Effect of Hydrogen on Spot-Welded Tensile Properties in Automotive Ultrahigh-Strength TRIP-Aided Martensitic Steel Sheet

Abstract

Effect of hydrogen on spot-welded tensile properties in ultrahigh-strength TRIP-aided martensitic steel (TM steel) sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 1 mm/min (strain rate of 2.8×10⁻⁴/s), using base metal and spot-welded specimens with or without hydrogen charging.

The results are as follows.

(1) The difference between the tensile strength (TS) of 1532 MPa for base metal specimen without hydrogen charging and the maximum stress (TS-H) of 1126 MPa for the base metal specimen with hydrogen charging (ΔTS-H=TS−TS-H) in the TM steel was smaller than that of hot stampted steel (HS1 steel) and superior to that of HS1 steel. On the other hand, the TS-H of 725 MPa for the base metal specimen with hydrogen charging was halved in comparison with the TS of 1438 MPa for base metal specimen without hydrogen charging in the HS1 steel. It is considered that this was because the retained austenite suppressed the strength reduction due to the hydrogen embrittlement of the TM steel.

(2) The amount of hydrogen decreased in the order of the HS1 steel, the TM steel, and the tempered martensitic steel (HS7 steel), and the HS1 steel was the highest. This is thought to be due to the high dislocation density of the HS1 steel.

(3) The difference between the maximum stress (TS-W) of the spot-welded specimen without hydrogen charging and the maximum stress (TS-WH) of the spot-welded specimen with hydrogen charging (ΔTS-WH=TS-W−TS-WH) in the TM steel and that of the HS1 steel were similar. It was considered that this is partly due to the effect of the stress concentration on heat affected zone (HAZ) softening of the hardness distribution of the spot-weld.

1. Introduction

In recent years, 1470 MPa grade hot stamped steel (HS steel) have been widely used for the center pillar (B pillar) and the bumper beams for the purpose of the weight reduction and the improvement of the crash safety¹⁾ due to the ultrahigh-strengthening of the automotive steel sheets. On the other hand, the TRIP²⁾-aided martensitic steels (TM steels)^3,4) are expected to an ultrahigh-strength steel sheets with the tensile strength of 1470 MPa or more which exhibits an excellent balance of the strength and the ductility, and a lot of research has been reported.⁵⁾

For example, Senuma et al. have investigated the effects of the alloying elements such as Mn, Nb, Ti, and Mo on the delayed fracture properties of the ultrahigh-strength martensitic steels.^6,7,8,9) In those research, the specimens with a thickness of 1 mm with a notch, where the maximum stress of 1.3 GPa was applied, were immersed in an aqueous solution containing ammonium thiocyanate, and the time to the fracture was measured. Yamazaki et al.¹⁰⁾ have conducted a delayed fracture test using the U-bent samples with the bolt tightening, and the U-bent samples were charged with hydrogen using a cathode charging. The results have been reported that the greater the amount of the retained austenite γ_R of the sample before U-bending, the higher the delayed fracture registance sensitivitiy. In order to evaluate the hydrogen embrittlement properties of ultrahigh-strength steel sheets for automobiles simulating practical use, a U-bending-cathode charging test of the 1470 MPa grade TM steel sheets were carried out and hydrogen embrittlement tests have been conducted using the U-bent specimens with a plastic strain applied by U-bending of a bending radius at 3 to 15 mm with a applied stress of 0–1000 MPa with hydrogen charging (electrolytic hydrogen charging, 3% NaCl + 5 g/L NH₄SCN aqueous solution, current density 10 A/m²).

In the tempered martensitic steels with a tensile strength of 1470 MPa grade with changing the additive amount of Si, occurring the quasi-cleavage and the intergranular fractures due to hydrogen, the effects of the strain rate, the hydrogen concentration and the temperature on the fracture strength and the fracture morphology of the round bar notched specimen have been investigated.¹¹⁾ In addition, in the evaluation of the side collision of automobiles, the effect of the heat affected zone (HAZ) softening on the strength and the elongation of the spot-welded tensile specimens is considered since the flange of the B pillar on the side of the car body may occur the in-plate tensile deformation mode.¹²⁾ However, the sufficient research has not been conducted on the hydrogen embrittlement resistance of the spot-welded TM steels.

