High-Speed Tensile Deformation Behavior of a Metastable 18Cr–6Ni–0.2N–0.1C Steel

Abstract

The present study investigated the high-speed deformation behavior of 6Ni–0.2N–0.1C steel. 0.2% proof stress (0.2% PS) of the 6Ni–0.2N–0.1C steel increased with an increase in strain rate () but tensile strength (TS) indicated almost the same value at above 10⁻¹ s⁻¹. Uniform elongation (U.El) largely decreased with an increase in . TS and U.El of the 6Ni–0.2N–0.1C steel at 10³ s⁻¹ were almost the same as those of SUS304 steel. When the effect of on mechanical properties was compared between the 6Ni–0.2N–0.1C and SUS304 steels, the strain rate dependence on 0.2% PS was larger in the 6Ni–0.2N–0.1C steel and that on TS was different at above 10⁰ s⁻¹. And the decrease of U.El with an increase in was larger in the 6Ni–0.2N–0.1C steel. The decrease of U.El at 10³ s⁻¹ was discussed from the viewpoint of change of flow stress at the maximum load point with an increase in . The estimated results proposed that austenite phase hardly transformed into deformation-induced martensite (α′) up to the maximum load point at 10³ s⁻¹ in the 6Ni–0.2N–0.1C steel. The tensile properties of the 6Ni–0.2N–0.1C steel are largely influenced by the higher strength of α′. It is difficult for the 6Ni–0.2N–0.1C steel to produce TRIP effect at high strain rates because the deformation-induced martensitic transformation is suppressed.

1. Introduction

Metastable austenite (γ) phase transforms into deformation-induced martensite (α′) by deformations, improving the mechanical properties including ductility.^1,2,3) This is well-known as the transformation-induced plasticity (TRIP) effect.⁴⁾ Various studies on the TRIP effect have been reported for TRIP-aided multi-microstructure steels (TRIP steels) developed mainly for steel sheet for automobiles.^5,6,7) Recently, many studies on the TRIP effect of high entropy alloys with metastable γ phase have also been reported.^8,9) Studies using γ single-phase steels are expected to play an important role in understanding the TRIP effect.^{10,11,12,13,14)}

In the previous study, we investigated the effect of deformation temperature on the tensile properties of Fe–18%Cr–6%Ni–0.2%N–0.1%C (6Ni–0.2N–0.1C) steel and discussed the role of α′ in the TRIP effect of metastable austenitic steels.¹⁰⁾ When the mechanical properties obtained by tensile tests at various deformation temperatures between 123 and 373 K were compared with those of SUS304 steel, the 0.2% proof stress (0.2% PS), tensile strength (TS), and uniform elongation (U.El) of the 6Ni–0.2N–0.1C steel were larger than those of the SUS304 steel at all temperatures. The mechanical stability of γ for the 6Ni–0.2N–0.1C steel was higher than that for the SUS304 steel. We also conducted neutron diffraction experiments at room temperature and showed that the improvements in the mechanical properties of the 6Ni–0.2N–0.1C steel were associated with larger work hardening of γ and higher strength of α′.¹⁰⁾ The increase in strength of α′ with the addition of N and C leads to better mechanical properties due to the TRIP effect, despite the smaller amount of deformation-induced martensitic transformation (DIMT).

On the other hand, both the TRIP effect and DIMT behavior also depend on the strain rate ($\dot{\epsilon }$).^12,13,15) Because the temperature rise of a specimen during adiabatic deformation¹⁶⁾ should be considered at high strain rates above about 10⁰ s⁻¹, it is difficult to understand the TRIP effect on mechanical properties and DIMT behavior.^17,18) Considering that TRIP steels have been used for automobiles, it is worth studying the effect of $\dot{\epsilon }$ on mechanical properties for metastable austenitic stainless steels to understand the TRIP effect on γ single-phase steels.^12,13) Therefore, this study examined the high-speed tensile deformation behavior including the effect of $\dot{\epsilon }$ on mechanical properties for the 6Ni–0.2N–0.1C steel. The dependence of tensile properties on $\dot{\epsilon }$ was also studied by using SUS304 steel^10,11,12) as a comparison. Especially, we focused on the tensile deformation behavior at 10³ s⁻¹ which corresponds to an automobile crash and examined the mechanical properties at 10³ s⁻¹ from the viewpoints of volume fraction of α′ (Vα′) and the dependence of flow stress (σ) on $\dot{\epsilon }$ in the 6Ni–0.2N–0.1C steel.

