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* Corresponding author: E-mail: kinugasa.junichiro@kobelco.com

Method for Evaluating Hydrogen Embrittlement of High-Strength Steel Sheets Considering Press Formation and Hydrogen Existence State in Steel

Junichiro KINUGASA,1)* Yosuke FUJITA,2) Kosuke SHIBATA,1) Takuya HIRAMATSU,1) Makoto KAWAMORI1) and Fumio YUSE1)

1) Materials Research Laboratory, Kobe Steel, Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe, Hyogo, 651-2271 Japan.2) Material Solution Division, Kobelco Research Institute. INC, 1-5-5 Takatsukadai, Nishi-ku, Kobe, Hyogo, 651-2271 Japan.

(Received on December 27, 2019; accepted on April 22, 2020; J-STAGE Advance published date: July 22, 2020)

High strength steel sheets are increasingly being used due to the growing need to improve fuel effi-ciency by reducing vehicle weight. Steel sheets are usually used as automotive parts formed by pressing, etc., and complicated strain is generated in the steel sheet after forming. This strain is thought to affect the occurrence of hydrogen embrittlement. In this study, a new hydrogen embrittlement evaluation method using a forming limit diagram was devised. A forming limit diagram of a steel sheet with a strength level adjusted to 1 470 MPa was prepared for uniaxial, plane strain and biaxial modes, and stress and hydrogen were applied to the specimens formed with respective strain modes to evaluate the occur-rence of fracture. In order to examine the relationship between the hydrogen content and the cracking, evaluation of the hydrogen content by Thermal Desorption Analysis, visualization of hydrogen by Second-ary Ion Mass Spectrometry and microstructure analysis by Electron BackScatter Diffraction were carried out. It was found that cracks are generated in the strain mode in which hydrogen accumulates locally in the steel even when the apparent hydrogen content is small, and it was clarified experimentally that evaluation of local hydrogen concentration is important for the evaluation of hydrogen embrittlement.

KEY WORDS: delayed fracture; hydrogen embrittlement; diffusible hydrogen; stress; strain; concentration; steel sheet; press forming; forming limit diagram.

1. Introduction

With the aim of improving the fuel consumption and col-lision safety of vehicles by reducing the body weight, a high strength steel exceeding 1 180 MPa is being extensively used in vehicles.1) Accordingly, the application of high strength sheets to automotive parts, from door impact beams shaped predominantly by bending, to pillars with compli-cated shapes, have been developed. Above this strength grade, “delayed fracture (hydrogen embrittlement),”2) appar-ently caused by hydrogen absorbed by steel while these parts are in use, is involved.

Hydrogen embrittlement is caused by three factors: mate-rial, environment, and stress. When using high-strength steel sheets that are formed into a complicated shape by press-forming, it is necessary to consider the hydrogen embrittlement properties that arise due to strain and residual stress during the process. In one of the proposed evaluation

methods for hydrogen embrittlement properties of formed steel sheets, a steel sheet is bent into a U-shape in a similar way to a bent part, and a stress is applied to the bent portion by tightening the foot of the part with a bolt and a nut.3–5) Subsequently, the presence or absence of a crack formed by hydrogen addition is evaluated. However, the conventional evaluation method, which only considers bending, might not be enough to properly evaluate the effects of strain on automotive parts with a complicated shape, such as a pil-lar, as they are under complex strain conditions owing to the processing modes utilized, e.g., deep drawing, bending, brattice, and stretch flange. Therefore, it is necessary to further investigate the potential evaluation method for the hydrogen embrittlement process involving automotive parts that have complicated shapes.

Although an evaluating method for hydrogen embrittle-ment that also considers the press-forming process has been proposed,6) the evaluation method in this paper involves pre-charging the samples with hydrogen and electro-galvanized to prevent the effusion of hydrogen during forming/bend-

© 2021 The Iron and Steel Institute of Japan. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs license (https://creativecommons.org/licenses/by-nc-nd/4.0/).