In this study, the hydrogen embrittlement properties of the spot-welded TM steel sheets were investigated to clarify the effects of the spot welding and the hydrogen on the tensile properties of the ultrahigh-strength steel sheets for the automobiles in order to improve the deformability (the ductility and the toughness) near the spot-welded part after the hot stamping in the ultrahigh-strength steel sheets that occurs in the case of a side collision of the automobile.

2. Experimental Procedure

Table 1 shows the chemical composition of the steel sheets used. Cold-rolled steel sheets (t=1.4 mm) with the different amount of silicon content were used in this study (Table 1). The steel sheets were produced by the rough hot rolling (thickness t=60→t=30 mm), then the finishing rolling (thickness t=30→t=4 mm), followed by the pickling and the cold rolling (thickness t=4→t=1.4 mm). The steel sheets were annealed at the austenite region of 900°C for 1200 s and isothermal transformation treated at 250°C for 200 s to produce the TRIP-aided steels with martensite matrix (TM steels). For comparison, two types of hot stamping (HS) steels were also produced by heating at 900°C for 240 s, then underwent the die-quenching for 15 s and by those processes followed by tempering at 700°C for 1 h with air cooling. The HS steels which only conduct the die-quenching process were called as HS1, whereas the HS steels that underwent the die-quenching and the tempering processes were designated as HS7.

Table 1. Chemical composition of steels used (mass%).

steel	C	Si	Mn	Ti	Cr	B
TM	0.22	1.51	1.51	0.020	0.21	0.003
HS1, HS7	0.22	–	1.21	0.038	0.25	0.004

Figure 1 shows a spot-welded tensile test specimen.¹³⁾ The spot-welded test specimen was prepared by the spot welding of the tab sheet to the center of parallel part of the tensile test specimen. Table 2 shows the condition for the spot welding. For the spot welding, a stationary DC inverter spot welding machine (Daihen, SLAI 65-601(S-1)) and the Cu–Cr alloy was used as an electrode (DR16×60, 40R). The spot welding was performed at an electrode force of 3.0 kN, a welding time of 333 ms, and a welding current at 6.5 kA.

Fig. 1.

Geometry of spot-welded specimen.¹³⁾

Table 2. Condition of spot welding.

Electrode cap	Electrode force	Welding time	Welding current (I)
Cu–Cr DR16×60, 40R	3.0 kN (0.25 MPa)	333 ms (20 cycles/60 Hz)	6.5 kA

The microstructure was observed by using the scanning electron microscope (SEM). The volume fraction of retained austenite (f_γ, vol%) was quantified from the integrated intensities of the diffraction peaks of (200)_α, (211)_α, (200)_γ, (220)_γ, and (311)_γ measured using CuKα radiation of the X-ray diffractometry.¹⁴⁾ The carbon concentration in the retained austenite (C_γ, mass%) was estimated by the following equation¹⁵⁾ using a lattice perimeter (a_γ, ×10⁻¹⁰ m) measured from (200)_γ, (220)_γ, and (311)_γ diffraction peaks of CuKα radiation.   

$$\begin{array}{l}{a}_{\gamma }=3.5780+0.0330C_{\gamma }+0.00095M{n}_{\gamma }\\ \hspace{1em}\hspace{0.5em}+0.0056A{l}_{\gamma }+0.0220N_{\gamma }\end{array}$$(1)

Here, Mn_γ, Al_γ and N_γ represent the concentration of each element in the austenite. In this study, the chemical compositions of the steel were adopted.

The tensile tests were carried out on a tensile testing machine using the base metal specimen with a width of 20 mm, a length of 60 mm and a gauge length of 50 mm in the parallel part, and the spot-welded specimen (welded with tab sheet of 20×20×1.4 mm)¹⁾ as the tensile test specimens at a crosshead speed of 1 mm/min, i.e., a strain rate of 2.8×10⁻⁴/s (Fig. 1).

The Vickers hardness tests were performed on a dynamic ultra micro-Vickers hardness tester at a load of 98.1 mN, and a holding time of 5 s and the hardness distribution at an interval of x = 0.1 mm on the surface of the steel sheet in the parallel direction at t/4 = 0.35 mm from the overlapped surface of the steel sheet was measured.