2. Experimental Procedures

We primarily studied two types of metastable austenitic stainless steel, 6Ni–0.2N–0.1C steel and JIS-SUS304 steel, whose chemical compositions are listed in Table 1. The Ni content of the 6Ni–0.2N–0.1C steel is 2 mass% lower than that of SUS304, and 0.2 mass% of N and 0.1 mass% of C are added as γ formers. The details of the 6Ni–0.2N–0.1C steel and JIS-SUS304 steel have been reported elsewhere.¹⁰⁾ We also prepared metastable austenitic steels with different Ni contents (5Ni–0.2N–0.1C, 7Ni–0.2N–0.1C and 8Ni–0.2N–0.1C steels) to control the mechanical stability of γ as seen in Table 1. Figure 1 shows optical micrographs of the 6Ni–0.2N–0.1C steel (a) and the SUS304 steel (b). The average γ grain sizes estimated by the conventional linear intercept method for the 6Ni–0.2N–0.1C and SUS304 steels were 52 and 18 μm, respectively. The average γ grain sizes of 5, 7 and 8 Ni–0.2N–0.1C steels were also approximately 50 μm.

Table 1. Chemical compositions (mass%) of the 6Ni–0.2N–0.1C, SUS304, 5Ni–0.2N–0.1C, 7Ni–0.2N–0.1C and 8Ni–0.2N–0.1C steels.

	C	Si	Mn	P	S	Ni	Cr	N
6Ni–0.2N–0.1C	0.099	0.4	0.99	0.002	0.0004	6.0	18.2	0.20
SUS304	0.05	0.4	0.98	0.03	0.008	8.2	18.2	0.023
5Ni–0.2N–0.1C	0.098	0.4	1.0	0.002	0.0007	5.0	18.2	0.20
7Ni–0.2N–0.1C	0.097	0.4	1.0	0.002	0.0004	7.0	18.3	0.20
8Ni–0.2N–0.1C	0.096	0.4	1.0	0.002	0.0005	8.0	18.2	0.21

Fig. 1.

Optical micrographs of the 6Ni–0.2N–0.1C steel (a) and the SUS304 steel (b).

Tensile tests were conducted at 296 K with various $\dot{\epsilon }$ between 3.3 × 10⁻⁶ s⁻¹ and 10³ s⁻¹. The tensile test specimens were prepared with the rolling direction parallel to the longitudinal direction. Tensile tests at $\dot{\epsilon }$ from 3.3 × 10⁻⁶ to 3.3 × 10⁻¹ s⁻¹ were conducted using a gear-driven tensile test machine (Instron, 5900 model).^10,11,12) Test specimens with a thickness of 1.5 mm, gage width of 5 mm, and gage length of 25 mm were prepared. Test specimens for high-speed tensile tests (10⁰ to 10³ s⁻¹) had a thickness of 1.2 mm, gage width of 2 mm, and gage length of 6 mm; these tests were conducted using a sensing block type testing machine (Saginomiya, High Speed Material Test System).^12,13) Moreover, tensile tests at $\dot{\epsilon }$ of 10⁰ to 10³ s⁻¹ for the SUS304 steel were also performed using another high-speed tensile testing machine (Shimadzu, HITS-T10 Hydroshot High-Speed Tensile Testing Machine) in order to calculate Vα′ in the fractured specimens. Test specimens with a thickness of 1.5 mm, gage width of 4 mm and gage length of 10 mm were prepared. Vα′ was calculated from X-ray diffraction (XRD) patterns obtained for specimens deformed up to the maximum load point (3.3 × 10⁻⁴ to 3.3 × 10⁻² s⁻¹) and for those after fracture (10⁰ to 10³ s⁻¹). Quantitative estimations of γ and α′ volume fractions by XRD experiments were based on the principle that the total integrated intensity of all the diffraction peaks for each phase in a mixture is proportional to the volume fraction of that phase.^11,12,13,14) The microstructure observation of fractured specimen at 10³ s⁻¹ was conducted by using a scanning electron microscope equipped with a measurement system of electron back scattering diffraction (EBSD). The EBSD pattern was analyzed as the possible phases were set as face-centered-cubic (FCC) and body-centered-cubic (BCC), and the scanning pitch was 0.1 μm.

