites are shown in Figs. 3 and 4, respectively. The microstructure of the steel after spheroidizing annealing is composed of ferrite and homogeneously dispersed spheroidized cementites. Since the HV hardness of all samples ranged from 253 to 258, hardly any effect of up to 0.05 mass% Nb on the hardness was observed. Regarding the size distribution of spheroidized cementites, hardly any effect of Nb was also observed, the average particle size of which ranged from 0.62 to 0.69 μm.
3.2. Effect of Addition of Trace Nb on Hardenability
Figures 5(a) and 5(b) show the effects of the soaking time and Nb content on the HV hardness of samples as-quenched after soaking at 780°C and 800°C, respectively. At both soaking temperatures, the hardness decreases upon soaking for 2 min but increases rapidly upon prolonging the soaking time. It is considered that the decrease in hardness upon heating for 2 min or less is partly due to the growth of ferrite grains under the insufficient α → γ reverse transformation. At the heating temperature of 780°C, only Steel-B has HV higher than 700 after soaking for 4 min, whereas the Steel-A and Steel-C have HV of 400–500 and Steel-D has HV less than 400. Upon heating at 800°C, HV values promptly increased and exceeded 700 after soaking for 4 min for all steels. In particular, the HV of Steel-B exceeded 700 after soaking for 3 min, indicating that the hardenability is improved by the addition of 0.01 mass% Nb irrespective of the soaking temperature.
Figure 6(a) shows the SEM images of samples soaked at 780°C for 4 min, for which a large difference in hardness was observed among the steels. In Steel-A, Steel-C and Steel-D, ferrite grain boundaries were clearly observed, whereas martensite laths were clearly observed in Steel-B. From these results, it is considered that the reverse transformation from the γ-phase to the γ-phase was retarded in Steel-A, Steel-C and Steel-D. Figure 7 shows the SEM images of all the steels : the HV steadily increased to over 800 upon quenching after soaking at 800°C for 10 min. No significant difference was observed in the distribution of undissolved cementites with the Nb content among the steels.
Next, to confirm the effect of Nb on the quenching elongation, the percentage changes in the length of specimens of Steel-A and Steel-B are shown in Fig. 8. In Steel-B with 0.01 mass% Nb, the mean elongation ratio (mean-el) was decreased by 0.023% and the standard deviation (σ) was decreased by 0.8 × 10−5 compared with those in Steel-A without Nb.
To interpret the effect of the adding Nb on reducing the amount of quenching elongation from the change in crystalline texture, the textures of Steel-A and Steel-B before and after quenching were compared with the (001) pole figures obtained by EBSD analysis as shown in Fig. 9. Before quenching, since the cold-rolled sheet was subjected to spheroidizing annealing under equilibrium conditions by box-annealing, the γ-fibers recrystallization texture, which is composed of <111>//ND grains, developed as well as that of the low-carbon ferritic steel. In Steel-A, the intensity of {111} <112> components is relatively high within the γ-fiber texture, whereas in Steel-B, it exhibits nearly random γ-fiber comprising <111>//ND grains. The texture after quenching appears to be almost random since the ferrite matrix changes to martensite through a reverse transformation to the austenite phase, but the texture of martensite in Steel-B appears to be more random than that in Steel-A as well as that in the as-annealed steel. It is suggested that the martensite derived from the ferrite matrix that randomized within the plane upon the addition of 0.01 mass% Nb has the tendency that the orientation of the c-axis of the martensite lattice is distributed more randomly, which probably decreases the elongation in a specific direction upon quenching.
Next, the change in hardness upon LTT at 150 to 350°C is shown in Fig. 10. The hardness of the as-quenched steel is almost the same as that of the steel obtained after tempering at 150°C. On the other hand, in the temperature range from 150 to 350°C, the hardness decreases linearly with the tempering temperature. Hardly any dependence of the change in hardness upon LTT on the amount of Nb addition was observed up to 0.05 mass%. In particular, the difference in HV hardness between 200 and 250°C, which are the commonly used tempering temperatures of the knitting needles, is within the range of 13 to 17, indicating that the effect of Nb is very small.
To understand the change in hardness in relation to the state of fine-carbide precipitation in martensite, the precipitation state of fine carbides in martensite subjected to tempering at 200 to 350°C was observed by SEM for Steel-A and Steel-B. The results are shown in Fig. 11. The precipitation of fine carbides was not observed after tempering at 200°C for every steel, but when the tempering temperature was raised to 250°C or higher, the precipitation of fine carbides was observed at the interfaces of martensite laths, and fine carbides were observed clearly after tempering at 300°C and above. The softening due to tempering can be explained by the precipitation of fine carbides from supersaturated C in solution, but no morphological changes in the fine carbides due to Nb addition were confirmed by SEM observation.
