Improving mechanical properties and high-temperature oxidation of press hardened steel by adding Cr and Si

1.   Introduction

Press hardened steels (PHSs), one of the advanced high-strength steels (AHSSs), are widely used in automotive body structural applications, such as B-pillars and anticollision beams, due to the ultra-high strength levels, high dimension accuracy, and low spring-back [1–4]. Currently, the ultimate tensile strength of the 22MnB5 steel has reached above 1500 MPa. However, with the high ultimate tensile strength, the elongation of 22MnB5 steel decreased to less than 7%. This is because ductility typically degrades as strength increases [5–6]. To achieve excellent mechanical properties, before being transferred to the pressing die, the PHS sheets should be heated in a furnace exceed the austenization temperature for several minutes and then quickly cooled to obtain martensite [7]. One of the most critical challenges for the transfer process is the severe surface oxidation of the steel at high temperature [7–8]. On the one hand, severe surface oxidation can increase the difficulty and cost of removing the oxide scales from the substrate, and reduce the dimensional accuracy of the steel surface. On the other hand, during the press hardening process, the oxide scales can change the friction coefficient between the steel sheet and the mold, and cause severe wear of the mold. In addition, during the production process, it is necessary to promptly clean the oxide scales inside the mold, which reduces production efficiency. These are not conducive to improving steel processing efficiency, energy efficiency, and energy consumption reduction [9]. Therefore, it is of great significance to study the oxidation behavior of the press hardening steel for achieving better industrial application.

To date, the oxidation behavior of low alloy steels [10–12] and stainless steels [13–15] at elevated temperature has been extensively studied. The oxide scales on the low alloy steel surface mainly comprise Fe₂O₃, Fe₃O₄, and FeO [10–12]. While the typical triplex oxide scale (Fe₂O₃, Fe₃O₄, and Cr₂O₃) forms on the austenitic steels due to high Cr content [13–15]. The oxidation mechanism is that the growth of the outer oxide scale is owing to the outward diffusion of cations, while the growth of the inner oxide scale is resulted from the inward diffusion of oxygen [16].

Previous studies have demonstrated that surface coating technologies such as Al–Si coating and Zn coating can prevent high-temperature oxidation [8,17]. Nevertheless, the issues associated with the Al–Si coating are low bendability, high cost, and international patent restrictions [8,18]. At the same time, the Zn coating leads to liquid metal embrittlement (LME) cracking during the resistance spot welding (RSW) process [19–20]. Recently, Hou et al. [21–22] studied the effect of induction heating (100°C/s) on the air oxidation behavior of Cr and Si alloyed PHS. The results indicated that it is possible to reduce the soaking time during the hot stamping process by induction heating, thereby improving the oxidation resistance (oxide thickness is less than 5 μm). However, achieving such a rapid heating rate during the press hardening process is difficult. Zhao et al. [23] reported a novel 22MnB5 by increasing carbon, chromium, silicon, and other micro-alloy elements content. The steel has high strength (2160 MPa), good total elongation (12%), and excellent oxidation resistance (oxide thickness was 12.4 μm). However, when the content of C element is above 0.35wt% in steel leads to inferior welding performance.

In addition to the above technologies, the high-temperature oxidation resistance of PHSs can improved by adding Cr and Si elements. The existence of Cr and Si improves the oxidation resistance of steels by hindering the diffusion of iron cations and oxygen anions [24–25]. Traditionally, Cr element (>13wt%) is added to stainless steels to improve the oxidation resistance, because a dense Cr₂O₃ oxide scale can reduce the diffusion rate of O and Cr elements [26–28], meanwhile, due to the cost constraints and the requirements for the press hardening process, the Cr element content in PHS should be lower than 3wt% [21–23]. Yuan et al. [29] suggested that the Si content should be less than 1.2wt%, because the net-like Fe₂SiO₄ forms in the innermost layer of the oxide scales during the hot rolling process. The net-like Fe₂SiO₄ is challenging to remove after the hot-rolled process. Thus, developing a novel PHS with low C, Si, and Cr contents could be a novel method to address the oxidation issue of the steel. However, few studies have focused on the oxidation mechanism of the PHS during the press hardening process. The PHS with low C and Si content during the press hardening process, particularly, has not been well understood.

