NIPPON STEEL TECHNICAL REPORT No. 126 MARCH 2021

Technical Report

Abstract

1. Introduction

7, 8)
The stability of the phase is pursued by theNi equivalent equation (Hirayama’s equation): 12.6C + 0.35Si + 1.05
Mn + Ni + 0.65Cr + 0.98Mo. Nippon Steel Stainless Steel has com­
mercialized the heavy plate and the sheet products of SUS316L(316L­HNi) of which the aforementioned Ni equivalent was en­

In partnership with Nippon Steel Corporation (ex­Nippon Steel& Sumitomo Metal Corporation), Nippon Steel Stainless Steel Cor­
poration participated in a project of the New Energy and Industrial
Technology Development Organization (NEDO, an independent ad­
ministrative institution), and promoted the R&D of the stainlesssteel used in the high pressure hydrogen gaseous environment. 1–3)
SUS316L (17.5Cr­12 – 14Ni­2Mo) and/or SUS316 that containmore than 12% Ni are representative stainless steels that are lessprovided for the high pressure hydrogen gaseous environment. Fur­thermore, in the aforementioned NEDO project, we have proposedaustenitic stainless steel with reduced Nickel and Molybdenum(hereinafter referred to as austenitic steel with reduced Ni and Mo)(STH2) for hydrogen use that has applicability and compatible highstrength for use in the hydrogen gaseous environment. 3, 9)
Against this background, Nippon Steel Stainless Steel has devel­
oped austenitic steel with reduced Ni and Mo STH™2 *1 wherein
the rare metals such as Ni and Mo are reduced. This article describes
the concept of the material design for hydrogen use and the princi­
pal result of our research and development pertaining to the alloy
design of STH2. 3, 10) Finally, we introduce the various properties of
STH2 steel plate actually produced for the test based on the result of

stainless steels, as compared with SUS304 (18Cr­8Ni), the transfor­
mation from the austenite phase ( ) to the martensite phase ( ’) by aplastic deformation­like tensile test is suppressed. The hydrogengaseous embrittlement of stainless steel is gauged by correlating itwith the stability of the austenite ( ) phase. 1–3) Recently, in the aus­­
tors pertaining to the lamination defect, epsilon ( ) phase, and atom
vacancy lattice defect on the hydrogen gaseous embrittlement has­
4–6)

Currently, the operating condition of steel materials of a hydro­­
gen gas, and the use of SUS316L and SUS316 is provided as an ex­**1

STH stands for Stainless Steel with T
Hydrogen Energy Systems, and is a registered trademark of Nippon
Steel Stainless Steel Corporation.

Chief Researcher, Dr. Eng., Function Creation Research & Development Div., Research & Development Center, Nippon Steel Stainless Steel Corporation

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NIPPON STEEL TECHNICAL REPORT No. 126 MARCH 2021
this R&D.
respective element was pursued. Although the amount of C was notintentionally changed, to take into account the dispersion of carbon
in smelting, proof strength was analyzed by multiple regression
analysis with respect to C + N. The multiple regression equation of

2. Concept of Material Design for Hydrogen Use
The 0.2% proof stress of SUS316L is relatively small with re­spect to the structural design standard, and is disadvantageous infurther reducing the weight of high hydrogen pressure equipment
and/or enhancing the operating pressure thereof. To date, hydrogengaseous embrittlement has been evaluated in various high strength
stainless steels. In the tensile strength test conducted in high pres­sure hydrogen gas, in SUS630 of the martensitic stainless steel, brit­
tle fracture appearance is observed on lath martensite, 11) and in SUS
329J4L of the ferritic­austenitic duplex stainless steel, brittle frac­ture appearance is observed mainly in the ferrite phase. 12) In either
case, the crystal structure of body­centered­cubic (bbc) of martens­
ite and/or ferrite is suggested to cause the embrittlement. On thesolving element N, and further, Cr and Mn also interacted asstrengthening elements. On the other hand, Ni and Cu have negativevalue coefficients, and are estimated as having small interaction
the respective element was generally similar to that in the experi­
steel 15) that conventionally employs Ni as the major phase stabiliz­
ing element.
Figure 2 shows examples of analysis of deformation microstruc­tures of austenitic steel with reduced Ni and Mo by the electron­(17.2Cr­14.6Mn­1.3Ni­0.37N) wherein Ni is replaced by Mn, and N
is added has high strength, and the phase is stabilized, it is readily
embrittled by hydrogen gas. 13) From our research result, we con­base steel (15Cr­9Mn­6Ni), b) N­added steel (0.1%N), and c) Cu­
added steel (1%Cu). The microstructure analysis by the EBSD
method was conducted for the tensile test pieces of JIS 13 B provid­
ed with uniform elongation of 30% at room temperature in atmo­

caused just by the formation of deformation­induced martensitic
structure ( ’ phase), but also has a strong correlation with the plastic
deformation of the phase proper, and is attributed to the localiza­tion of strain by hydrogen. 4, 6) Based on this, we considered that thekey to gaining excellent hydrogen gaseous embrittlement resistanceis the material design that provides the highly deformation­stablephase, does not readily allow the formation of the ’ phase, and iscontrollable to form a deformation microstructure that suppresses
strain localization even when strength is enhanced.

