Introduction

Current requirements on the quality of metal products and structures are leading to a rapid increase in the production of alloy steels as a percentage of all of the iron alloys that are made.

Recent years have seen significantly more interest being shown in high-nitrogen alloy steels (HNSs), particularly those that are resistant to corrosion [1]. One of the main advantages of these steels compared to analogous conventional steels is their higher strength. This makes it possible to reduce the volume of production of high-alloy steels 10 – 20% as a result of a reduction in the working cross section of machine parts, mechanisms, and structures, since such a reduction also decreases the amount of material that these elements contain.

The process of alloying iron alloys with nitrogen in large quantities requires the use of special production processes, which is one of the factors that raises the steelmaking costs. However, since nitrogen has a high capacity for stabilizing austenite, alloying with nitrogen makes it possible to reduce stainless steels’ content of austenite-forming elements — nickel and manganese — by a factor of 1.5 – 2 or even avoid adding these elements at all. The introduction of nitrogen into steel not only makes them stronger and saves alloying elements, but it also solves environmental problems. Being a component of air, nitrogen is inexpensive, and the process of extracting it from air does not require the surface and subsurface mining necessary to obtain ores.

At the same time, strengthening austenitic steels with nitrogen inevitably increases the load on the equipment that is used to process those steels. The high nitrogen content can significantly alter aging processes — particularly strain aging — in high-alloy steels and thus influence the choices made for the processing regimes and areas of application. In particular, the high nitrogen content can affect decisions to use such steels in applications which require that the steel have high ratings for certain special properties — especially resistance to corrosion.

The goal of the investigation being discussed in this article was to study how commercial nitrogen-bearing austenitic steels alloyed by different systems behave mechanically during hot and warm deformation.

Methods of Study

The chemical composition of the steels that were studied is shown in Table 1.

Table 1 Chemical Composition of the Steels
Full size table

All of the steels were obtained by industrial refining and were in different initial states, i.e. they differed in their preliminary processing: casting; standard hot rolling; high-temperature thermomechanical treatment (HTMT) with different degrees and schemes of deformation; quenching, cold rolling after HTMT (Table 2).

Table 2 Initial State (Treatment) of the Investigated Steels
Full size table

The hot longitudinal rolling performed as part of the HTMT cycle (HTMT LR) was done on a 210 hot-rolling mill. The 2 × 25 × 75 × 100 mm specimens, in the form of parallelepipeds, were subjected to hot rolling with ε = 50% (in one pass) and 80% (in two passes): 50% + 30% (total reduction). They were then cooled in water or air. Rolling speed was 0.1 m/sec.

Cold longitudinal rolling was performed on a 160 coldrolling mill. The 5 × 5 × 65 × 100 mm specimens, which were cut from other specimens that had been hot-rolled in the HTMT cycle, were subjected to cold rolling with a 25% reduction in two passes (23% + 2% total reduction).

Hot radial-shear rolling in the HTMT cycle (HTMT RSR) was done on a “25” mill. The temperature to which the specimens were heated prior to RSR corresponded to the temperatures to which the steel was heated prior to hot longitudinal rolling in the HTMT cycle. The total deformation ε = 65% was attained in five passes, with conversion of the initial 23.5-mm-diam. round into a 14.5-mm-diam. final product. The size of the reduction was the same in each pass. Rolling speed was 0.01 m/sec. The specimens were kept in the furnace for 5 – 10 sec between each pass.

The subsequent processing operations were:

a) quenching from 1050 – 1150°C (the exact temperature depending on the grade of steel) in water;

b) hot deformation in compression on a “Gleeble System 3800” unit;

c) hot + warm deformation in compression on the Gleeble System 3800;

After each of these operations, the specimens were measured to determine their hardness HV and were subjected to metallographic analysis.

Hot upsetting (compression) of the specimens was done at a rate of 1 sec – 1 in a vacuum and was followed immediately by cooling in air to room temperature; the true strain (e) was 0.51 (ε = 40%).

The dimensions of the cylindrical specimens that were used to conduct tests: ∅ 5 × 5 mm and ∅ 12 × 12 mm. The preheating and deformation temperatures were within the range 1050 – 1150°C, the exact temperature depending on the grade of steel.

Warm deformation of the specimens was also done in compression, after hot compression at temperatures in the range 1050 – 1150°C (depending on the grade of steel) with e = 0.22 (ε = 20%) and rapid (at a rate of 200 K/sec) cooling to 410°C. The only exception was steel 12Kh18N10T, which was cooled to 650°C. The true strain in the warm deformation was e = 0.11 (ε = 10%). The testing was done in a vacuum and was followed immediately by cooling in air to room temperature. The rate of warm deformation was 0.01 sec–1.

Results and Discussion

Figure 1 and Table 3 present the stress-strain curves of the specimens in compression and the results obtained from their analysis.