3. Results and Discussion

3.1. Effect of Strain Rate on Mechanical Properties

Figure 2 shows nominal stress (s)–nominal strain (e) curves of the 6Ni–0.2N–0.1C steel (a) and the SUS304 steel (b) obtained by tensile tests at various strain rates. With an increase in $\dot{\epsilon }$, 0.2% PS increased and U.El and total elongations severely decreased. TS also decreased when $\dot{\epsilon }$ increased from 3.3 × 10⁻⁴ to 3.3 × 10⁻² s⁻¹ and increased again a little at $\dot{\epsilon }$ above 3.3 × 10⁻² s⁻¹.^12,13) Figure 3 shows 0.2% PS, TS, and U.El as a function of $\dot{\epsilon }$ in the 6Ni–0.2N–0.1C steel (a) and the SUS304 steel (b). 0.2% PS increased with an increase in $\dot{\epsilon }$ and the strain rate dependence on 0.2% PS was larger in the 6Ni–0.2N–0.1C steel. TS decreased up to $\dot{\epsilon }$ of 10⁻² s⁻¹ with an increase in $\dot{\epsilon }$. At the $\dot{\epsilon }$ above 10⁰ s⁻¹, TS of the SUS304 steel increased again with an increase in $\dot{\epsilon }$^12,13) whereas that of the 6Ni–0.2N–0.1C steel indicated almost the same value. U.El decreased with an increase in $\dot{\epsilon }$. This time, U.El at $\dot{\epsilon }$ below 3.3 × 10⁻³ s⁻¹ were larger in the 6Ni–0.2N–0.1C steel but the difference in U.El between the two steels became smaller at $\dot{\epsilon }$ above 3.3 × 10⁻² s⁻¹. Figure 4 shows the TS–U.El balance of various austenitic stainless steels at $\dot{\epsilon }$ of 10³ and 3.3 × 10⁻⁴ s⁻¹.^10,12,13) At 3.3 × 10⁻⁴ s⁻¹, the 6Ni–0.2N–0.1C steel indicated a better balance of TS and U.El.¹⁰⁾ The TS–U.El balance became smaller from 3.3 × 10⁻⁴ to 10³ s⁻¹ because U.El at 10³ s⁻¹ drastically decreased. In Fig. 4, both TS and U.El decreased with an increase in $\dot{\epsilon }$ in the metastable austenite steels, and TS increased but U.El decreased in the stable austenitic stainless steels.

Fig. 2.

Nominal stress–nominal strain curves of the 6Ni–0.2N–0.1C steel (a) and the SUS304 steel (b) obtained by tensile tests at various strain rates. (Online version in color.)

Fig. 3.

0.2% proof stress, tensile strength and uniform elongation as functions of strain rate in the 6Ni–0.2N–0.1C and SUS304 steels. (Online version in color.)

Fig. 4.

Tensile strength–uniform elongation balance of various austenitic stainless steels at 10³ and 3.3 × 10⁻⁴ s⁻¹. (Online version in color.)