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https://doi.org/10.2355/isijinternational.ISIJINT-2019-822Toward Suppression of Hydrogen Absorption and Hydrogen Embrittlement for Steels

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ing tests. It cannot be said to be a method considering the actual usage situations, because processing was carried out on the steel sheet in which hydrogen is added. As a prediction method for the limit of forming of steel sheets, a method using forming limit diagram is known.7) This is a technique that predicts the forming limit of parts by carry-ing out press-forming in each strain mode (uniaxial, plane strain, and biaxial), and plotting the calculated crack limit diagram of the material. The forming limit diagram is used to investigate the forming of complicated parts, but there are only a few examples utilized for evaluating hydrogen embrittlement.8)

In order to propose a new evaluation method for the hydrogen embrittlement of steel sheets, which also consid-ers the press-forming process, in this study, we examine the effects of the strain mode, stress, and hydrogen content on hydrogen embrittlement by adding hydrogen and stress to specimens manufactured in accordance with the forming limit diagram. In addition, it is assumed that any difference in the strain mode affects the strain state in the steel sheet, which in turn affects the behavior of hydrogen in the steel. There have only been a few reports on the behavior of hydrogen in steel that considers this complicated strain state. Thus, we examine the effects of the uniaxial and biaxial strain on the presence of hydrogen in steel and the hydrogen embrittlement properties.

2. Experimental Procedure

2.1. MaterialWe used martensitic steel sheet mainly containing C, Si

and Mn whose strength level was adjusted to 1 500 MPa grade by quenching and tempering.

2.2. Forming Method (Marciniak Method)We selected the Marciniak method, in which the form-

ing surface is uniformly smooth, and cut out the steel sheet in the following sizes, 40 mm × 175 mm × 1.2 mm thick, 135 mm × 175 mm × 1.2 mm thick, and 175 mm × 175 mm × 1.2 mm thick, to shape them using the uniaxial, plane strain, and biaxial strain, respectively. Figure 1 shows a schematic of the press-forming process. A backing plate was placed on the test piece, and press-forming was per-formed by applying a wrinkle pressing force of 40 t using a flat head punch of φ 100 mm. Figure 2 shows the appear-ance after the press-forming in each strain mode. As shown in Fig. 3, the amount of deformation of a 1/4 inch scribed circle that was transcribed on the test surface beforehand was measured as the minimum and maximum strains, and the forming limit for each strain mode was calculated. The maximum strain (ε1) and minimum strain (ε2) were cal-culated from the maximum deformation amount (d1) and minimum deformation amount (d2) after the forming. The forming limit diagram, shown in Fig. 4, was made by plot-ting the minimum strain on the X-axis and the maximum distortion on the Y-axis at the fracture point.

We evaluated the hydrogen embrittlement of the speci-men made under the press conditions immediately before

Fig. 1. Schematic of the press-forming process.

Fig. 2. Sample appearance after press forming in each strain mode.

Fig. 3. Scribed circle method.

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the fracture limit for the uniaxial, plane strain, and biaxial on the forming limit diagram. We also examined whether it is possible to perform a hydrogen embrittlement evalua-tion that considers press-forming, such as the change in the fracture limit and the hydrogen embrittlement properties resulting from the strain mode. Furthermore, to evaluate the effects of the amount of equivalent plastic strain, we made an additional specimen, to which 2% equivalent plastic strain in the uniaxial and biaxial strain modes was applied. The strain modes significantly affect the hydrogen embrittle-ment properties.

2.3. Evaluation of Hydrogen EmbrittlementIn order to evaluate the hydrogen embrittlement proper-

ties of the uniform and smooth surface at the center of the specimen made by the Marciniak method, the portion not in the central part as a test surface was masked with resin to avoid hydrogen absorption by the unnecessary portion. A jig was attached to apply stress on the central section. The stress on the evaluation part was set at two levels. One level

was only for residual stress caused by the press forming, and the other level was 1 300 MPa which was equivalent to 1.0 σys (100% proof stress). For the equivalent plastic strain of 2%, hydrogen was introduced for 48 hours in advance to homogenize hydrogen concentration in the specimen, and then a stress of 1 300 MPa was applied by four-point bending. Note that the stress was calculated from the strain gauge values at the surface and the Young’s modulus. To examine the distribution of hydrogen in the steel, a speci-men of 35 mm × 10 mm was cut out from the uniform and smooth surface made by Marciniak method, and 1 300 MPa was uniformly applied to the outside of the bending by four-point bending using a titanium jig. Note that the test piece and jig were electrically insulated by having a ceramic bar inserted between them. A strain gauge for fracture detection was placed on the jig, and the crack generation was detected by using a strain gauge and by visual observation.