In this work, a novel PHS with superior oxidation resistance by adding lower Cr and Si content was developed. The microstructure and mechanical properties of the Cr–Si steel and 22MnB5 steel after press hardening were investigated using the field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), and tensile testing machine. The reasons for the increase in elongation are investigated with uncoated 22MnB5 as the benchmark. Moreover, the oxidation behavior during the press hardening process was studied by FE-SEM, TEM, and X-ray diffraction (XRD). The oxidation model was proposed for the Cr–Si micro-alloyed PHS by analyzing its oxidation behavior and the selective oxidation mechanism.

2.   Experimental

2.1   Materials and process

The chemical compositions of the Cr–Si micro-alloyed press hardened steel (Cr–Si steel) and 22MnB5 steel are shown in Table 1. The equilibrium transition temperature (Ae₃) of the Cr–Si steel was 817°C calculated using the commercial Thermo-Calc software, as shown in Fig. 1(a). Fig. 1(b) shows the expansion curve of the Cr–Si steel. The Ac₁, Ac₃, M_s, and M_f of the Cr–Si steel are 680, 872, 383, and 220°C, respectively.

Table  1.  Chemical compositions of the Cr–Si steel and 22MnB5 steel wt%

Steel	C	Si	Mn	B	Cr	Ti	Fe
Cr–Si steel	0.23	1.15	1.11	0.003	3.06	0.047	Bal.
22MnB5	0.22	0.30	1.12	0.003	0.21	0.045	Bal.

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Fig. 1.  (a) Equilibrium phase fractions of the Cr–Si steel at different temperature and (b) dilatometric curve of the Cr–Si steel during heating and cooling process.

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The Cr–Si steel and 22MnB5 steel were melted into an ingot in a 50 kg vacuum induction furnace and forged into billets with 30 mm thickness. Then, the billets were heated to 1200°C for 1 h, further hot rolled to 4.0 mm, and the final rolling and coiling temperature was 900 and 680°C, respectively. After that, the hot rolled sheets were cold rolled to 1.0 mm, annealed at 680°C for 2 h, and then cooled to room temperature. Finally, the sheets were heated to 930°C for 5 min and cooled to room temperature at 30°C/s for press hardening.

2.2   Mechanical test

Dog-bone tensile specimens (180 mm × 20 mm × 1 mm) were extracted from the sheets after press hardening by wire-electrode cutting, and then mechanically ground surfaces and side surfaces before testing. The tensile specimens were tested at room temperature using a CMT 6505 machine at a cross speed of 2 mm/min. The microstructure of the Cr–Si steel and 22MnB5 steel after press hardening was etched with 4% nitric acid alcohol, and observed by FE-SEM (ZEISS ULTRA 55). To confirm the volume fraction of austenite and martensite, the specimens were analyzed by XRD (Bruker D8-Advance) with Cu-K_α radiation at a voltage of 40 kV and current of 40 mA (scanning speed: 1°/min, scanning angle: 40°–90°). The volume fraction of martensite and austenite was calculated from the integrated intensities of the (110)_α, (200)_α, and (221)_α martensite peaks and (220)_γ peak of austenite. The carbides and austenite in the Cr–Si steel were characterized using TEM (JEOL JEM-2100UHR) at an operation voltage of 200 kV.