3.2. Volume Fraction of Deformation-induced Martensite in the High-speed Tensile Deformation Behavior

Table 2 represents Vα′ at the maximum load point and those at the fracture specimens in the 6Ni–0.2N–0.1C and SUS304 steels obtained by the XRD experiments. Vα′ decreased with an increase in $\dot{\epsilon }$ but changed little at $\dot{\epsilon }$ above 10⁰ s⁻¹.^12,13,15) When Vα′ at a given $\dot{\epsilon }$ was compared between the two steels, Vα′ was smaller in the 6Ni–0.2N–0.1C steel¹⁰⁾ but the difference became smaller at $\dot{\epsilon }$ above 10⁰ s⁻¹. Figure 5 shows the SEM-EBSD inverse pole figure and phase map of the fractured specimen deformed at 10³ s⁻¹ in the 6Ni–0.2N–0.1C steel. In the phase map, green and red indicate γ and α′ phases, respectively. Approximately 10% of Vα′ was observed from the phase map and was in agreement with the Vα′ calculated by XRD experiment in Table 2. In Fig. 5, both annealing and deformation twins were observed. In the previous studies,^19,20,21,22) α′ was formed at the intersection of the ipsiron martensite band or planar slip band in the SUS304 steel. When the stacking fault energy decreases with an increase in N content, the shear band is composed of deformation twinning or ipsiron martensite band.²¹⁾ In the 6Ni–0.2N–0.1C steel, α′ was found to form at the intersection of deformation twins.^{3,19,20,21,22)}

Table 2. Volume fractions of deformation-induced martensite (Vα′) at the maximum load point (3.3 × 10⁻² to 3.3 × 10⁻⁴ s⁻¹) and those at the fracture specimens (10⁰ to 10³ s⁻¹) in the 6Ni–0.2N–0.1C and SUS304 steels.

Strain rate (s−1)	Vα′ (%)
	(a) 6Ni–0.2N–0.1C	(b) SUS304
103	8.5	8.6
102	7.9	9.0
101	7.1	7.9
100	8.8	6.0
3.3 × 10−2	9.8	16.2
3.3 × 10−3	18.6	46.8
3.3 × 10−4	30.5	48.0

Fig. 5.

Inverse pole figure map (a) and EBSD phase mapping image (b) in the 6Ni–0.2N–0.1C steel deformed at 10³ s⁻¹. (Online version in color.)

3.3. Tensile Strength and Uniform Elongation at 10³ s⁻¹ in the 6Ni–0.2N–0.1C Steel

Next, the high-speed deformation behavior at $\dot{\epsilon }$ of 10³ s⁻¹ in the 6Ni–0.2N–0.1C steel is discussed. As seen in Figs. 3 and 4, the U.El and TS–U.El balance of the 6Ni–0.2N–0.1C steel decreased with an increase in $\dot{\epsilon }$. Figure 6 shows s–e curves of the 6Ni–0.2N–0.1C and SUS304 steels at $\dot{\epsilon }$ of 10³ and 3.3 × 10⁻⁴ s⁻¹. The 6Ni–0.2N–0.1C steel indicated better TS and U.El at 3.3 × 10⁻⁴ s⁻¹.¹⁰⁾ But TS and U.El at 10³ s⁻¹ were almost the same between the 6Ni–0.2N–0.1C and SUS304 steels whereas the 0.2% PS was larger in the 6Ni–0.2N–0.1C steel. The increase of σ from yielding to the maximum load point was smaller in the 6Ni–0.2N–0.1C steel at 10³ s⁻¹. This is associated with TS and U.El at 10³ s⁻¹ in the 6Ni–0.2N–0.1C steel from the viewpoint of yield ratio (YR).²³⁾ Figure 7 shows U.El vs. YR in various austenitic stainless steels obtained by the tensile tests at various strain rates.^10,12,13) U.El became smaller with an increase in YR, i.e., the decrease in σ from 0.2% PS to TS. In the metastable austenitic stainless steels, TS is largely dependent on Vα′, i.e., the DIMT behavior after yielding.^4,11,12,13) As seen in Table 2, Vα′ decreased with an increase in $\dot{\epsilon }$ and those at $\dot{\epsilon }$ above 10⁰ s⁻¹ were 10% or less. The decrease in Vα′ with more than 10% leads to the decrease of TS. Because the strength of α′ was found to be larger in the 6Ni–0.2N–0.1C steel from the previous study,¹⁰⁾ the effect of Vα′ on σ must be larger in the 6Ni–0.2N–0.1C steel. On the other hand, σ generally increases with an increase in $\dot{\epsilon }$.^13,24) Figure 8 shows σ at various true strains (ε) vs. $\dot{\epsilon }$ in the 6Ni–0.2N–0.1C and SUS304 steels. The slope in Fig. 8 is the strain rate sensitivity exponent (m) and σ can be described as a function of $\dot{\epsilon }$ by using the m-value as follows,^24,25)   

