Studying the Effect of Cr and Si on the High-Temperature Oxidation-Resistance Mechanism of Hot Stamping Steel

: The surface of hot stamping steel is severely oxidized during heating, holding, and transfer from the heating furnace to the stamping die in the production of traditional automotive parts. Coating-free hot stamping steel with Cr and Si elements exhibits excellent oxidation resistance during hot stamping without the protection of a surface coating. This paper investigates the oxidation behavior of three types of hot stamping steel at 800–1200 ◦ C. The results show that although Cr-Si hot stamping steel performs excellently short-term ( ≤ 7.5 min) for oxidation resistance, its long-term ( ≥ 15 min) or high-temperature ( ≥ 1100 ◦ C) oxidation resistance is much lower than that of the conventional hot stamping steel 22MnB5, affecting the production and surface quality control of the new coating-free Cr-Si hot stamping steel. By analyzing the oxidation kinetics and characterizing the structure of oxide layers in hot stamping steel, it was found that the structural change in the Cr and Si element enrichment layer between the oxide scale and the substrate varied in oxidation performance at different temperatures. When the oxidation temperature was below 1000 ◦ C, the solid Cr and Si enrichment layer acted as a barrier to prevent the diffusion of Fe ions. When the oxidation temperature exceeded 1100 ◦ C, the molten Cr and Si enrichment layer effectively adapted to the substrate and avoided blistering. Meanwhile, Fe 2 SiO 4 penetrated the Fe oxide layer along the grain boundary and became a rapidly diffusing channel of Fe ions, contributing to a signiﬁcant increase in the oxidation rate.


Introduction
Hot stamping steel [1][2][3], as a form of advanced high-strength steel, is widely used in the modern automobile industry for lightweight bodies and safety.The manufacturing process of thermoformed parts includes both austenitizing heating and hot stamping, with resulting oxidation.Therefore, the deterioration of surface quality and the increase in the surface treatment process are problems that should be addressed when processing thermoformed parts [3,4].Widely used Al-Si-coated hot-forming steel [5,6] has excellent oxidation resistance.However, the coating is not conducive to the subsequent welding process and is much more costly.
Alloy elements affect the oxidation resistance of steel [7,8].By forming protective oxidation layers (Cr 2 O 3 , FeCr 2 O 4 , SiO 2 , Fe 2 SiO 4 , etc.), Cr [9][10][11] and Si [12] significantly improve the oxidation resistance of steel.Jae-Young Park et al. [13] found that the oxidation of Fe-Si-Cr alloys at 650 • C produced a double-layer structure oxide with Cr 2 O 3 in the outer layer and SiO 2 in the inner layer.Tianyuan Wang et al. [14] found that Si makes the oxide layer flat and reduces spalling, which improves the antioxidant performance at 800 • C more than five times.Wen Zeng et al. [15] studied the antioxidant properties of steel with different Mn contents and found that the composite oxide MnCr 2 O 4 enhanced antioxidant properties.However, a high Mn content leads to a decreased critical concentration of Cr to form Cr 2 O 3 , which deteriorates antioxidant properties.As for hot stamping steel, adding Cr and Si elements in hot stamping steel widens the process window and promotes the formation of dense SiO 2 , Cr 2 O 3 , and Fe 2 SiO 4 , resulting in an improvement in oxidation resistance [16][17][18][19][20]. Thus, previous researchers generally developed coating-free hot stamping steels by increasing Cr or Si.In addition to excellent mechanical properties and oxidation resistance, coating-free Cr-Si hot stamping steel also produces good surface quality, cost efficiency, and reliability.Li et al. [21] suggested that adding Cr to medium-manganese steel produces a great improvement in surface quality due to the formation of Cr 2 O 3 after hot stamping.Mn and Cr enhance the hardenability of the steel, which effectively reduces the austenitizing temperature of the hot stamping process and further resists surface oxidation.Chai et al. [22] prominently improved oxidation resistance by forming Cr-rich and Si-rich oxides on the surface of coating-free Cr-Si hot stamping steel.
Previous research has mostly focused on the oxidation behavior of hot-stamping steel during the hot-stamping process (a temperature range of 800-950 • C and a heating time range of 0-10 min).Oxidation behavior at high temperatures should also be considered for coating-free hot stamping steel, as heating, hot rolling, and coiling prolong the oxidation time and increase oxidation temperatures as well.Especially for steel with increasing Cr and Si content, as Si increases, the formation of Fe 2 SiO 4 [23][24][25][26][27] occurs between the oxide scale and the substrate during the hot rolling process.Fe 2 SiO 4 increases the adhesion of the oxide scale, which is difficult for hot rolling and descaling, resulting in the severe deterioration of surface quality in the hot rolled plate and even surface defects.
In this paper, the oxidation behavior of coating-free Cr-Si hot stamping steel and traditional hot stamping steel were investigated for 120 min at 800-1200 • C. The oxidation behavior and mechanism of Cr and Si on coating-free Cr-Si hot stamping steel during hot rolling were also investigated.