2.3   High-temperature oxidation test

The rectangular specimens (10 mm × 20 mm × 1 mm) were prepared from the annealed sheet for the high-temperature oxidation test, which were conducted in a chamber electric furnace. Before the high-temperature oxidation test, the surface of the rectangular specimens was ground on SiC papers up to 2000 grit. Then, the specimens were washed with alcohol and dried. The specimens were heated to 930°C and held for 5, 10, 20, 30, and 40 min, then cooled to room temperature at a cooling rate of 30°C/s. The mass changes of the specimens before and after the high-temperature oxidation test were weighed with an electronic balance (±0.0001 g precision).

The surface and cross-sectional morphologies of the oxide scales were observed by FE-SEM. To further understand the compositions of the oxide scales, site-specific TEM specimens were prepared using a focused ion beam (FIB, FEI Scios 2 DualBeam) after depositing protective Pt layer on the oxidized section. The outmost surface oxides formed on the specimens were analyzed by XRD.

3.   Results

3.1   Mechanical properties

The engineering stress–strain curves of the Cr–Si steel and 22MnB5 steel after press hardening are illustrated in Fig. 2(a). The yield strength (YS), ultimate tensile strength (UTS), total elongation (TE), and strength–ductility balance (UTS × TE) of the steels are shown as in Fig. 2(b). Compared with 22MnB5 steel, the UTS and YS are increased by 26 and 56 MPa, respectively. The Cr–Si steel exhibits higher TE (9.16%) than that of the 22MnB5 steel (6.44%). The ductility of the Cr–Si steel improves significantly. Moreover, the UTS × TE value of the Cr–Si steel achieves 13.28 GPa·%, which is about 1.5 times than that of the 22MnB5 steel (9.11 GPa·%).

Fig. 2.  (a) Engineering stress–strain curve and (b) mechanical properties of the Cr–Si steel and 22MnB5 steel.

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Fig. 3 shows the microstructure of the Cr–Si steel and 22MnB5 steel after the press hardening process. The grain size decreases after adding the Cr and Si elements, showing that the average prior austenite grain sizes measured by the linear intercept method are 5.79 and 11.25 μm for the Cr–Si steel and 22MnB5 steel, respectively. It is evident that the microstructure of the Cr–Si steel is martensite with a high dispersion of carbides, whereas the 22MnB5 steel is fully martensite with no carbides. The carbides in the Cr–Si steel are mainly located along the martensitic lath with a size of approximately 180 nm, and are rich in Cr, Mn, and Fe. The carbides are determined to be M₂₃C₆ (M = Cr, Mn, and Fe) carbides by the energy dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED), as shown in Fig. 4(a)–(b), which is consistent with the thermodynamic calculation results (Fig. 1(a)). In addition, 1.57% retained austenite (RA) is detected in the Cr–Si steel, as shown in Fig. 4(c)–(d).

Fig. 3.  SEM images of (a) Cr–Si steel and (b) 22MnB5 steel after press hardening.

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Fig. 4.  (a–b) Bright field image and SAED pattern and corresponding EDX analysis of M₂₃C₆ carbide, (c) bright field image and SAED pattern of austenite, and (d) XRD pattern of the Cr–Si steel.

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3.2   Oxides mass gain analysis

Fig. 5 shows the mass gain versus time plots and macro-morphology for the Cr–Si steel and 22MnB5 steel oxidized at 930°C for 5–40 min. As shown in Fig. 5(a), the mass gain for both specimens always increased upon the increasing oxidation time. In the early period of oxidation, the mass gain increased rapidly; however, upon extending the oxidation time, the mass gain decreased gradually. Throughout the entire procedure, the mass gain of the Cr–Si steel is lower than that of the 22MnB5 steel. Fig. 5(b) demonstrates the macro-morphology of the oxidized specimens. The specimen of the Cr–Si steel oxidized for 5 min still has scratches, indicating the oxidation degree of the Cr–Si steel oxidized at 930°C for 5 min is relatively slight. As the oxidation time prolongs, a light gray oxide scale appears on the surface of the Cr–Si steel, which gradually transforms into a dark gray oxide scale. The oxide scale formed on the Cr–Si steel oxidized for 5–10 min exhibited better adhesion, and no spallation was observed. In contrast, the specimens of the 22MnB5 steel oxidized for 5–40 min have a dark gray surface. Besides, bubbles and spallation appear on the oxidation surface of the 22MnB5 steel oxidized for 5 min. The spallation tendency of the oxide scales increases as the oxidation time increases. The results of mass gain and macro-morphology mean that the high-temperature oxidation resistance of 22MnB5 steel is significantly improved by adding Cr and Si elements.