The hydrogen introduction method into steel was referred to previous reports.9,10) One was acid immersion in hydro-chloric acid adjusted to pH 1 for 100 hours or more [Test (a)]. The other was the cathode charge method, in which hydrogen was introduced by passing a predetermined elec-tric current for up to 48 hours in 3% NaCl aqueous solution containing NH4SCN as a poison [Tests (b) and (c)]. To control hydrogen content introduced into the steel, three different combinations of NH4SCN concentration and cur-rent density (3 g/L − 1.0 × 10 −3 A/cm2, 30 g/L − 0.1 × 10 −3 A/cm2, and 30 g/L − 1.0 × 10 −3 A/cm2) were used. Figure 5 shows schematics of the hydrogen embrittlement tests. For the analysis of hydrogen content, a specimen of 5 mm × 5 mm was cut out from the stress application part, and the analysis was performed using a Thermal Desorption Analysis (TDA). The heating rate was set at 12°C/min, and the hydrogen content was evaluated as integration value of hydrogen released from room temperature to 300°C.

2.4. Hydrogen Visualization by SIMSThe distribution of hydrogen was observed using 0.7 mm

Fig. 5. Schematics of the hydrogen embrittlement test. (Online version in color.)

Fig. 4. Forming limit diagram for test samples.

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thick specimens, uniaxially and biaxially formed, in which the equivalent plastic strain was 2% and immediately before fracture limit prepared by the method described as 2-2. A specimen was cut out from the uniform central part of the press-formed sample which were then ground on both sides up to 0.7 mm. Hydrogen was introduced to the steel as follows: (1) the specimen was embedded in an epoxy resin, (2) the plate-thickness-direction cross section (the observation surface) was polished to a mirror finish by wet-polishing, and (3) hydrogen was introduced using the cathodic electrolysis method with an applied current density of 0.1 × 10 −3 A/cm2 in a 3% NaCl D2O solution containing 30 g/L of NH4SCN as a poison. By using deuterium (D) instead of hydrogen (H), the background resulting from the naturally existing water (H2O) can be reduced. An analysis of the hydrogen distribution in steel was performed using Secondary Ion Mass Spectrometry (SIMS) with deuterium employed as a tracer. Note that the difference in the sec-ondary ion yield, depending on the crystal orientation of the substrate Fe, affects the apparent intensity of 2D–.11) In order to clarify the hydrogen existing position, 72FeO– was acquired as the background of Fe and the intensity of 2D– obtained was divided pixel-by-pixel using 72FeO– to normal-ize the secondary ion image (2D–/72FeO–).

2.5. Microstructure Evaluation by Electron BackScat-ter Diffraction Analysis

A technique using a Kikuchi pattern obtained with a Transmission Electron Microscope (TEM)12) or a Back Scattering Electron Diffraction image method (EBSD)13) has been proposed for measuring the plastic strain of a material. Since EBSD can obtain a wider range of data more easily than a TEM,14) the microstructual deformation and strain state for the individual forming modes were analyzed using the EBSD in this study. EBSD system (OIM system, ver. 5.2, TSL solution, Japan) attached to the SEM (JSM-6500F, JEOL Ltd. Japan) was used and the measured area and the analysis step were 50 μm × 50 μm and 0.125 μm, respec-tively. The results obtained were evaluated by IPF map to

understand the influence of the forming mode.15) Among various parameters proposed to evaluate the degree of deformation and strain for individual crystalline grain,16,17) grain average misorientation (GAM)13) was chosen in this study in order to conduct the evaluation with a focus on the crystalline grain.

3. Results

3.1. Hydrogen Embrittlement Evaluation in Each Forming Mode

Figure 6 shows examples of crack detection on a uniaxial mode specimen during the hydrogen embrittlement tests. While immersed in acid [Test (a)], no sudden change in the value of the strain gauge was observed, and no crack was observed even by visual observation. On the other hand, during the cathode charge [Test (b)], the value of the strain gauge decreased rapidly after 3.5 hours, showing cracking in the specimen.

Fig. 6. Examples of crack detection during hydrogen embrittle-ment tests in the uniaxial mode.

Table 1. Result of the hydrogen embrittlement test and the hydrogen content after experiment.