Fig. 1
figure 1

Stress-strain curves of the investigated steels from compression during hot rolling at a speed of 1 sec – 1 (the numbers next to the curves denote the numbers of the steels in Table 1): 1, 2, 3, 4) deformation temperature T d = 1050°C; 5, 6, 7) T d = 1150°C.

Full size image
TABLE 3 Parameters of the Stress-Strain Curves from Hot Compression (\( \dot{\upvarepsilon} \) = 1 sec–1)
Full size table

The following can be concluded by analyzing the stress-strain curves (Fig. 1), characteristics calculated on the basis of those curves, and the results obtained from metallographic studies of the structure of the specimens (Table 3).

The stress-strain curves of the steels alloyed with nitrogen (curves 2, 4, and 7) often have a maximum. The curve of steel 3 has a steady-state stage: σmax = σsts , while cast steel 6 continuously hardens to e = 0.65 during its hot deformation under the chosen conditions. Softening processes are dominant during the final stages of deformation for most of the investigated steels (except for steels 3 and 6). The diagrams that describe the hot deformation of the nitrogen-alloyed steels have an upper yield point at 1050°C and a yield-point plateau (curves 2 and 4).

The softening which takes place during deformation depends both on the rate of diffusion and on the stress level and the initial structure. Thus, the value of σmax that corresponds to dynamic equilibrium between strain-hardening and softening of the steels changes in a complex manner.

Increasing the contents of nitrogen and carbon in steels with a similar alloying base helps softening begin earlier. For example, the value of σmax is reached twice as quickly on the curve of steel 4 (0.3% N) as it is on the curves of steels 2 (0.127% N) and 3 (0.135% N). This obviously occurs due to the greater strain-hardening which takes place.

When analyzing the parameters of the diagrams that describe hot deformation of the steels and comparing steels with a similar alloying base, it becomes apparent that, other conditions being equal, nitrogen increases resistance to hot deformation and thereby elevates all of the stress characteristics: σ0.2 , σmax , etc. In other words, nitrogen strengthens the solid solution in the high-temperature state as well. For example, the maximum attainable deformation resistance in steel 10Kh23N18 (curve 5) is nearly twice as low as in its nitrogen-bearing analog — steel 10Kh23AN18 (curve 6) — for the same degree of deformation.

On the whole, the resistance of a steel to hot deformation is affected mainly by the steel’s composition. The higher its nitrogen content and the greater the ratio C/N, the greater the deformation resistance and the earlier softening begins. It was shown in [2] that the initial structure has a marked effect on softening processes in the steel, with this occurring mainly as a result of the effect of initial grain size on the process of recrystallization.

Strain-aging effects may be seen during the hot deformation of high-nitrogen austenite. High-nitrogen austenite is even more likely to undergo aging at the temperatures corresponding to warm deformation. The aging of traditional stainless steels of type Kh18N10 is often used to increase their strength, especially when they are used as heat-resistant (high-temperature) steels and the aging is done within the temperature range 700 – 750°C. Increasing the steel’s nitrogen content and its total content of nitrogen and carbon can shift the temperature range in which carbonitrides are precipitated to lower temperatures. In connection with this, we performed a calorimetric analysis of the processes that occur during the heating of these steels after quenching to a solid solution from 1050°C. The same type of analysis was performed for steel 10Kh23AN18 quenched from 1100°C (Fig. 2). The calorimetric curves are represented as difference curves which describe the heating of the quenched specimens, their repeat heating to 550°C in the calorimeter, and their cooling to room temperature.

Fig. 2
figure 2

Calorimetric curves describing the heating of steels Kh18N10 (1) and 08Kh18AN10 (2) after quenching from 1050°C and the heating of steel 10Kh23AN18 (3) after quenching from 1100°C. The numbers next to the curves indicate the thermal effect.

Full size image

Slight thermal effects were seen in the nitrogen-bearing steels within the range 340 – 450°C, these phenomena probably be related to the precipitation of nitride and carbonitride phases. Such effects might be seen at lower temperatures — temperatures within the range 365 – 420°C — in steel 10Kh23AN18, which has a higher content of nitrogen and chromium. Figure 2 also shows the calorimetric heating curve of nitrogen-free steel Kh18N10 (0.02% C; 18.2% Cr; 10.2% Ni; 1.5% Mn; 0.25% Mo; 0.4% Si); no precipitation or heat-absorption sections are seen on this curve over the entire range of temperatures to which the steel was heated (200 – 550°C).

The different absolute values of the effects and the different forms of the peaks on the DSC curves of the different steels might be related to the fact that dissolution and the precipitation of excess phases might proceed to different degrees of completion in each case. However, we will not discuss the indices pertaining to these effects further in this article, in light of the inadequate sensitivity of the method that was used. It is important to note that no phase transformations were detected calorimetrically in the nitrogen-free steel-analog when it was heated to temperatures below 550°C.

We did not investigate the phase precipitation that usually takes place in nitrogen-free steel Kh18N10 at higher temperatures.

Measurement of the Vickers hardness of the steel specimens that were tempered by the heating regimes used in the calorimeter showed the following.