Fig. 5.  (a) Mass gain versus time plots and (b) macro-morphology for the oxidized specimens of the Cr–Si steel and 22MnB5 steel.

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3.3   XRD analysis of the oxide scales

The XRD patterns of the Cr–Si steel and 22MnB5 steel after high-temperature oxidation at 930°C for 5 min are shown in Fig. 6. It is observed that the oxide scales on both steels are composed of Fe₂O₃, Fe₃O₄, SiO₂, FeCr₂O₄, and Fe₂SiO₄. In addition, the diffraction peak of Fe and Cr₂O₃ also exist in the XRD pattern of Cr–Si steel. The existence of diffraction peaks in Fe indicates that the oxide scales are relatively thin, and Cr₂O₃ appears due to its high Cr content. However, different from the Cr–Si steel, the FeO phase is detected in the 22MnB5 steel.

Fig. 6.  XRD patterns of the Cr–Si steel and 22MnB5 steel after high-temperature oxidation.

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3.4   Surface morphology of the oxide scales

The surface morphologies of the Cr–Si steel and 22MnB5 steel after high-temperature oxidation at 930°C for 5 min are shown in Fig. 7, and the corresponding chemical compositions of the oxides are shown in Table 2. After high-temperature oxidation of 5 min, the surface of the Cr–Si steel is covered with thin and dense oxides, as shown in Fig. 7(a). These oxides exhibit clusters and nanometer-sized particles. The clustered oxides consist of petal-liked Fe₂O₃ and mesh-liked Fe₃O₄, and are rich in Cr, Si, and Mn. The chemical compositions of Fe, Cr, and Si contents in the fine granular oxides are higher than the clustered oxides. Different from the Cr–Si steel, the entire surface of the 22MnB5 steel is covered with micrometer-sized Fe₂O₃ grains, with some cracks (Fig. 7(b)). The oxide size of the Cr–Si steel is smaller than that of the 22MnB5 steel. Usually, the oxide scales with compact structures and small-size oxides could exhibit good high-temperature oxidation resistance [30]. Thus, the high-temperature oxidation resistance is improved after adding Cr and Si elements.

Fig. 7.  SEM images for the oxide scales of different specimens after high-temperature oxidation at 930°C for 5 min: (a) Cr–Si steel; (b) 22MnB5.

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Table  2.  EDS analysis at different points in Fig. 7 at%

Region	O	Fe	Cr	Si	Mn
A1	68.00	30.76	0.46	0.23	0.55
A2	58.98	36.58	2.56	0.92	0.96
A3	19.00	74.92	2.63	2.44	1.01
B1	60.83	39.17	—	—	—
B2	62.77	37.23	—	—	—

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3.5   Cross-sectional morphology of the oxide scales

Fig. 8(a) and (c) displays the morphology and energy dispersive spectroscopy (EDS) elemental maps for the cross-sectional oxides of the Cr–Si steel and 22MnB5 steel. The oxide thickness of the Cr–Si steel is much thinner (3.74 μm) than that of 22MnB5 steel (45.50 μm), confirming the excellent oxidation resistance of Cr–Si steel. This indicates that the oxidation rate of the Cr–Si steel is significantly lower than that of the 22MnB5 steel, which is also consistent with the mass gain comparison shown in Fig. 5(a). It is noted that internal oxidation exists in the Cr–Si steel and is embedded in the substrate. The EDS scanning lines of the cross-sectional microstructures after high-temperature oxidation are shown in Fig. 8(b) and (d). The EDS detection results in Fig. 8 are listed in Table 3.