Mode Stress (MPa) Test condition

NH4SCN (g/L)

Current density (A/cm2) Fracture Hydrogen content

(wt. ppm)

Uniaxial

Residual stress(a) – – No 0.3

(b) 3 1×10 −3 No 6.6

1 300(a) – – No 0.3

(b) 3 1×10 −3 Yes 1.2

Plane strain

Residual stress(a) – – No 0.6

(b) 3 1×10 −3 No 6.6

1 300(a) – – No 0.4

(b) 3 1×10 −3 Yes 3.7

Biaxial

Residual stress(a) – – No 0.6

(b) 3 1×10 −3 No 9.4

1 300

(a) – – No 0.4

(b) 3 1×10 −3 No 3.6

(b) 30 1×10 −3 Yes 4.1

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Table 1 shows the result of the hydrogen embrittlement test and the hydrogen content measured after experiment using the processed specimen at the point immediately before the fracture in each strain mode. In all these strain modes, no crack occurred under acid immersion or cath-ode charge when the stress on the test surface was only the residual stress caused by press forming. Note that the residual stress caused by press forming was approximately 100 MPa when evaluated using a cut-and-open test. When the stress on the testing surface was set at 1 300 MPa, no crack occurred in either strain mode in acid immersion, but a crack occurred in cathode charge. A crack occurred when stress was applied and the hydrogen content in the steel was increased. In addition, after the test (b) in which fracture occurred, there were differences in the hydrogen content, depending on the strain mode – 1.2 ppm in the uniaxial, 3.7 ppm in the plane strain, and 4.1 ppm in the biaxial; therefore, it was suggested that different hydrogen embrittle-ment properties appeared depending on the strain mode.

3.2. Hydrogen Visualization by SIMSFigure 7 shows the secondary-ion images of 2D–/72FeO–

by SIMS of the specimens uniaxially and biaxially formed to the point of immediately before fracture limit. No stress was loaded on the as-formed specimens. Although the sig-nals were weak in most of the observed field of the uniaxi-ally formed specimen, there were some parts with strong signals indicating local existence of D. On the other hand, in the biaxially formed specimens, the signals were strong and distributed uniformly over the entire field. These results sug-gested that the presence of hydrogen in steel would depend on the strain modes when the specimens were formed near the fracture limit.

3.3. Effects of Strain Mode and Degree of Equivalent Plastic Strain on Hydrogen Embrittlement Prop-erty

Table 2 shows the result of fracture time, hydrogen content and equivalent plastic strain when hydrogen was introduced into steel by cathode charge and using the

Fig. 7. 2D −/72FeO − map obtained by SIMS of the specimens uniaxially and biaxially formed to the point immediately before the fracture limit. No stress was loaded on the as-formed specimens.

Fig. 8. Microstructure analysis results obtained using EBSD for the test pieces in the state just before fracture in uni-axial and biaxial modes, and the un-deformed area as a reference.

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Table 2. Results of fracture time, hydrogen content and equiva-lent plastic strain when hydrogen was introduced into steel by cathode charge.

Mode Stress (MPa)

Fracture time (h)

Hydrogen content (wt. ppm)

Equivalent plastic strain (%)

UniaxialResidual

stress – 5.43.9

1 300* 16 8.8

BiaxialResidual

stress – 7.722

1 300* 27 13.5

*Applied stress: 4-point bendSolution: 3%-NaCl + 30 g/L-NH4SCN/Current density: 0.1×10 −3 A/cm2

Table 3. Results of fracture time, hydrogen content when hydro-gen was introduced by cathode charge to uniaxial and biaxial specimens with equivalent plastic strain of 2% and applied stress of 1 300 MPa.

Mode Stress (MPa)

Fracture time (min)

Hydrogen content (wt. ppm)

Equivalent plastic strain (%)

Uniaxial1 300*

3.5 2.4 2

Biaxial No 7.2★ 2

*Applied stress: 4-point bendSolution: 3%-NaCl + 30 g/L-NH4SCN/Current density: 0.1×10 −3 A/cm2

★Hydrogen content after 48 hours (No fracture)

level of equivalent plastic strain. Figure 9 shows the strain distribution for each grain in each forming mode. Note that in evaluating the distribution of the GAM value, the GAM values were classified in increments of 0.25. While the GAM values of the uniaxially formed specimen with an equivalent plastic strain of 3.9% were concentrated between 0.5 and 1.0 with a peak at approximately 0.75, the value of the biaxially formed specimens with an equivalent plastic strain of 22% were distributed between 0.75 and 2.0 with a peak at approximately 1.25.