$$\sigma =K{\dot{\epsilon }}^m$$(1)

where K is a constant. The m-value was larger in the 6Ni–0.2N–0.1C steel, similar to the case of 0.2% PS as seen in Fig. 3. But the m-values between the 6Ni–0.2N–0.1C and SUS304 steels converged as ε increased. In Fig. 8, σ for the 8Ni–0.2N–0.1C and SUS310S^26,27) steels, which are stable at room temperature, are also shown as reference data. In Fig. 8(a), the m-values are almost the same between the 6Ni–0.2N–0.1C and 8Ni–0.2N–0.1C steels. This is also true for the SUS304 and SUS310S steels in Fig. 8(b). σ at lower $\dot{\epsilon }$ were slightly larger in the metastable austenitic steels with an increase in ε because of the DIMT of γ. From Fig. 8, we considered the strain rate dependences on σ of γ phase in the 6Ni–0.2N–0.1C and SUS304 steels to be the same as those for the 8Ni–0.2N–0.1C and SUS310S steels.

Fig. 6.

Nominal stress–nominal strain curves of the 6Ni–0.2N–0.1C and SUS304 steels at 10³ and 3.3 × 10⁻⁴ s⁻¹. (Online version in color.)

Fig. 7.

Uniform elongation as a function of yield ratio in various austenitic stainless steels. (Online version in color.)

Fig. 8.

Double logarithmic plots of flow stresses at various true strains vs. strain rate for the 6Ni–0.2N–0.1C and 8Ni–0.2N–0.1C steels (a) and SUS304 and SUS310S steels (b). (Online version in color.)

U.El decreased with an increase in YR as seen in Fig. 7. This means that the change of σ after yielding is related to U.El. The strain rate dependence on 0.2% PS was larger in the 6Ni–0.2N–0.1C steel as seen in Fig. 3. The 0.2% PS can be considered as the σ of γ phase whose strain rate dependence becomes larger with increasing of N content.^26,28) On the other hand, it is important for TS to consider the effect Vα′ on σ in addition to the strain rate dependence on σ in the case that $\dot{\epsilon }$ increased as seen in Fig. 6. Based on the above-mentioned concept, we discussed TS at 10³ s⁻¹ as σ at the maximum load point. TS at 10³ s⁻¹ is almost the same between the 6Ni–0.2N–0.1C and SUS304 steels as seen in Fig. 6. The difference in σ between 10³ s⁻¹ and 3.3 × 10⁻⁴ s⁻¹ is closely associated with the effects of Vα′ and $\dot{\epsilon }$ on σ.^29,30) Therefore, the change of σ at the maximum load point from 3.3 × 10⁻⁴ to 10³ s⁻¹ can be determined from the change of σ due to the decrease in Vα′ (Δσ_V) and that with an increase in $\dot{\epsilon }$ ($\mathrm{\Delta }{\sigma }_{\dot{\epsilon }}$),   

$${\sigma }_{TS\left(3\right)}={\sigma }_{TS\left(-4\right)}-\mathrm{\Delta }{\sigma }_V+\mathrm{\Delta }{\sigma }_{\dot{\epsilon }}$$(2)