Materials and Methods
The nominal chemical composition of three hot stamping steels used in this work is provided in Table 1.22MnB5 is the conventional hot stamping steel.1# and 2# are two types of coating-free Cr-Si hot stamping steel containing different Cr and Si elements.The hot-rolled sheets were processed into 10 mm × 10 mm × 4 mm specimens by wire cutting.The observed surfaces were ground with W7 abrasive paper to ensure that no oxide remained.Specimens were cleaned with anhydrous ethanol and dried thoroughly before oxidation tests.Throughout the experiment, each specimen was placed in a dried Al 2 O 3 crucible for oxidation and measurement to avoid the test error caused by oxide shedding.A resistance furnace with an air oxidation atmosphere was used for the high-temperature oxidation experiment.There were enough small holes on the back of the box furnace to ensure sufficient air in the furnace.The samples were swiftly transferred into the resistance furnace upon reaching the designated temperatures, ensuring a quick isothermal temperature.The heat-treated samples were removed from the furnace at specific intervals and then air-cooled.The oxidation temperature was chosen between 800 and 1200 • C, and the oxidation time was selected between 0 and 120 min.The exact weight of the samples was measured using an electronic balance with an accuracy of 0.0001 g.The ULTRA 55 ZEISS field emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Baden-Württemberg, Germany) was used to observe the surface and cross-section of the samples.The elemental distribution was characterized using INCA X-MAX 50 X-ray Energy Dispersive Spectroscopy (EDS, Oxford Instruments, Oxford, UK).

Effect of Cr and Si on Oxidation Kinetics
Figure 1 shows the thermogravimetric curves at 800-1200 • C for the hot stamping steel.It can be seen that the oxidation weight gain of the three steels increased continuously as the oxidation time increased.Among them, the changing trends of oxidation weight gain in 1# and 2# were similar, and both were different from that of 22MnB5.At 1000-1200 • C, the oxidation rates of all three steels changed gently with the increasing oxidation time.At 800-900 • C, the oxidation rates of 1# and 2# were slower than that of 22MnB5 with increasing oxidation time.
Metals 2023, 13, x FOR PEER REVIEW 3 of 13 field emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Baden-Württemberg, Germany) was used to observe the surface and cross-section of the samples.The elemental distribution was characterized using INCA X-MAX 50 X-ray Energy Dispersive Spectroscopy (EDS, Oxford Instruments, Oxford, UK).