to the phase (fcc) and the red color regions correspond to the ’phase (bcc). In the base steel, about 10% of the ’ phase wasformed. The N­added steel and the Cu­added steel consist of the­orientation, when observed with focus on the deformation micro­
structure of the phase, in the base steel, as compared with the N­
added steel and the Cu­added steel, misorientation becomes larger(green to yellow) in the local sections in the vicinity of crystal grainboundaries and/or within crystal grains, and localization of strain isobserved. Namely, in the austenitic steel with reduced Ni and Mo,
phase sta­bility and suppressing the localization of strain.
Thus, the basic composition of the austenitic steel with reduced
Ni and Mo was determined as 15Cr­9Mn + Ni + N + Cu with Ni

3. Alloy Design of STH2
3.1 Enhancement of strength and deformation microstructure
control
The austenitic steel with reduced Ni and Mo was selected as a
guideline for the alloy design of STH2, and a chemical composition
was studied that compatibly realizes: (1) enhancement of strengthfrom the level of SUS316L and (2) deformation microstructure thatsuppresses the localization of strain. Specifically, Cr­Mn­Ni steel
with the chemical composition as shown in Table 1 was vacuum­
smelted, and 0.2% proof stress and the deformation microstructure
were evaluated. In this study, Mn, N, and Cu were considered assolid­solving elements to enhance phase stability. As the index toindicate phase stability of a material, the Ni equivalent equationthat includes coefficients of N and Cu (Sanga’s equation 14)): Ni +of achieving 0.2% proof stress exceeding 300 N/mm2 and of sup­pressing the strain localization.

used. The Ni equivalent of the prepared test material was in therange of 27.5 to 31.5.
Figure 1 shows the relationship between the alloying elements
and 0.2% proof strength of austenitic steel with reduced Ni and Mo.
0.2% proof stress was measured by the JIS 13 B tensile strength testwith specimens of 2 mm thick cold­rolled and annealed sheets withcrystal grain size adjusted to GS No. 8. The measured 0.2% proof
stress was analyzed by multiple regression analysis with respect toTable 1 Chemical compositions of test materials(mass%)
C
Si
Mn
Cr
Ni
Cu
N
Nieq
0.06 0.5 8.0–12.0 14.0–16.0 4.0–7.5 0–3.0 0.03–0.25 27.5–31.5

Fig. 1 Relationship between alloying elements and 0.2% proof stress ofsteels reduced amount of nickel and molybdenum

Nieq

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NIPPON STEEL TECHNICAL REPORT No. 126 MARCH 2021
pieces fractured in atmospheric air exhibit cup and corn type frac­ture mode, and the ’ phase ratio decreases to 2.8% from 28.2%
along with the increase of Ni equivalent from 29.0 to 30.7. In the Ni
equivalent range from 29.0 to 29.6, the fractured test pieces in 70
exhibit a decrease of elongation as compared with those in2
atmospheric air, and side cracks as shown by the arrows in the photo
are observed. The fractured test pieces with Ni equivalent of 30.2 to
30.7 are free of such side cracks, and exhibit the same fracture mode
as with those of the test pieces tested in atmospheric air. The phaseratios of ’ of these fractured test pieces hardly change from thoseof the test pieces tested in atmospheric air. In, the fracturesurfaces of the test pieces shown in Fig. 3 are shown. Test pieces
tested in atmospheric air show dimpled ductile fracture surfaces,
and about 80% of the reduction of area. Test pieces with Ni equiva­
exhibit pseudo cleavage
2
fracture, 1–3) and the reduction of area drops to 35 to 40%. With Ni
equivalent of 30.2 to 30.7, pseudo cleavage fracture does not appear,
and the ductile fracture and the reduction of area almost equal tothose of the test pieces tested in atmospheric air appear.
The above result reveals that the hydrogen gaseous embrittle­­