where σ_TS(3) and σ_TS(−4) are σ at the maximum load point at 10³ and 3.3 × 10⁻⁴ s⁻¹, respectively. We estimated Δσ_V and $\mathrm{\Delta }{\sigma }_{\dot{\epsilon }}$ for the 6Ni–0.2N–0.1C and SUS304 steels to verify Eq. (2). Δσ_V was estimated by using Vα′ in Table 2 and the phase stress of α′.^5,10,31) Here, Vα′ at 10³ s⁻¹ in Table 2 was used for the reference data because those were calculated using the fractured test specimens.^13,15) The phase stresses of α′ for the 6Ni–0.2N–0.1C and SUS304 steels were assumed to be 3 and 2.2 GPa, respectively from the previous study.¹⁰⁾ In the previous study,³²⁾ the effect of deformation temperature on the phase stress of α′ was very small. Thus, the phase stress of α′ at 10³ s⁻¹ was assumed to be the same as that obtained at the low strain rate.¹⁰⁾ $\mathrm{\Delta }{\sigma }_{\dot{\epsilon }}$ was estimated by the change of σ for the stable austenitic stainless steels in Figs. 8(a) and 8(b). Vα′ and the parameters used in the estimations based on Eq. (2) are summarized in Table 3. As can be seen, the estimated σ at 10³ s⁻¹ using Eq. (2) was almost the same as the measured σ for SUS304 steel. In terms of the 6Ni–0.2N–0.1C steel, the estimated σ was larger than the measured σ. It is thereby necessary for the 6Ni–0.2N–0.1C steel to revise the value of Δσ_V because $\mathrm{\Delta }{\sigma }_{\dot{\epsilon }}$ is estimated based on the experimental results in Fig. 8(a). The values of σ for various ε at 10³ s⁻¹ in Fig. 8(a) were almost the same between the 6Ni–0.2N–0.1C and 8Ni–0.2N–0.1C steels. But the mechanical stability of γ is higher in the 8Ni–0.2N–0.1C steel because of the Ni content.^32,33) We also investigated the tensile deformation behavior with a wide range of $\dot{\epsilon }$ including 10³ s⁻¹ using the 5, 7 and 8Ni–0.2N–0.1C steels in Table 1 to compare the experimental results of 6Ni–0.2N–0.1C steel. Figure 9 shows 0.2% PS, TS, and U.El as a function of $\dot{\epsilon }$ in the 5, 6, 7 and 8Ni–0.2N–0.1C steels. The addition of Ni changed the TS and U.El at a given $\dot{\epsilon }$ whereas the strain rate dependence on 0.2% PS were almost the same. Because the Ni content affects the mechanical stability of γ,^33,34) TS and U.El among 4 types of metastable austenitic steels are influenced by their DIMT behavior.^14,34) In Fig. 9, TS and U.El for the 6, 7, and 8Ni–0.2N–0.1C steels at 10³ s⁻¹ were almost the same. We confirmed that γ phases of the 7 and 8Ni–0.2N–0.1C steels were not transformed into α′ from the XRD experiments using the fractured specimens deformed at 10³ s⁻¹. Judging from those experimental results using the 6, 7 and 8Ni–0.2N–0.1C steels, we guessed γ phase of the 6Ni–0.2N–0.1C steel was stable up to the maximum load point at 10³ s⁻¹. If Vα′ at the maximum load point with $\dot{\epsilon }$ of 10³ s⁻¹ in the 6Ni–0.2N–0.1C steel is assumed to be zero, Δσ_V would be estimated to be 915 MPa. By using this revised Δσ_V, σ of the maximum load point at 10³ s⁻¹ using Eq. (2) was estimated to be 1118 MPa in the 6Ni–0.2N–0.1C steel, which was almost the same as the measured σ (1050 MPa) in Table 3. Therefore, it is concluded that DIMT do not occur in the 6Ni–0.2N–0.1C steel at 10³ s⁻¹ up to the maximum load point. We could calculate the decreases of TS from 3.3 × 10⁻⁴ to 10³ s⁻¹ based on Eq. (2). Those calculated results can explain that TS and U.El at 10³ s⁻¹ were almost the same between the 6Ni–0.2N–0.1C and SUS304 steels. In the previous study,¹⁰⁾ we verified that the better tensile properties of the 6Ni–0.2N–0.1C steel in the static tensile test largely affect the higher strength of α′.¹⁰⁾ The TRIP effect can no longer be expected at high strain rates in the 6Ni–0.2N–0.1C steel because DIMT was suppressed.²⁹⁾ In the high-speed deformation behavior at $\dot{\epsilon }$ more than 10⁰ s⁻¹, temperature rise of a specimen during adiabatic deformation¹⁶⁾ was considered by the following equation.   