When steel 10Kh23AN18 was tempered at 360, 420, 450, 520, and 550°C, its hardness remained at about 220 HV throughout the range of tempering temperatures. The hardness of steel 08Kh18AN10 also remained at roughly 210 HV during and after tempering at 300, 420, and 500°C. The fact that the hardness of these steels did not change is probably related to compensation for the precipitation of strengthening particles of carbides, nitrides, and carbonitrides and a reduction in the strengthening of the solid solution as a result of its decomposition.

To study strain-aging during warm deformation, we constructed diagrams depicting the warm deformation of certain chromium-nickel steels at 410 and 650°C (Fig. 3).

Fig. 3
figure 3

Stress-strain curves from the warm deformation of steels at a rate of 0.01 sec – 1 (Td = 650°C for steel 12Kh18N10 (1) and Td = 410°C for the other steels): 1, 2, 3, 4, 5) steels 12Kh18N10, 04Kh18N10AT, 08Kh18AN10, 08Kh18N10AT, and 10Kh23N18, respectively.

Full size image

The diagrams of all of the steels had the usual form—an increase in stress with strain and a gradual decrease in strain-hardening. In addition, the stress-strain curve of steel 04Kh18N10AT (curve 2) — the calorimetric curve of which clearly showed aging — contained a readily visible yieldpoint plateau. There was no upper yield point on the stress–strain curves of the other steels, although inflections were seen at low levels of deformation. However, these features were difficult to identify on the curves we obtained.

Table 4 shows calculated values of the parameters of the stress-strain curves plotted during warm deformation. It is apparent that grain size was usually larger in this case than after hot deformation with 40% strain (e = 0.51) (Table 3). The size of the austenite grains should not be changed by warm deformation, and in our case it should be equal to the grain size seen after the previous hot deformation with a reduction of 20% (e = 0.11). Grain size in all of the steels was larger after warm deformation than after the preliminary 40% deformation; for example, grain size in steel 04Kh18N10AT after hot deformation with ε = 40% (e = 0.51) was almost three times smaller than after hot deformation with ε = 20% (e = 0.11) and subsequent warm deformation, i.e. complete recrystallization does not take place at ε = 20%.

Table 4 Parameters of the Stress-Strain Curve from Warm Deformation at a Rate of 0.01 sec–1 and Grain Size after Deformation
Full size table

Together with the composition of the steel, the structure of the hot-deformed austenite determines the resistance of the steel to warm deformation. The greater the steel’s nitrogen content, the higher its resistance to small (σ0.2) and large (σε) thermal strains. Resistance to large strains increases more when the steel has a high value for the ratio C/N and a small grain size on the order of about 5 μm.

The final structure and properties of the hot-deformed austenite are determined solely by the conditions under which the steel undergoes hot deformation.

Figure 4 shows results from measurement of the hardness of the investigated steels in the initial state, after deformation by different schemes, and after quenching from 1050°C. It is apparent that the hardness of the initially thermomechanically strengthened steels decreases substantially after subsequent hot compression. If the preliminary treatment is quenching, then the change in hardness after hot compression will be small (or nonexistent, if the steel does not contain nitrogen; hardness can decrease if the steel does contain nitrogen). Subsequent warm deformation will increase the hardness of initially quenched steels and decrease the hardness of initially thermomechanically strengthened nitrogen-bearing steel 08Kh18AN10.

Fig. 4
figure 4

Hardness of the investigated steels in the initial state (1), after quenching from 1050°C (2), after hot deformation (3) (see Table 3), and after warm deformation (4) (see Table 4): 1) initial state (see Table 2): HTMT RSR (ε = 65%, T d = 1070°C) for steel 08Kh18AN10; hot-rolled for steel 55Kh20G9AN4; cast for steel 10Kh23AN18; quenched for the remaining steels.

Full size image

Conclusions

1. The resistance of nitrogen-bearing austenitic steels to hot deformation is affected mainly by their composition. The higher their nitrogen content and the higher the ratio C/N, the greater the steels’ deformation resistance and the earlier softening processes will begin. The initial structure of the steel significantly affects these processes because of the effect of grain size on recrystallization. The final structure and properties of the hot-worked austenite are determined solely by the conditions under which its hot deformation takes place.

2. Resistance to warm deformation is determined by the structure of the previously hot-worked austenite and the composition of the steel. The higher the steel’s nitrogen content, the greater its resistance to small (σ0.2) and large (σε) strains. Its resistance to large strains is greater in the case of higher values of the ratio C/N and a fine grain size of about 5 μm.

3. Increasing the nitrogen content of chromium-nickel austenitic steels reinforces the effects of strain-aging during hot and warm deformation, which can alter the steels’ resistance to corrosion and heat.

This study was conducted with financial support from the Research and Development Program of the Moscow State Institute of Steel and Alloys and the AVTsP (target program) “Development of the Scientific Potential of Higher Education.”