Figure 2 shows the apparatus set-up for the tensile test after the cathodic hydrogen charging. The cathodic hydrogen charging was conducted in the 400 ml-aqueous solution composed of 3 wt% NaCl and 5 g/L NH₄SCN mixture. The hydrogen charging current density was kept at 10 A/m² and the hydrogen charging duration was set for 48 h before the tensile tests. Platinum wire was used as a counter electrode. The hydrogen charging area was defined as the total surface area of both side of the tab specimen (20 mm×20 mm×2) and the side area (20 mm×1.4 mm×4) after the unnecessary area for the hydrogen charging in the specimen was masked.

Fig. 2.

Appearance of tensile test after hydrogen charging. (Online version in color.)

Thermal desorption spectrometry (TDS) was used to measure the hydrogen content of the specimens.¹⁶⁾ The specimens were charged with the hydrogen in the same composition of the aqueous solution at the same charging current density as tensile tests for 48 h. Then, the hydrogen-charged specimens were heated to a temperature of 800°C at a heating rate of 100°C/h. The hydrogen desorption rate was calibrated by using the standard leak gas system, and the hydrogen content was calculated as the hydrogen desorption rate integrated over the time and divided by the weight of the specimen. The hydrogen content desorbed from the room temperature to 300°C was considered as the diffusible hydrogen, whereas the hydrogen that desorbed at the temperature more than 300°C was considered as the non-diffusible hydrogen. Since the diffusible hydrogen affects the hydrogen embrittlement, the diffusible hydrogen content was analyzed¹⁷⁾ in this study.

3. Results and Discussion

3.1. Microstructure and Mechanical Properties

Figure 3 shows the microstructure (SEM) of samples. Figures 3(a), 3(b) and 3(c) show the microstructure of the TM steel, the HS1 steel, and the HS7 steel, respectively. According to the microstructure etched by 3% nital etchant, the microstructure of the TM steel consists of the martensite and the retained austenite (f_γ (volume fraction of γ_R) = 1.52 vol%, C_γ (carbon concentration in γ_R) = 0.79 mass%), the microstructure of the HS1 steel consists of martensite, and the microstructure of the HS7 steel consists of the tempered martensite, respectively. Although both the TM steel and the HS1 steel possess the martensite matrix, it was confirmed that some cementite particles were precipitated in the relatively wide martensite lath of the HS1 steel than that in the TM steel.

Fig. 3.

Scanning electron micrographs of (a) TM, (b) HS1 and (c) HS7 steels.¹³⁾

Table 3 shows the mechanical properties and the carbon equivalent (C_eq)¹⁸⁾ of the base metal specimens. The carbon equivalent (C_eq) is a parameter indicating the weldability, which is calculated from the following Eq. (2) and is in a range of 0.47 to 0.58 mass%. Here, [C], [Si], [Mn] and [Cr] indicate the chemical composition (mass%).   

$$Ceq=\left[\text{C}\right]+\frac{\left[\text{Si}\right]}{24}+\frac{\left[\text{Mn}\right]}{6}+\frac{\left[\text{Cr}\right]}{5}$$(2)

Table 3. Mechanical properties of steels used.

steel	YS (MPa)	TS (MPa)	UEl (%)	TEl (%)	TS×TEl (GPa %)	Ceq (mass%)
TM	1180	1532	5.6	9.4	14.4	0.58
HS1	1095	1469	6.5	7.7	11.3	0.47
HS7	521	559	13.7	22.4	12.5	0.47

YS: yield stress or 0.2% offset proof stress, TS: tensile strength, UEl: uniform elongation, TEl: total elongation, TS×TEl: strength-ductility balance and C_eq: carbon equivalent.

Table 4 shows the mechanical properties of the spot-welded specimen. Here, the tensile strength difference (ΔTS) was defined as ΔTS=TS-W−TS, and the total elongation difference (ΔTEl) was defined as ΔTEl=TEl-W−TEl, using the tensile strength (TS) and the total elongation (TEl) of the base metal specimen, the maximum stress (TS-W) and the fracture elongation (TEl-W) of the welded specimen (Tables 3, 4). The tensile strengths of the base metal specimens of each steel were 1470 MPa-grade for the TM steel and the HS1 steel, and 590 MPa-grade for the HS7 steel. On the other hand, the HS7 steel exhibited the larger total elongation than the TM steel and the HS1 steel. In addition, the TM steel possessed the larger total elongation than the HS1 steel due to the strain-induced transformation of the retained austenite (γ_R) even though the tensile strength of the TM steel and the HS1 steel were almost same level.