Fig. 8.  Cross-sectional SEM images and corresponding EDS analysis: (a, b) Cr–Si steel; (c, d) 22MnB5 steel.

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Table  3.  EDS analysis at different points in Fig. 8(a) and (c) at%

Region	O	Fe	Cr	Si	Mn
C1	61.47	36.90	0.35	0.47	0.81
C2	54.06	43.08	0.80	1.35	0.71
C3	50.59	36.06	5.68	7.19	0.48
D1	53.87	45.79	—	—	0.34
D2	46.35	52.01	0.30	0.25	1.09
D3	47.13	51.56	0.32	0.21	0.78
D4	56.09	34.37	2.32	5.92	1.29

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EDS analysis illustrates that the oxide scales of the Cr–Si steel consist of an outer scale with Fe-rich oxide and an inner scale with Fe–Si–Cr-rich oxide, as shown in Fig. 8(b). However, the oxide scales on the 22MnB5 steel can be morphologically divided into three parts: outer and middle scale with Fe-rich oxide, inner scale with Fe–Si–Cr-rich oxide (Fig. 8(d)). The outer scale of the Cr–Si steel and 22MnB5 steel are rich in Fe and O elements, and lack of Cr and Mn elements. Therefore, these Fe-rich regions (outer scales in Fig. 8(b) and (d)) are mainly composed of Fe₃O₄ and Fe₂O₃. The middle scale of the 22MnB5 steel is FeO, as shown in D2 and D3 in Table 3. The inner scale of the Cr–Si steel and 22MnB5 steel are rich in Fe, O, Cr, and Mn elements, so these Fe–Si–Cr-rich regions (inner scales in Fig. 8(b) and (d)) are Fe₂SiO₄ and FeCr₂O₄ spinel. The thickness ratio of the inner scale in the Cr–Si steel is higher than that of the 22MnB5 steel.

To further understand the structure of the oxide scales at the inner and middle oxide scale, TEM specimens of the Cr–Si steel and 22MnB5 steel are prepared using the FIB/SEM technique. The positions for the TEM specimens of the Cr–Si steel and 22MnB5 steel are extracted from the boxed area in Figs. 9(a) and 10(a), respectively. The microstructure of TEM specimens of the Cr–Si steel and 22MnB5 steel are shown in Figs. 9(b) and 10(b), respectively. The interfaces between the different scales are marked with blue dashed lines.

Fig. 9.  Microstructure of oxide scales of the Cr–Si steel after oxidation at 930°C for 5 min: (a, b) FIB/SEM secondary electron images; (c) high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of oxide scale and corresponding EDS elements mapping, corresponding selected area electron diffraction (SAED) pattern of positions 1–3, high-resolution TEM (HRTEM) image of position 4 and corresponding fast fourier transform (FFT) pattern of positions 5 and 6.

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The cross-sectional SEM images show that the hole appears at the substrate/inner oxide scale interface in the Cr–Si steel (Fig. 9(b)), while an obvious gap appears at the same position in the 22MnB5 steel (Fig. 10(b)). The inner oxide scale of the Cr–Si steel is divided into the small grains near the substrate and the columnar grains near the outer oxide scale, as shown in Fig. 9(c). The small grains region and the columnar grains region are identified as the mixed spinel oxide (FeCr₂O₄ and Fe₂SiO₄) (point 1 and 2 in Fig. 9(c)). The outer oxide scale is identified as Fe₃O₄ (point 3 in Fig. 9(c)). The oxides close to the substrate are identified as SiO₂ and amorphous SiO₂ (points 5 and 6 in Fig. 9(c)). Nevertheless, the inner oxide scale of the 22MnB5 steel is composed of some small grains and identified as the mixed spinel oxide (FeCr₂O₄ and Fe₂SiO₄) (point 7 in Fig. 10(c)). While the middle oxide scale is identified as FeO (point 8 in Fig. 10(c)), which is consistent with the SEM results in Fig. 8(c)–(d).