4. Discussion

4.1. Hydrogen Embrittlement Evaluation Using the Marciniak Method

The hydrogen content in steels under atmospheric corro-sion reported in previous papers was about 0.2 ppm,18,19) and the hydrogen content in the steel immersed in a hydrochloric acid solution of pH 1, where no cracks were observed, was 0.28–0.39 ppm. This strongly suggests that the larger the hydrogen content, the more easily hydrogen embrittlement occurs in the specimen under the same processing condi-tions. Therefore, it seems likely that hydrogen embrittlement does not occur at the hydrogen content of the atmospheric corrosion condition even if the steel is processed within the forming limit line and a stress of 1 300 MPa or less is applied based on HC (the hydrogen content at which crack occur) and HE (the hydrogen content absorbing in the envi-ronment).20,21) On the other hand, according to the results of the cathode charge tests, hydrogen content in the fractured specimen is 1.2 ppm for the uniaxial strain, 3.7 ppm for the plane strain, and 4.1 ppm for the biaxial strain, respectively. Despite of the fact that the specimens deformed with the plane strain and the biaxial strain have a larger hydrogen content than the specimen with uniaxial strain, fracture of the specimens with the plain strain and biaxial strain is less likely to occur that the specimen with uniaxial strain. The interpretation of these results will be discussed in Sections 4.2 and 4.3, focusing on the cases involving the uniaxial and biaxial strains, where the difference in the hydrogen content

cut-out test pieces before the fracture limit of the uniaxial and biaxial forming. In the uniaxially formed specimen, the equivalent plastic strain was 3.9%, and the hydrogen content in the steel was 5.4 ppm without stress loading and 8.8 ppm with the stress load of 1 300 MPa applied to the outside of the bending. On the other hand, in the biaxially formed specimen, the equivalent plastic strain was 22%, and the hydrogen contents in specimens without loading and with the stress of 1 300 MPa were 7.7 ppm and 13.5 ppm, respectively. The hydrogen content near the fracture limit was higher in the biaxially deformed specimen with the larger equivalent plastic strain than in the uniaxially deformed specimen with smaller equivalent plastic strain.

Table 3 shows the results obtained when a stress of 1 300 MPa was applied to the outside of the bending, by using uniaxial and biaxial specimens with an equivalent plastic strain of 2%. The uniaxial specimen fractured after 3.5 minutes (hydrogen content: 2.4 ppm) while the biaxial specimen did not fracture for 48 hours, and the hydrogen content of the biaxial specimen measured after 48 hours of the test was higher than that of the uniaxial specimens measured after fracture.

3.4. Microstructure Evaluation by EBSDFigure 8 shows the microstructure analysis results

obtained using EBSD for the test pieces in the state just before fracture in uniaxial and biaxial modes, and the un-deformed area as a reference. The average GAM of the non-deformed area was 0.62. On the other hand, the GAM values were 0.72 and 1.2 for the specimens formed uni-axially with an equivalent plastic strain of 3.9% and those formed biaxially with an equivalent plastic strain of 22%, respectively. That is, the GAM value increased with the

Fig. 9. Distribution of GAM in specimens to un-deformed and just before the fracture limit of uniaxial and biaxial forma-tion.

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and the fracture time between them was significant.Figure 10 shows a three-dimensional map of the hydro-

gen embrittlement properties considering the press form-ing (applied stress at 1 300 MPa), with the forming limit diagram drawn along the x- and y-axes and the hydrogen content plotted against the z-axis. By using this evaluation methods, it is possible to visually express the evaluation of crack occurrence in each forming mode and forming limit diagram considering the hydrogen embrittlement properties.

4.2. Microstructure Evaluation by EBSDSince a material deforms in only one direction in the uni-

axial, there is a possibility that strain accumulates in local such as intergranular and grain boundary due to tension even in martensitic steel, which has a generally uniform struc-ture. On the other hand, there is a possibility that the local accumulation of strain applied to the microstructure may be canceled out for some reason as a result of deformation in the x and y directions in the biaxial mode unlike that in the uniaxial mode. Therefore, we examined the reason why the resistance to hydrogen embrittlement was different in rela-tion to the difference of strain mode.

As shown in Fig. 8, indicating the GAM value mapping obtained by means of EBSD, a larger strain is applied to the biaxially formed specimens than to the uniaxially formed specimen just before the fracture. This result is correspon-

dent to the difference in the equivalent plastic strain in the respective specimens. In addition, as shown in Fig. 9, more strained grains are distributed throughout the structure in the biaxially formed specimens than in the uniaxially formed specimens. The hydrogen existing sites in the press-formed steel sheet have been shown to be lattice defects, such as dislocation and vacancy, mainly due to the strain.23–26) The local hydrogen accumulation in the local regions where the small number of strained grains cause an increase in hydrogen content is remarkable in the uniaxially formed specimen. Therefore, although the average hydrogen content obtained by means of TDA is higher in the biaxially formed specimens, the local hydrogen enrichment was not observed by SIMS. It is likely that the dislocation density increases as the number of sites with a high GAM value increases, result-ing in an increase in the hydrogen content. The experimental results suggest that the hydrogen embrittlement is affected by the local hydrogen enrichment, that is, the hydrogen content difference from the surroundings.