Next, the effect of the Nb content and tempering temperature on the Charpy impact value is shown in Fig. 12. When tempering at 200°C or lower, hardly any recovery of the toughness value was observed. Toughness recovery was clearly observed upon tempering at 250°C and above. In particular, the toughness recovery of Steel-B was significantly greater than that of the other steels in the temperature range of 250 to 300°C. On the other hand, the toughness recovery of Steel-D was delayed as compared with that of the other steels. When the tempering temperature was raised to 350°C and the steel was softened to about 600 HV, the toughness value decreased with increasing Nb content. It is clear that Steel-D exhibits the lowest toughness value at every tempering temperature, suggesting that 0.05 mass% Nb is a too large amount to maintain the sufficient toughness of needles.
Next, the effect of the Nb content on the fatigue life with the tension-tension loading mode for Steel-A, Steel-B and Steel-C tempered at 250 and 300°C is shown in Fig. 13. In Steel-B, the time to fracture is longer than that in the other steels at every tempering temperature. Comparing the fatigue life for 250°C tempering with that for 300°C tempering, the fatigue life and the toughness value are greater for 300°C, and the increase in the fatigue life from 250 to 300°C is larger in Steel-B than in the other steels. This is consistent with the improvement of the toughness value shown in Fig. 12.
From the above results, it was confirmed that the addition of 0.01 mass% Nb to the hypereutectoid steel used to make knitting needles is effective for improving hardenability, reducing the quenching elongation during the heat treatment of needles, and increasing the impact toughness and fatigue durability when used in a knitting machine.
4. Discussion
There have been few studies focusing on the effect of adding a trace amount of Nb (0.01 mass%) to steels, especially hypereutectoid steels, while many studies have been carried out on the low-carbon steels containing 0.02 to 0.20 mass% Nb.16) The present study revealed that the addition of 0.01 mass% Nb to the hypereutectoid steel used for knitting needles was effective for improving their overall performance.
In this section, the effect of adding a trace amount of Nb on the α → γ reverse transformation rate at the soaking temperature prior to quenching and that of adding a trace amount of Nb on the precipitation of supersaturated C in solution during the LTT process are discussed on the basis of the results of TEM observation and APT analysis.
4.1. Effect of Trace Nb on Hardenability
As shown in Figs. 6(a) and 6(b) and, it was presumed that the α → γ reverse transformation at the soaking temperature was promoted by adding 0.01 mass% Nb. Then, focusing on the difference between the amount of residual C in solution in the ferrite matrix and the dissolution rate of spheroidized cementite, the effect of a trace amount of Nb is discussed on the basis of additional experiments.
First, for all steels after spheroidizing annealing, the Snoek damping (Q−1max), measured by using lateral-vibration-type internal-friction testing equipment, is shown in Fig. 14. When Nb was added, the Snoek peaks shifted to a slightly higher temperature and became broad, and a slight increase in Q−1max was recognized, i.e., 2.4 × 10−4 for Steel-A, 3.3 × 10−4 for Steel-B, 3.3 × 10−4 for Steel-C and 4.0 × 10−4 in Steel-D. The amounts of interstitial solid solution elements were calculated using the empirical formula of Aoki et al., that is C + N (mass%) = 0.0043 · T · Q−1max.17) Values of about 3.3 ppm for Steel-A, about 4.8 ppm for Steel-B, about 5.0 ppm for Steel-C, and about 5.9 ppm for Steel-D were obtained, which indicate that Nb addition increases the amounts of residual interstitial elements (C and N) in the ferrite matrix after spheroidizing annealing. On the other hand, the broadening of the Snoek peak cannot be clearly interpreted, but for Steel-C and Steel-D, for which broadening was prominent, the peak associated with the cold-worked damping disappeared at a temperature above 150°C. Taking into account the recent discovery of a new broad peak due to the C pair in ε-carbide by Shimotomai,18) it seems that the basic damping process due to the C in solution may change in the steel to which 0.02 mass% or more Nb has been added. It is not yet known how Nb affects the dissolution of spheroidized cementite or the α → γ reverse transformation behavior during heating.
Furthermore, for Steel-B, Steel-C and Steel-D to which Nb was added, the TEM images and the EDX analyses of the precipitates of the samples quenched after soaking at 780°C for 4 min are shown in Fig. 15, for the case of the largest hardness change observed in Fig. 5(a). In Steel-B, the presence of Nb was confirmed from the contrast, which appeared to be in the form of NbC precipitates in martensite, whereas in Steel-C and Steel-D, Nb precipitates were clearly observed in the untransformed ferrite phase. It was confirmed by EDX analysis that the greater the amount of Nb added, the larger the precipitates and the greater the Nb content. This result indicates that the greater the Nb content, the more C exists as NbC in the ferrite matrix before quenching. We estimated the amount of C fixed as NbC from the stoichiometry of Nb/C to be 13 ppm in Steel-B, 26 ppm in Steel-C, and 65 ppm in Steel-D.