Fig. 10.  Microstructure of oxide scales of the 22MnB5 steel after oxidation at 930°C for 5 min: (a, b) FIB/SEM secondary electron images; (c) HAADF-STEM image of oxide scale and corresponding EDS elements mapping, and corresponding SAED pattern of position 7 and 8.

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4.   Discussion

In this work, the addition of Cr and Si to the PHSs is to improve the high-temperature oxidation resistance and maintain excellent mechanical properties. After adding Cr and Si elements to the 22MnB5 steel, the UTS and YS are increased slightly. Moreover, the TE increased significantly from 6.44% to 9.16%. Since the addition of Cr and Si elements has an important effect on the mechanical properties of 22MnB5 steel, the microstructure of the Cr–Si steel will be thoroughly analyzed in the following, and the reasons for its improved mechanical properties will be explained through the microstructure. Through micro-alloying of the elements, such as Cr and Si, a large amount of M₂₃C₆ carbide plays a pinning effect, inhibiting the migration of prior austenite grain boundaries (PAGS) and refining the prior austenite grains [23,31–33]. Luo et al. [34] reported that the refinement of prior austenite grains, packets, and blocks could induce an increase in YS and toughness. In addition, Hou et al. [35] also showed similar insights that the refinement of prior austenite grains by microalloying is beneficial to obtaining nanoscale austenite. The nanometer-sized film-shaped RA can continuously provide a transformation-induced plasticity (TRIP) effect and improve elongation [36–38]. Therefore, adding Cr and Si alloy elements to 22MnB5 helps to improve both strength and plasticity.

Compared to the minor effect on the mechanical properties, the high-temperature oxidation resistance is obviously improved with the increase of Cr and Si content in 22MnB5 steel. It can be seen that the increase of Cr and Si content in the steel reduces the thickness of the oxide scales, prevents the FeO phase of the oxide scales, and promotes the formation of FeCr₂O₄ and Fe₂SiO₄. During the high-temperature oxidation of PHSs, the formation of the oxides is determined by the standard Gibbs free energy (Δ${G}^{\ominus}$) of the oxidation reaction. The value of Δ${G}^{\ominus}$ can be calculated using HSC Chemistry 9.0 software and its related thermodynamic databases [39–40], as shown in Fig. 11. When the temperature is between 600 and 1000°C, the Δ${G}^{\ominus}$ of the oxides from low to high are SiO₂, Cr₂O₃, FeCr₂O₄, Fe₂SiO₄, FeO, Fe₃O₄, Fe₂O₃, and MnO₂ [40–41]. A lower Δ${G}^{\ominus}$ value indicates a greater affinity for oxygen and is more prone to oxidation. Thus, SiO₂ and Cr₂O₃ will preferentially form than other oxides.

Fig. 11.  Gibbs free energy of the reaction between the oxygen and elements in the Cr–Si steel and 22MnB5 steel.

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To illustrate the effects of Cr and Si content on the high-temperature oxidation resistance of 22MnB5 steel, the oxidation mechanisms at 930°C for 5 min are illustrated in Fig. 12. For the Cr–Si steel, the oxygen can penetrate the steel substrate and react with Si and Cr elements to generate SiO₂ and Cr₂O₃ at the initial stage of oxidation, as shown in Fig. 12(a1) and (a2). The diffusion coefficient of Si in α-Fe (${D}_{\mathrm{S}\mathrm{i}\;\mathrm{i}\mathrm{n}\; \text{α} -\mathrm{F}\mathrm{e}}$) and Cr in α-Fe (${D}_{\mathrm{C}\mathrm{r}\;\mathrm{i}\mathrm{n}\;\text{α}-\mathrm{F}\mathrm{e}}$) can be expressed as follows [42–43]:

Fig. 12.  Schematic for oxidation at 930°C of different specimens: (a1–a4) Cr–Si steel; (b1–b4) 22MnB5.