4.3. Effects of Strain Mode and Equivalent Plastic Strain on Hydrogen Embrittlement Property

Figure 11 shows the observation results of 2D–/72FeO– using SIMS uniaxially and biaxially with the same equiva-lent plastic strain of 2%. According to the result of the hydrogen content indicated in the figure, the signal of the biaxially formed specimen is stronger than that of the uni-axial specimen. In any of the strain modes, unlike those formed up to just before the fracture shown in Fig. 7, no local hydrogen accumulation was observed. In addition, EBSD measurement was carried out even in the sample in which equivalent plastic strain was adjusted to 2%, and the concentration of the strain was not clearly observed in the structure.

The reason why the hydrogen content in the uniaxial (2.4 ppm) was lower than that in biaxial (7.2 ppm) is likely to be the effect of the fracture time in the hydrogen embrittlement tests, as described in Table 3. Since uniaxial specimen frac-tured 3.5 minutes after starting the test, it appeared that the hydrogen content was less than that of the uniaxial speci-men which did not fracture for 100 hours due to the short hydrogen charging time, as compared in Fig. 7. Takagi et al. examined the effects of the forming mode on the hydrogen

Fig. 10. Three-dimensional map of the hydrogen embrittlement properties considering the press forming (applied stress at 1 300 MPa).

Fig. 11. 2D–/72FeO– map obtained using SIMS for the uniaxially and biaxially specimens deformed with a constant equivalent plastic strain of 2%.

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embrittlement of 1 180 MPa grade steel sheet using U-bend and deep drawing specimens.27) According to the report, the average hydrogen content measured by TDA can be classi-fied by equivalent plastic strains. Regardless of the strain modes, the hydrogen embrittlement properties could not be uniquely classified by the equivalent plastic strains and hydrogen content, and it is proposed that it is necessary to evaluate for each strain mode so that further investigation is required to understand the roles of these factors. Accord-ing to the results of this study, when no large plastic strain is introduced, there is no large difference in the local strain state in the material between the strain mode at the same equivalent plastic strain. Therefore, we concluded that there is no difference in the hydrogen content. On the other hand, when a large amount of equivalent plastic strain just before fracture is applied, the strain state generated locally in the material will differ depending on the strain mode, and the hydrogen content due to localized strain will also differ. In equivalent plastic strain of 2%, considering from the form-ing limit diagram shown in Fig. 4, the biaxial strain has more latitude for the fracture limit line than the uniaxial strain. This may have affected the presence or absence of cracks. However, the difference between the two has not been detected by the hydrogen measurement, SIMS, and EBSD carried out this time, and the study of the mechanism is a future subject.

Based on the above results, in order to evaluate the hydro-gen embrittlement properties of press-forming products with complicated shapes using high-strength steel sheets, it is sometimes not possible to obtain appropriate evaluation results simply by using the amount of hydrogen or equiva-lent plastic strain as an index. One of the countermeasures to this problem is to add the effect of strain modes.

5. Conclusions

• Forming limit diagrams were prepared for the steel sheet whose strength level was adjusted to 1 470 MPa, and stress and hydrogen were applied to each forming mode (uniaxial, plane strain, biaxial) to evaluate the occurrence of cracks. As a result, the difference between the resistance of hydrogen embrittlement and the resistance to hydrogen embrittlement depending on the strain mode could be evalu-ated.• From the results of the EBSD and SIMS analyses of

the hydrogen distribution, we concluded that hydrogen was locally concentrated in a part with a large amount of strain (variant) in the microstructure in the uniaxial specimens. On the other hand, in the biaxially formed specimens, strain was averagely high and strain distribution was more homo-

geneous, so that the local hydrogen enrichment to local area is unlikely to occur.• These results suggest that the local strain (deforma-

tion quantity) in the structure may affect the resistance to hydrogen embrittlement even in the same steel sheet.

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and T. Heller: Int. J. of Auto. Eng., 2 (2010), 7.

Appendices

Figures 1–5 and 8 have been reprinted from the “Zairyo-to-Kankyo, 68 (2019), 116.” by the courtesy of Japan Soci-ety of Corrosion Engineering.