Table 4. Mechanical properties of spot-welded specimen.

spot welded test specimen	TS-W (MPa)	TEl-W (%)	TS-W×TEl-W (GPa%)	ΔTS (MPa)	ΔTEl (%)
TM	1450	7.0	10.2	−82	−2.4
HS1	1388	6.2	8.6	−81	−1.5
HS7	557	21.7	12.1	−2	−0.7

TS-W: maximum stress of welded specimen, TEl-W: fracture elongation of welded specimen, TS-W×TEl-W: strength-ductility balance of welded specimen, ΔTS: tensile strength difference (ΔTS=TS-W−TS) and ΔTEl: total elongation difference (ΔTEl=TEl-W−TEl).

In the TM steel and the HS1 steel, the tensile strength and the total elongation of the welded specimens were lower than those of the base metal specimens (Tables 3, 4). The fracture occurred at the periphery of the spot-welded part in the welded specimens of the TM steel and the HS1 steel. On the other hand, in the HS7 steel, the TEl-W slightly decreased, and the fracture at the base metal part was observed although the TS-W of the welded specimen was hardly changed compared to that of the base metal specimen (Table 4). Thus, the maximum stress (TS-W) = 1450 MPa and the fracture elongation (TEl-W) = 7.0% of the welded specimen of the TM steel are superior to the fracture elongation of the HS1 steel, and exhibited the same strength level of the base metal specimen of the HS1 steel (TS = 1469 MPa) (Tables 3, 4). In addition, the TM steel exhibits an excellent strength-ductility balance (TS × TEl) = 14.4 GPa% (TS = 1532 MPa, TEl = 9.4%), and the strength-ductility balance (TS-W × TEl-W) = 10.2 GPa% of the welded specimen of the TM steel is also superior to that of the HS1 steel.

3.2. Effect of Hydrogen on Tensile Properties

Table 5 shows the maximum stress after the spot welding and the hydrogen charging of each steel. The tensile strength of the base metal specimen is called TS, the maximum stress of the welded specimen is called TS-W, the maximum stress of the hydrogen-charged base metal specimen is called TS-H, and the maximum stress of the welded and hydrogen-charged specimen is called TS-WH, respectively.

Table 5. Tensile strength and maximum stress of TM, HS1 and HS7 steels.

specimen	TS (MPa)	TS-W (MPa)	TS-H (MPa)	TS-WH (MPa)
TM	1532	1450	1126	929
HS1	1469	1388	725	811
HS7	559	557	507	533

TS: tensile strength of base metal specimen,

TS-W: maximum stress of welded specimen,

TS-H: tensile strength of base metal specimen with hydrogen charging, and TS-WH: maximum stress of welded specimen with hydrogen charging.

Figure 4 shows the base metal specimens after the tensile tests. The TM steel base metal specimen (a), the TM steel hydrogen-charged base metal specimen (b), the HS1 steel base metal specimen (c), and the HS1 steel hydrogen-charged base metal specimen (d) are called TM-B, TM-H, HS1-B and HS1-H, respectively. The base metal specimens are considered to be the ductile fracture because the fractures occur along 45-degree of the maximum shear stress direction in the tensile direction (Figs. 4(a), 4(c)). On the other hand, the fracture mode of brittle fracture might be dominant in the hydrogen-charged specimens because the fracture occurred toward the perpendicular to the tensile direction (Figs. 4(b), 4(d)).

Fig. 4.

Base metal specimens after tensile test ((a) TM-B, (b) TM-H, (c) HS1-B, (d) HS1-H). (Online version in color.)

Figure 5 shows the welded specimens after the tensile tests. The welded specimen of the TM steel (a), the welded and hydrogen-charged specimen of the TM steel (b), the welded specimen of the HS1 steel (c), and the welded and hydrogen-charged specimen of the HS1 steel (d) are called TM-W, TM-WH, HS1-W and HS1-WH, respectively. The tensile properties of the welded specimens were evaluated in the same manner as the base metal specimens because the tensile tests of the welded specimens were assumed as an in-plane tensile test.¹²⁾ In the welded specimens without the hydrogen charging, the ductile fracture is considered to be dominant because the fracture occurs toward 45-degree of the maximum shear stress direction from the tensile direction (Figs. 5(a), 5(c)). In the TM-WH specimen, it is considered to be occurred the ductile fracture and the brittle fracture since the fractures propagated toward the perpendicular to the tensile direction in the right side and 45-degree of the maximum shear stress direction from the tensile direction in the left side (Fig. 5(b)). The HS1-WH is considered to occur the brittle fracture because the fracture propagated toward the perpendicular to the tensile direction (Fig. 5(d)).