Conventionally, the hydrogen gaseous embrittlement of austenit­
ic stainless steel becomes apparent not at room temperature but at a16)This phenomenon is ex­
plained by the relationship between the fraction of the ’ phase andbased on the aforementioned basic composition. For concrete evalu­
ation, 30 to 45 kg of 13 types of steel in which Ni equivalent wasvaried in the range of 29.0 to 31.5 by adding Ni, N, and Cu within
The vacuum­smelted materials were hot­forged and hot­rolled to 15
then water­quenched. Round bar test pieces 3 mm in diameter in theparallel section with a length of 20 mm in the rolling direction weretaken from the solution­treated materials. The slow strain rate ten­sion test (SSRT) was conducted at the 5.0 × 10 s strain rate at. 7)
2
Figure 3 shows the examples of side views of the fractured test
pieces after SSRT. In the top left, the ratio value of the ’ phase(amount of ferrite) measured by a ferrite meter is shown. All test

Fig. 2 Analysis of deformation structure of steels reduced amount of nickel and molybdenum by EBSD method

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NIPPON STEEL TECHNICAL REPORT No. 126 MARCH 2021

tain Ni equivalent equal to or above 30.2 by Sanga’s equation.

This chapter describes the advantages of various properties of
the STH2 steel sheet (Ni equivalent: 30.5) actually produced fortesting based on the concept of its alloy design stated in the previous
chapter. For the assessment of properties, steel sheet 1.2 mm in
Figure 6 shows the result of the measurement of 0.2% proof
stress and tensile strength. For the measurement, the JIS 13 B tensile
test was conducted (L direction) at a strain rate of 8.3× 10 /s (inof STH2 is 1.2 to 1.5 times higher than that of SUS316L (17.5Cr­
12Ni­2Mo). Furthermore, the fracture elongation of STH2 is about

Fig. 5 Multiple regression analysis of relative reduction of area after

Figure 7. SSRT was conducted at the strain rate of 3 × 10 /s for2
sheet type tensile test pieces (L direction), each having a parallel
section of 20 mm in length and a width of 4 mm. STH2 realized

duced Ni and Mo with Ni equivalent of above 30.2.
The relative reduction of area (relative reduction of area tested
divided by the reduction of area tested in atmospheric2­higher than 60%. In comparison with SUS316L, tensile strength in­creased by 1.3, and fracture elongation increased to the same level.
Based on the result of our past research and its mechanism, 17) wephase also con­tributes to the high strength and high ductility of STH2 in addition
to the rise of the dislocation density by cross­slip at the slow strain
rate tension test. Furthermore, the relative fracture strength of STH2

interaction of the added Ni, N, and Cu on the hydrogen gaseous em­
brittlement was assessed. Figure 5 shows the result of the multiple
regression analysis of the relative reduction of area of the 13 steelswith respect to the amounts of Ni, N, and Cu. The relationship be­
tween the relative reduction of area and the amounts of addition ofthese elements is as follows, obtained by multiple regression analy­2­

STH2 has realized high strength and high ductility in SSRT at, and is equipped with excellent hydrogen gas­
2
eous embrittlement resistance.
Figure 8experiment and observation examples of the microstructures of theanalysis are 6.24 and 0.78, respectively, which generally agree withphase stability. From this, the hydrogen gaseous embrittlement re­sistance of the austenitic steel with reduced Ni and Mo has goodcorrelation with phase stability, and N and Cu in addition to Ni areembrittlement resistance.
Based on the above results, in the alloy design of STH2, in order

are frequently used for pre­coolers to cool high pressure hydrogengas. 18)­
lowing manner. The surfaces of the test samples were miller­fin­

and Cu were added to the basic composition (15Cr­9Mn + Ni) to ob­

furnace, and held for three hours under a bearing pressure of 0.5

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NIPPON STEEL TECHNICAL REPORT No. 126 MARCH 2021

Fig. 6 0.2% proof strength and tensile strength of STH2 and SUS316L

Fig. 9 Optical microscope structure of TIG welding material

Fig. 7 Stress­elongation curves after SSRT in gaseous hydrogen at 70

evaluation surface #600

cordingly, STH2 is judged to be TIG­weldable the same as SUS
316L.
Figure 10 shows the appearance of the test pieces after a com­bined cycle test conducted pursuant to the Japanese Automotive
Standards Organization (JASO, 30 cycles). One cycle of the test is 8

microscope structure of bonding interface

to 10
Crystal grains of STH2 and SUS316L were observed across thebonded interface. This observation result showed that the bonded­
ty almost equal to that of SUS316L.
In Fig. 9, examples of observation of the cross section micro­
structure of TIG­welded material (tungsten inert gas welding) are
shown. TIG welding was conducted under the condition that theweld bead is applied in the direction of rolling and the width of theback bead becomes about 2.0 mm (welding current: 200 A, welding
speed: 150 cm/min). As the shielding gas, Ar gas was used. In the­bles were not observed. In addition, the width of the back bead of
SUS316L was about 2.0 mm under the same welding condition. Ac­