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where R is the universal gas constant (8.3145 J·mol⁻¹·K⁻¹), T is the absolute temperature (K). When the T is selected at 930°C, the ${D}_{\mathrm{S}\mathrm{i}\; \mathrm{i}\mathrm{n}\; \text{α} -\mathrm{F}\mathrm{e}}$ and ${D}_{\mathrm{C}\mathrm{r}\; \mathrm{i}\mathrm{n}\; \text{α} -\mathrm{F}\mathrm{e}}$ are 4.69 × 10⁻¹⁴ and 1.12 × 10⁻¹⁴ m²/s, respectively [42]. Because Si ions show higher diffusion coefficient than Cr ions in the substrate, SiO₂ particles can form at the interface between the substrate and oxide before Cr₂O₃ at the beginning of oxidation (point 5 of Fig. 9(c)). As the oxidation time prolongs, SiO₂ grows and aggregates.

For the Cr–Si steel and 22MnB5 steel, the dense and continuous Cr₂O₃ oxide scales cannot be generated on the substrate surface, since Cr contents are lower than 12wt% [39]. However, Si acts as a reactive element in the Cr–Si steel and 22MnB5 steel, and reduces the threshold content required to form a dense Cr₂O₃ scale [44]. Therefore, the previously formed SiO₂ serves as the nucleation site for Cr₂O₃, accelerating the nucleation of Cr₂O₃ [44–45].

Once the SiO₂ and Cr₂O₃ are generated in the oxide scales, even if the oxide scales are not continuous, the oxygen partial pressure on the steel surface can be reduced. Thus, the iron oxide formation is suppressed [46]. On the other hand, the diffusion coefficient of Fe³⁺ in SiO₂ (${D}_{{\mathrm{F}\mathrm{e}}^{3+}\; \mathrm{i}\mathrm{n}\; \mathrm{S}\mathrm{i}{\mathrm{O}}_{2}}$) and Cr₂O₃ (${D}_{\mathrm{F}\mathrm{e}\; \mathrm{i}\mathrm{n} \; {{\mathrm{C}\mathrm{r}}_{2}\mathrm{O}}_{3}}$) can be expressed as follows [47–48]:

When the T is selected at 930°C, the ${D}_{{\mathrm{F}\mathrm{e}}^{3+}\; \mathrm{i}\mathrm{n}\; \mathrm{S}\mathrm{i}{\mathrm{O}}_{2}}$ and ${D}_{\mathrm{F}\mathrm{e}\; \mathrm{i}\mathrm{n} \; {{\mathrm{C}\mathrm{r}}_{2}\mathrm{O}}_{3}}$ are 1.70 × 10⁻¹⁹ and 1.61 × 10⁻¹⁸ m²/s, respectively. The diffusion rates of iron ions in SiO₂ and Cr₂O₃ are slow, resulting in a decrease in oxidation rate. When Si and Cr elements are depleted, FeCr₂O₄ and Fe₂SiO₄ are generated before iron oxides. Besides, the oxide close to the substrate is identified as amorphous SiO₂ (position 6 in Fig. 9(c)). The amorphous SiO₂ is characterized by the absence of grain boundaries and low defect concentration, leading to a low diffusion rate [49–50]. The amorphous SiO₂ can act as a diffusion barrier, preventing the outward diffusion of Fe, Cr, and Mn [51]. Hence, the oxidation reaction time of Cr and Si elements during the Cr–Si steel oxidation process is longer than that of 22MnB5 steel. The time for the Fe oxidation reaction is shorter than the 22MnB5 steel, resulting in a very small amount of FeO. The small amount of FeO can react with SiO₂ or Cr₂O₃ to generate Fe₂SiO₄ or FeCr₂O₄, and further combine with Fe₂SiO₄ to form a dense and continuous Si-rich phase, blocking the diffusion path of the Fe ions and decreasing the growth of the iron oxides [52–53].