Fig. 5.

Spot-welded specimens after tensile test ((a) TM-W, (b) TM-WH, (c) HS1-W, (d) HS1-WH). (Online version in color.)

Figure 6 shows the hydrogen-charged HS7 steel specimens. The hydrogen-charged base metal specimen of the HS7 steel (a) is called HS7-H, and the welded and hydrogen-charged specimen of the HS7 steel (b) is called HS7-WH. For the HS7 steel, the ductile fracture occurs because the fracture propagated toward 45-degree of the maximum shear stress direction from the tensile direction (Figs. 6(a), 6(b)).

Fig. 6.

Specimens after tensile test ((a) HS7-H, (b) HS7-WH). (Online version in color.)

Figures 7, 8 and 9 show the stress (σ)-strain (ε) curves of the base metal specimens and the welded specimens for the TM, HS1 and HS7 steels with and without hydrogen charging, respectively. From Figs. 7 and 8, the base metal specimens and the welded specimens of the TM and HS1 steels without hydrogen charging fractured after the plastic deformation whereas the fracture occurred at the elastic deformation region in both the base metal specimens and the welded specimens when the tensile test was performed with hydrogen charging. On the other hand, the HS7 steel in Fig. 9 exhibited the same maximum stress in the base metal specimens and the welded specimens with and without hydrogen charging whereas it can be seen that the fracture elongation is reduced due to the hydrogen charging.

Fig. 7.

Stress (σ)-strain (ε) curves of (a) base metal specimen with and without hydrogen charging, (b) spot-welded specimen with and without charging for TM steel. (Online version in color.)

Fig. 8.

Stress (σ)-strain (ε) curves of (a) base metal specimen with and without hydrogen charging, (b) spot-welded specimen with and without hydrogen charging for HS1 steel. (Online version in color.)

Fig. 9.

Stress (σ)-strain (ε) curves of (a) base metal specimen with and without hydrogen charging, (b) spot-welded specimen with and without and with hydrogen charging for HS7 steel. (Online version in color.)

Figures 10, 11 show the comparison of the tensile strength and the effect of hydrogen on the tensile strength of the steels. The tensile strength of the base metal sample of the HS1 steel with hydrogen charging was TS-H = 725 MPa, which was half compared with TS-B = 1469 MPa of the base metal sample for the HS1 steel, and the brittle fracture occurred at the hydrogen-charged part (Table 5, Fig. 4(d)). The tensile strength of the base metal sample for the TM steel with hydrogen charging is TS-H = 1126 MPa, which is less affected by the hydrogen embrittlement than the base metal sample for the HS1 steel after hydrogen charging of the TS-H = 725 MPa (Figs. 10, 11). The welded sample for the TM steel with hydrogen charging has TS-WH = 929 MPa, which is less affected by the hydrogen embrittlement and the spot-welding than the TS-WH = 811 MPa of the welded specimen of the HS1 steel after hydrogen charging (Figs. 10, 11). When the TM steel and the HS1 steel are charged with hydrogen, the hydrogen is mainly trapped at the prior austenite grain boundaries and the lath boundaries,¹⁹⁾ on dislocations,²⁰⁾ in retained austenite, or at the retained austenite/martensite boundaries²¹⁾ whereas it is considered that the hydrogen was mainly trapped at the martensite/cementite boundaries²²⁾ in addition to the prior austenite grain boundaries, lath boundaries, and dislocations in the HS1 steel in the same way as the TM steel. The hydrogen concentration at the vicinity of martensite which was transformed due to the martensitic transformation of the retained austenite during the tensile test increased because of the difference of the hydrogen absorption capacity between austenite (fcc) and martensite (bcc),²³⁾ and that caused cracking in the TM steel.¹⁶⁾ However, the strain-induced and stress-induced transformation of the retained austenite were suppressed because the stability of the retained austenite against the plastic strain was high due to the fine retained austenite which exists surrounding hard martensite in the TM steel. In addition, the crack growth was immediately suppressed because the size of the cracks initiated in the early stage of the deformation was small. Resultantly, it was considered that the decrease in the tensile strength of the TM steel was smaller than that of the HS1 steel. The base metal specimen for the HS7 steel with hydrogen charging had TS-H = 507 MPa, which was similar to TS= 559 MPa of the base metal specimen, and was fractured at the base metal. This is because the steels with the tensile strength of 1200 MPa or less are not susceptible for hydrogen (Figs. 10, 11). The TS-H = 1126 MPa of the base metal specimen for the TM steel with hydrogen charging and the TS-W = 1450 MPa of the welded specimen for the TM steel showed the similar strength as those of the HS1 steel (Fig. 10).