to moistening atmosphere with a relative humidity of 95% for 2 h.
of STH2 is slight as compared with that of SUS430 (17Cr), and isequivalent to the level of that of the representative austenitic stain­
less steel SUS304 (18Cr­8Ni).
Table 2 shows the time of the occurrence of stress corrosion
cracking (JIS G 0756). The test condition is: U­bent test pieces 75
mm in length and 10 mm in width are immersed in the boiling 42%
and 20% MgCl2 solutions for 100 h at the longest. The time of oc­
currence of stress corrosion cracking of STH2 is comparable to thatof SUS304, and stress corrosion cracking resistance of STH2 is al­

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NIPPON STEEL TECHNICAL REPORT No. 126 MARCH 2021
Table 2 Time of occurrence of stress corrosion cracking (JIS G 0576

STH2

Acknowledgement
The slow strain rate tension test (SSRT) in the high pressure hy­
drogen gaseous environment described in Chapter 3 was conductedin the NEDO­sponsored researches of “Development of Technolo­

SUS304

MgCl2

1

2

1

2

20%
42%

24 h

24 h

1h

1h

of “Research and Development on Hydrogen Utilization Technology­
tude to all concerned in the said sponsored research and develop­
ment.

most at the same level as that of SUS304.
Therefore, in comparison with SUS316L, the STH2 steel sheet(Ni equivalent: 30.5) actually produced for the test is compatibly
equipped with high strength and applicability to the high pressurehydrogen gaseous environment. STH2 is also equipped with bond­ability and weather resistance at the same level as those of SUS316L
and/or SUS304, and has material properties for application to practi­
cal use.

References

1) NEDO: Establishment of Codes and Standards for Hydrogen Economy
Society. 2005– 2009 Fiscal Year Report on Result of Research. 2010­
ery and Storage Systems. 2010 – 2012 Fiscal Year Report on Result ofResearch. 20133) NEDO: Research and Development on Hydrogen Utilization Technology
Research and Development. 2013 – 2017 Fiscal Year Report on Result ofResearch. 2018
4) Hatano, M. et al.: Acta Materialia. 67, 342 (2014)

5. Conclusion
This article summarized the result of major research pertaining
to the alloy design of austenitic steel with reduced Ni and Mo(STH2) for hydrogen use, and the various properties of the STH2
steel sheet practically produced for the test.
In the alloy design of STH2, to obtain hydrogen gaseous embrit­

Japan. 49 (10), 29 (2012)
Regulation. 20169) Hatano, M. et al.: JRCM NEWS. (375), 2 (2018)10) Hatano, M. et al.: Forum Symposium of Hydrogen Gaseous Embrittle­

or Cu to the basic chemical composition of (15Cr­9Mn + Ni). The
addition to enhancing phase stability was revealed. Hydrogen gas­
eous embrittlement resistance has good correlation with phase sta­bility (Sanga’s equation), and N and/or Cu in addition to Ni are ef­
fective elements for enhancing hydrogen gaseous embrittlement re­sistance. The STH2 steel sheet practically produced for the test (Niequivalent: 30.5) is compatibly equipped with high strength and ap­
plicability to the high pressure hydrogen gaseous environment.
Hereafter, with the promotion of the application of STH2, realiza­(2007)
14) Tsuchita, N. et al.: Journal of the Japan Institute of Metals and Materials.
72 (9), 769 (2008)15) Ohkubo, N. et al.: ISIJ Int. 34, 764 (1994)
16) Fukuyama, S. et al.: Journal of the Japan Institute of Metals and Materi­als. 67, 456 (2003)17) Hatano, M. et al.: Journal of the Japan Institute of Metals and Materials.
77, 593 (2013)18) Miura, S. et al.: Kobe Steel Eng. Rep. 64 (1), 49 (2014)

the future construction of the impending hydrogen energy society.

Masaharu HATANO
Chief Researcher, Dr. Eng.
Function Creation Research & Development Div.
Research & Development Center
Nippon Steel Stainless Steel Corporation

Mitsuki SUGEOI
Researcher
Function Creation Research & Development Div.
Research & Development Center
Nippon Steel Stainless Steel Corporation

743­8550
Kazuhisa MATUMOTOManager

Kenji HATTORI
Senior Manager

Nippon Steel Stainless Steel Corporation

Nippon Steel Stainless Steel Corporation

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