From the above results, it is difficult to specify the reason why the α → γ reverse transformation occurs in the steel with a Nb content of 0.010% when it is briefly heated. Before heating, however, the C in the steel is fixed as NbC upon the addition of Nb. Considering the above result and the behavior of the bulk concentration of P, which is increased by the addition of Nb as described later, it appears that the α → γ reverse transformation might be delayed by suppressing the dissolution and diffusion of C when over 0.02 mass% Nb is added. On the other hand, the reason why the hardenability is improved by the addition of 0.01 mass% Nb to the Nb-free steel is considered to be that the residual amount of dissolved C in the α-phase after the spheroidizing annealing tends to slightly increase with the Nb addition. It is considered or concluded that a trace amount of Nb may affect the thermal stability and dissolution behavior of spheroidized cementite.
4.2. Effect of Trace Nb on Low Temperature Tempering Behavior
To discuss why the toughness recovery was faster and the fatigue life was longer in the steel with 0.01 mass% Nb than in the other steels during the LTT process, the atomic-scale changes in the spatial distributions of C and P were studied by the APT.
For Steel-A and Steel-B, the atomic maps of C obtained by the APT analysis for the as-quenched samples (As-Q) and the samples tempered at 250°C (QT250) and 300°C (QT300) after quenching from 800°C are shown in Fig. 16. Even in the as-quenched sample, a somewhat non-uniform distribution of C atoms was observed, indicating that the atomic-scale distribution of C atoms has already been affected by the martensitic transformation proposed by Sharby et al.19) The addition of Nb tends to make the contrast of C atoms slightly clearer, but it cannot be concluded from this C atom map that this effect is due to Nb. On the other hand, tempering at 250°C or higher clearly increased the concentration of C atoms. This probably involved a process in which supersaturated C in solution precipitated as θ-carbides or ε-carbides, and the contrast of C atoms thus become clearer upon the addition of Nb. After tempering at 300°C, a large region where C atoms were sparsely distributed appeared in Steel-A, whereas a region where C atoms disappeared was clearly observed in Steel-B, thus clarifying the change in the concentration of C atoms due to Nb addition.
Next, to understand the change in the concentration of C atoms more quantitatively, Fig. 17 shows the one-dimensional concentration profile of C atoms along a length of 200 nm detected in a rectangular region of 5 nm × 5 nm × 200 nm evaluated from the data in Fig. 16. Fluctuations in the concentration of C atoms were observed in both steels, even in the as-quenched samples. After tempering at 250°C, the clustering of C atoms with a concentration of about 10 to 15 at% was observed in Steel-A, as also reported by Caballero et al.,20) whereas in Steel-B a peak at a concentration of 10 at% or less and a peak at a concentration of about 25 at%, which appears to correspond to θ-carbides, were observed instead of clusters with a C concentrations of about 15 at% observed in Steel-A. Next, after tempering at 300°C, C concentration peaks of 20 to 30 at%, which are considered to correspond to θ-carbides, were observed for both steels, but Steel-B has a larger amount of precipitated θ-carbides. Focusing on C concentrations of less than 5 at%, while a small fluctuation of C of 5 at% or less was still observed in Steel-A, the fluctuation of C in Steel-B was nearly zero.
The above results suggest that the addition of 0.01 mass% Nb promotes the precipitation of ε- and θ-carbides during the LTT process. In particular, in Steel-A, the fact that clusters of C with a concentration of about 15 at% were observed before ε- and θ-carbide precipitation during the LTT process is in agreement with our previous result5) on the effect of trace amounts of P. In the present study, by adding a trace amount of Nb to steels with the same P content as that Ref. 5, the clusters of C with a concentration of about 15 at% disappeared and carbides with a C concentration of about 25 at% were observed, which is the same as the effect of reducing the P content. This suggests that the delay in the C precipitation process caused by the existence of a trace amount of P was alleviated by adding a trace amount of Nb. This result suggests that ε- and θ-carbides may have precipitated directly from supersaturated C in solution in Steel-B with a trace amount of Nb added, similarly to that observed in the steel with a very low P content.