Fig. 10.

Tensile strength (TS) and maximum stress of TM, HS1 and HS7 steels, in witch “B”, “W”, “H” and “WH” denote “base sample”, “spot-welded sample”, “hydrogen charged sample” and “spot-welded and hydrogen charged sample”, respectively.

Fig. 11.

Difference between maximum stress after hydrogen charging (TS-H) and tensile strength (TS-B) in base metal samples (ΔTS-H), and difference between maximum stress after hydrogen charging (TS-WH) and maximum stress (TS-W) in spot-welded samples (ΔTS-WH) of TM, HS1 and HS7 steels.

3.3. Effect of Hardness Distribution at Spot-welded Portion on Tensile Properties

Figure 12 shows cross-sectional image of the spot-welded TM steel. Figures 13, 14 and 15 show the Vickers hardness (HV) distribution at cross section of welded part, respectively.

Fig. 12.

Cross-sectional image of spot-welded TM steel.¹³⁾ (Online version in color.)

Fig. 13.

Vickers hardness (HV) distribution for TM stee.¹³⁾ (Online version in color.)

Fig. 14.

Vickers hardness (HV) distribution for HS1 steel.¹³⁾ (Online version in color.)

Fig. 15.

Vickers hardness (HV) distribution for HS7 steel.¹³⁾ (Online version in color.)

According to the hardness distributions at the spot-welded part in the steels in Figs. 13 to 14, the TM and HS1 steels exhibited the hardness at the base metal of 500 HV–600 HV and 400 HV–500 HV, respectively, and hardness at the weld metal possessed similar value as that at the base metal. The decrease in the hardness at the HAZ portion of the TM steel was less than that of the HS1 steel although the hardness at the HAZ portion of the TM and HS1 steels was deteriorated compared with that of the base metal. On the other hand, the hardness at the fusion and the HAZ portions were 500 HV which is extremely higher than that at the base metal of 200 HV in the HS7 steel. The spot-welded TM and HS1 steels might be exhibited the low fracture elongation compared with those of the base metal because the crack initiated at the HAZ portion due to the low hardness in comparison with that at the base metal and the fusion portion which affect as the notch. In contrast, it was considered that the remarkable deterioration of the fracture elongation in the spot-welded HS7 steel was suppressed because the HAZ portion does not act as the notch due to the hardness at the base metal and the fusion portion of 200 HV and 500 HV, and the fracture occurred at the base metal. The suppression of the HAZ softening of spot-welded part did not occur in the TM steel. However, the strengthening of the part where the HAZ softening occurred was achieved due to the TRIP effect of the retained austenite during the tensile tests, and resultantly, the crack initiation was delayed at the HAZ portion.

Figures 16, 17 show the SEM images of the fracture surface of the TM steel and the HS1 steel. From Fig. 16, the TM steel with and without hydrogen exhibited the dimple fracture surface. However, it is considered that the brittle fracture surface of the hydrogen-charged specimen was dominant due to the shallow dimples with a flat bottom. Thus, the effect of hydrogen embrittlement of the TM steel is smaller than that of the HS1 steel, and the amount of the strength loss due to the hydrogen embrittlement was similar compared with that due to the spot welding, implying that the effect of the hydrogen embrittlement of the TM steel was similar to that of the spot welding (Fig. 16). In the TM steel, the dimples were observed in both the base metal specimen with and without hydrogen charging. However, the dimples with shallow and flat were dominant in the TM steel after hydrogen charging. Therefore, it is considered that the TM steel after hydrogen charging occurred the quasi-cleavage fracture which is the brittle fracture (Fig. 16(c)).

Fig. 16.

Scanning electron micrographs of fracture surface in TM steel. ((a) high magnification (×2000) of base metal, (b) low magnification (×47) of (a), (c) high magnification of base metal with hydrogen, (d) low magnification of (c)).

Fig. 17.