Therefore, the oxide scales of the Cr–Si steel are composed of Fe₂O₃, Fe₃O_4, mixed spinel oxide (FeCr₂O₄ and Fe₂SiO₄), and amorphous SiO₂. As a result of FeCr₂O₄ and Fe₂SiO₄ being denser and finer than iron oxides, the inner oxide scales of the steels are more protective than the outer and middle oxide scales (Figs. 9(c) and 10(c)). That is because FeCr₂O₄ and Fe₂SiO₄ can hinder the inward diffusion of oxygen ions and the outward diffusion of iron ions [21,52]. As a result, compared to 22MnB5 steel, the oxide scales of Cr–Si steel are substantially thinner and do not contain the FeO phase.

For the 22MnB5 steel, due to the lower content of Cr and Si elements, little SiO₂ and Cr₂O₃ are generated on the substrate surface at the beginning of oxidation. Thus, a thin mixed spinel oxide (FeCr₂O₄ and Fe₂SiO₄) can be formed later, which cannot prevent the inward diffusion of O and outward diffusion of cations. Besides, as the Si content is lower than 1wt%, the continuous amorphous SiO₂ cannot form on the substrate of the 22MnB5 steel [43–44]. The amorphous SiO₂ can fill the voids between the substrate and the mixed spinel oxide, and prevent cations from diffusion efficiently [44]. That results in some defects between the substrate and oxide of 22MnB5 steel. This may be the reason for the poor oxidation resistance of the 22MnB5 steel compared to the Cr–Si steel. In the subsequent oxidation process, when Si and Cr elements are depleted, O reacts with Fe to form a typical triplex oxide scale (Fe₂O₃, Fe₃O₄, and FeO) with enough time, as shown in Fig. 12(b3).

In addition, because more SiO₂ as the nucleation site has a smaller size than iron oxide, the size of the oxide formed after oxidation on the Cr–Si steel is smaller than that of the 22MnB5 steel. This indicates that a closer bonding between nearby oxide particles increases the density of the oxide scales [54]. On the other hand, dense oxide scales can improve the plastic deformation ability of the oxide, promote the release of thermal stress, and improve the adhesion between the oxide scale and the substrate [55].

5.   Conclusions

The mechanical properties and high-temperature oxidation behavior of the novel Cr–Si steel with low C and Si content are investigated through multiscale characterization technique. The effect of Cr and Si addition on the mechanical properties and high-temperature oxidation behavior of 22MnB5 press hardened steel are analyzed. The following conclusions can be made.

(1) Compared with the traditional 22MnB5 steel, the TE (9.16%) of the Cr–Si steel improved markedly, while the YS (1115 MPa) and UTS (1450 MPa) improved slightly. After press hardening, the presence of RA and M₂₃C₆ carbides is the reason for the increase in elongation of the Cr–Si steel.

(2) After oxidation at 930°C for 5 min, the oxide thickness of the Cr–Si steel is 3.74 μm, which is 1/12 thick of the 22MnB5 steel. The oxide scales of the Cr–Si steel are Fe₂O₃, Fe₃O₄, mixed spinel oxide (FeCr₂O₄ and Fe₂SiO₄), and amorphous SiO₂.

(3) The addition of Cr and Si results in better oxidation resistance due to the increase of the mixed spinel oxide, the appearance of amorphous SiO₂, and the disappearance of the FeO phase.

(4) The priority oxidation of SiO₂ is due to the lowest Gibbs free energy, and the SiO₂ serves as the nucleation site for Cr₂O₃, accelerating the nucleation of Cr₂O₃ oxide.