Then, to confirm the behavior of P atoms in martensite upon the addition of trace Nb, the one-dimensional concentration profile of P atoms in Steel-A, Steel -B and Steel -D is shown in Fig. 18. Despite the P contents of the three types of steel being almost the same (0.010 ± 0.001 mass% ~0.017 at%), not only the concentration fluctuation of P in martensite but also a higher average concentration in Steel-D can be clearly observed. Hardly any change in the one-dimensional concentration profile of P due to tempering at 250°C was observed among the three steels. Therefore, to compare the concentration of P atoms in martensite with the average bulk concentration, the analytical area was expanded to 25 nm × 25 nm × 100 nm, the result of which is shown in Fig. 19. To confirm the reproducibility of the results, multiple APT data were obtained for each sample: thus, there are multiple plots for the same sample in Fig. 19. In the as-quenched Steel-A, the P concentration was 0.03 at%, which was slightly higher than the atomic concentration of 0.017 at% calculated from the chemical composition shown in Table 1. Supposing that P is in solution in the martensite, since P is unlikely to be in solution in the spheroidized cementite, it is considered that the average concentration of P slightly increases at the atomic level. On the other hand, in the steels to which trace amounts of Nb had been added, there was no marked difference in the average concentration of P between Steel-A and Steel-B, whereas in Steel-D, the average concentration of P increased from 0.05 to 0.12 at%. Since the concentration increased immediately after quenching from 800°C, the average concentration of P in the γ-phase reached this range. Thus, it is considered that the concentration of P in the γ-phase may have been promoted by the preferential segregation of Nb at the γ-grain boundaries or the interface of the γ-phase and undissolved cementite. At present, no explanations have been given in the literature for this large concentration of P caused by the addition of up to 0.05 mass% Nb. Moreover, after tempering at 250°C, the average concentration slightly increased for Steel-B, but the effect of the Nb content remained the same with quenching.
The above results are consistent with the fact that P is already concentrated in the martensite in as-quenched Steel-D, and the toughness value is also lower than that of the other steels. The toughness behavior after tempering at 350°C, at which the recovery of the toughness of martensite depends on the amount of Nb, is considered to be caused by the difference in the average concentration of P in the martensite.
On the basis of the above results, the effect of trace Nb on mechanical properties, such as the toughness of hypereutectoid steel subjected to quenching and tempering is schematically shown in Fig. 20. As shown in Figs. 15 and 16, in the steel containing 0.01 mass% P, ε- or θ-carbides can be directly formed during the LTT without an intermediate clustering process of C in the vicinity of P atoms. It is considered that adding Nb promotes the tempering of martensite, which has a positive effect on toughness recovery. On the other hand, the increasing average bulk concentration of P with Nb addition revealed in this study is significant in steels containing more than 0.02 mass% Nb. Therefore, considering the adverse effect of P in solution on the mechanical properties, it is thought that the addition of excess Nb reduces the toughness of martensite. Taking the above two factors into consideration, the trend shown by the dotted line in Fig. 20 is derived. This tendency confirms the result that mechanical properties such as the toughness and fatigue characteristics of hypereutectoid steel examined in this research are improved by the addition of 0.01 mass% Nb.
5. Conclusions
As a result of examining the effect of trace Nb addition in the range of 0.01–0.05 mass% on the texture formation and mechanical properties of hypereutectoid steel subjected to quenching and tempering, which are used for textile machine parts such as knitting needles, the following conclusions were obtained.
(1) The hardenability upon brief heating was improved by adding 0.01 mass% Nb. However, under suitable soaking conditions, hardly any effect of the addition of trace Nb on the hardness was observed.
(2) Regarding the change in quenching elongation, the dimensional change and dispersion were reduced in the steel to which 0.01 mass% Nb had been added compared with those in Nb-free steel. This is presumably caused by the martensite texture after quenching, which was more randomized by the <111>//ND texture before quenching upon the addition of Nb.
(3) Hardly any effect of Nb on the softening behavior was observed in the low-temperature tempering process, but the increase in toughness of the steel containing 0.01 mass% Nb was considerable when the tempering temperature was increased from 250 to 300°C.
(4) After low-temperature tempering, the fatigue life of the steel with 0.01 mass% Nb was 30% to 50% longer than those of the Nb-free steel and the steel with 0.02 mass% Nb.
(5) Atomic scale analysis by APT revealed that supersaturated C in solution precipitated directly as ε- or θ-carbide for consistency during the low-temperature tempering process without forming carbon clusters with concentrations of about 10 to 15 at% in the steels with Nb addition. This suggests that the addition of Nb promotes the basic process of carbide precipitation during low-temperature tempering.
(6) For steels with the same P content, it was confirmed that the bulk average concentration of P in the martensite increases with the addition of Nb. This tendency becomes prominent when 0.05 mass% Nb is added to steels.
(7) The steels with 0.01 mass% Nb addition had improved properties after low-temperature tempering. The Nb content can be optimized by balancing the positive effect of carbide precipitation in the martensite and the negative effect due to the increase in the bulk average concentration of P with the addition of Nb.
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