Scanning electron micrographs of fracture surface in HS1 steel ((a) high magnification (×2000) of base metal, (b) low magnification (×47) of (a), (c) high magnification of base metal with hydrogen, (d) low magnification of (c)).

Figure 18 shows the hydrogen desorption curves of the TM, HS1 and HS7 steels obtained by thermal desorption spectroscopy (TDS) at a heating rate of 100°C/h using the tab plates (20 × 20 × 1.4 mm) with hydrogen charging. It can be seen that the diffusible hydrogen content of the TM steel is 2.0 ppm, which is less than that of 2.9 ppm in the HS1 steel. Since the hydrogen is trapped at dislocations, and the dislocation density increases with increasing the strength of the steels, it was considered that the amount of diffusible hydrogen is higher in the TM steel which exhibits higher strength level of the base metal. However, the amount of diffusible hydrogen in the TM steel was less than that in the HS1 steel. In the TM steel, the hydrogen is mainly trapped at the prior austenite grain boundaries and the lath boundaries,¹⁹⁾ on the dislocations,²⁰⁾ in the retained austenite, or at the retained austenite/martensite boundaries,²¹⁾ whereas the hydrogen trapping sites were at the martensite/cementite boundaries²²⁾ in addition to the prior austenite grain boundaries and the lath boundaries in the HS1 steel in the same way as those in the TM steel. The strength of the TM steel is obtained by the martensitic transformation of the retained austenite during the plastic deformation. On the other hand, it is considered that the strength of the HS1 steel was obtained by the increase in dislocation density in the matrix and a large amount of cementite precipitation in the martensite matrix. Therefore, it is considered that the HS1 steel exhibited a high hydrogen content because a large amount of hydrogen was trapped at the dislocations and the martensite matrix/cementite boundaries, which exists more than that in the TM steel. The diffusible hydrogen content in the HS7 steel is 0.4 ppm, which is extremely small compared to that in the TM steel and the HS1 steel. The high hydrogen content in the HS1 steel compared with that of the HS7 steel was considered to be high dislocation density of the HS1 steel and the precipitation of a large amount of cementite. Therefore, the HS7 steel fractured at the base metal regardless of the hydrogen charging because of the low hydrogen concentration.

Fig. 18.

Typical hydrogen desorption curves of TM, HS1 and HS7 steels.

4. Conclusions

The effect of hydrogen on the spot-welded tensile properties of the ultrahigh-strength TRIP-aided martensitic steel (TM steel) sheets for the automobiles was investigated. The main results are as follows.

(1) The difference between the maximum stress after hydrogen charging TS-H = 1126 MPa and the tensile strength TS = 1532 MPa in the base metal specimen of the TM steel (ΔTS-H=TS-H−TS=−406 MPa) was superior to that of the hot-stamped steel (HS1) (ΔTS-H=−744 MPa). This might be caused by the suppression of deterioration of the strength (i.e. hydrogen embrittlement) due to the retained austenite in the TM steel.

(2) The hydrogen content absorbed by hydrogen charging in the steels decreased in the order of the HS1 steel, the TM steel, and the tempered martensite steel (HS7 steel), and the HS1 steel exhibited the highest. This is considered to be due to the high dislocation density of the HS1 steel.

(3) The difference between the maximum stress after hydrogen charging (TS-WH) and the maximum stress (TS-W) in spot-welded specimen of the TM steel (ΔTS-WH=TS-WH−TS-W=−521 MPa) and that of the HS1 steel (ΔTS-WH=−577 MPa) was similar. It is considered that this is due to the stress concentration because of the HAZ softening of the hardness distribution at the spot-welded portion.

Acknowledgments

The part of this research was financially supported by The Amada Foundation and Suzuki Foundation. We would also like to thank Junya Naito of the Kobe Steel, Automobile Solution Center Ltd., Riki Fujisawa, Ryoya Hoshina, Haruyuki Ono, Yuto Miyazawa, Kotaro Shiozaki, Tukasa Sekizaki, Seigo Furutani, Kousuke Mitsui, Atsushi Mio, Masayuki Kato, and Takayuki Sato of National Institute of Technology (KOSEN), Nagano College. Finally, the authors wish to thank the 2019 Joint Research Project at the Institute for Materials Research, Tohoku University Foundation (Subject No. 19K0032), Institute for Materials Research, Tohoku University and National Institute of Technology (KOSEN), Nagano College Foundation for financial supports.

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