$$T=T_0+\frac{0.95}{\rho C_p}{\int }_0^{e}s\left(e\right)de$$(3)

where T₀ is the initial temperature (296 K), ρ is the mass density, C_p is the temperature dependent heat capacity of the specimen, and s and e are nominal stress and nominal strain, respectively. The DIMT of γ is suppressed by the temperature rise in the high-speed tensile deformation.^12,13,16) The temperature rises at the maximum load pint at 10³ s⁻¹ calculated by Eq. (3) in the 6Ni–0.2N–0.1C and SUS304 steels are 55 K and 50 K, respectively. Judging from the estimated temperature rise and the mechanical stability of γ at 10⁻⁴ s⁻¹ for the 6Ni–0.2N–0.1C steel,¹⁰⁾ γ phase of the 6Ni–0.2N–0.1C seems to be less transformed into α′ than SUS304 in the high-speed tensile deformation such as 10³ s⁻¹. On the other hand, it is interested that almost the same TS and U.El were obtained at 10³ s⁻¹ in the 6Ni–0.2N–0.1C and SUS304 steels despite the Vα′ at the maximum load point of SUS304 was about 8%. This seems to be associated with the increase of work hardening in γ phase due to the additions of N and C^20,35) in the 6Ni–0.2N–0.1C steel. Effective use of high strength α′ of the 6Ni–0.2N–0.1C steel in addition to the analysis of σ for γ are important issues in the high-speed deformation behavior.

Table 3. Estimations of true stress at the maximum load point at 10³ s⁻¹ based on Eq. (2) in the 6Ni–0.2N–0.1C and SUS304 steels.¹⁰⁾

		(a) 6Ni–0.2N–0.1C	(b) SUS304
● Vα′ at the maximum load point
3.3×10−4 s−1		30.5%10)	32%10)
103 s−1		8.5%	8.6%
● True stress at the maximum load point
1.	3.3×10−4 s−1	1853 MPa	1422 MPa
2.	103 s−1	1050 MPa	1085 MPa
3.	ΔσV	–660 MPa	–515 MPa
4.	$\mathrm{\Delta }{\sigma }_{\dot{\epsilon }}$	+180 MPa	+150 MPa
5.	1. + 3. + 4.	1373 MPa	1057 MPa

Fig. 9.

0.2% proof stress, tensile strength and uniform elongation as functions of strain rate in the 5, 6, 7 and 8Ni–0.2N–0.1C steels. (Online version in color.)

4. Conclusions

The present study investigated the high-speed deformation behavior including the effect of $\dot{\epsilon }$ on the mechanical properties of 6Ni–0.2N–0.1C steel. The tensile tests were conducted at $\dot{\epsilon }$ between 10³ s⁻¹ and 3.3 × 10⁻⁶ s⁻¹. The main conclusions are as follows:

(1) As the strain rate increased, the 0.2% PS of the 6Ni–0.2N–0.1C steel increased but TS indicated almost the same value at $\dot{\epsilon }$ above 10⁻¹ s⁻¹. U.El largely decreased with an increase in $\dot{\epsilon }$ and that at 10³ s⁻¹ was approximately 30%.

(2) The effect of $\dot{\epsilon }$ on mechanical properties was compared between the 6Ni–0.2N–0.1C and SUS304 steels. The strain rate dependence on 0.2% PS was larger in the 6Ni–0.2N–0.1C steel and that on TS was different at $\dot{\epsilon }$ above 10⁰ s⁻¹. The decrease of U.El with an increase in $\dot{\epsilon }$ was larger in the 6Ni–0.2N–0.1C steel. TS and U.El at 10³ s⁻¹ were almost the same between the 6Ni–0.2N–0.1C and SUS304 steels.

(3) TS at 10³ s⁻¹ were discussed as σ at the maximum load point from the viewpoints of change of Vα′ and strain rate dependence on σ in γ. It was proposed from the estimated results that γ phase hardly transformed into α′ up to the maximum load point at 10³ s⁻¹ in the 6Ni–0.2N–0.1C steel. This is the reason why TS and U.El at 10³ s⁻¹ are almost the same between the 6Ni–0.2N–0.1C and SUS304 steels.

Acknowledgments

The authors are grateful to Prof. Y. Tanaka of Kagawa University and Dr. R. Ueji of National Institute for Materials Science for their helps.

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