Effect of Cr and Si on Oxidation Kinetics
Figure 1 shows the thermogravimetric curves at 800-1200 °C for the hot stamping steel.It can be seen that the oxidation weight gain of the three steels increased continuously as the oxidation time increased.Among them, the changing trends of oxidation weight gain in 1# and 2# were similar, and both were different from that of 22MnB5.At 1000-1200 °C, the oxidation rates of all three steels changed gently with the increasing oxidation time.At 800-900 °C, the oxidation rates of 1# and 2# were slower than that of 22MnB5 with increasing oxidation time.The law of iron oxide scale growth is expressed by Equation (1): In the equation, Kp is the oxidation rate constant; ∆W is the weight gain mass per unit area (measured in mg/cm 2 ); and t is the oxidation time (measured in minutes).
Kp can be expressed using the Arrhenius formula: where Q is the oxidation activation energy, J/mol; t is the oxidation temperature, K; R is the gas constant, 8.314 J/(mol•K); and A is the model constant.
After logarithmic conversion on both sides of Equation (2), Based on Equation (3), the fitting plots are drawn in Figure 2. The oxidation activation energies of 1#, 2#, and 22MnB5 were 141.89 KJ/mol, 126.85 KJ/mol, and 101.93 KJ/mol, respectively.This result indicates that the oxidation activation energy increases in hot stamping steel due to the addition of Cr and Si.Further, the oxidation activation energy significantly increased with Si compared to Cr.The law of iron oxide scale growth is expressed by Equation (1): In the equation, K p is the oxidation rate constant; ∆W is the weight gain mass per unit area (measured in mg/cm 2 ); and t is the oxidation time (measured in minutes).
K p can be expressed using the Arrhenius formula: where Q is the oxidation activation energy, J/mol; t is the oxidation temperature, K; R is the gas constant, 8.314 J/(mol•K); and A is the model constant.
After logarithmic conversion on both sides of Equation ( 2), Based on Equation (3), the fitting plots are drawn in Figure 2. The oxidation activation energies of 1#, 2#, and 22MnB5 were 141.89 KJ/mol, 126.85 KJ/mol, and 101.93 KJ/mol, respectively.This result indicates that the oxidation activation energy increases in hot stamping steel due to the addition of Cr and Si.Further, the oxidation activation energy significantly increased with Si compared to Cr.Short-term (<7.5 min) oxidation resistance was significantly improved in Cr-Si hot stamping steel.From the thermogravimetric curves (within 7.5 min), the oxidation weight of 1# and 2# was substantially less than that of 22MnB5, which could be related to the selective oxidation of elements.The oxidation weight gain of hot stamping steel at 1000 °C and ≥7.5 min increased in the order of 1#, 2#, and 22MnB5, which is the same order as the oxidation activation energy.At 800 °C and 900 °C, both 1# and 2# exhibited a similar oxidation weight, which was significantly lower than that of 22MnB5.The weight gain was different between 1# and 2# at low excitation temperatures due to the slight weight change and the limited testing accuracy.For the long-term (>15 min) oxidation behavior, the oxidation resistance of Cr-Si hot stamping steel (1# and 2#) exclusively improved when oxidated below 1000 °C.The oxidation resistance of Cr-Si hot stamping steels is not as good as that of 22MnB5 when oxidated above 1000 °C.
It is worth noting that some Cr-Si hot stamping steel data deviated significantly from the fit line in Figure 2. Thus, ln Kp,t was fitted in segments at 800-1000 °C and 1000-1200 °C, respectively.However, the slope of the piecewise fitting line and origin data trend of 22MnB5 was slightly different from that of 1# and 2#.This trend suggests a different oxidation mechanism for coating-free Cr-Si hot stamping steel at 800-1000 °C and 1000-1200 °C, which needs further analysis and discussion.

Effect of Cr and Si on Oxide Surface Morphology
Figures 3-5 display the macroscopic surface morphology of three types of hot stamping steel after being oxidized at different temperatures.At 1200 °C, the surfaces of the oxide scale in the three types of steel were smooth and shiny.At 1100 °C, the oxide scale of 22MnB5 showed apparent local blisters 3-5 mm in size when the oxidation time increased, while 1# and 2# only showed a few wrinkles.At 1000 °C, the oxide surfaces of 1# and 2# for 7.5 min and 15 min were flat and started to break after 30 min oxidization; the oxide layer of 22MnB5 remained undamaged.When oxidized at 900 °C for 7.5 min and 15 min, the oxide layers peeled off during cooling for 1#and 2#, which exposed a yellow surface and some speckled oxides.When the oxidation time exceeded 30 min, flat oxide layers were observed in 1# and 2# and did not peel off during cooling, whereas the corresponding oxide layers of 22MnB5 were delaminated.For 7.5-15 min of oxidization at 800 °C, the surfaces of 1# and 2# exhibited minor yellowish-brown spots, and the surface of 22MnB5 was evenly covered by a reddish-brown oxide layer.As the oxidation time increased, the spotted oxide layer on 1# and 2# expanded to the entire surface, and the color also changed into reddish-brown, while 22MnB5 showed slight delamination.After observing this Short-term (<7.5 min) oxidation resistance was significantly improved in Cr-Si hot stamping steel.From the thermogravimetric curves (within 7.5 min), the oxidation weight of 1# and 2# was substantially less than that of 22MnB5, which could be related to the selective oxidation of elements.The oxidation weight gain of hot stamping steel at 1000 • C and ≥7.5 min increased in the order of 1#, 2#, and 22MnB5, which is the same order as the oxidation activation energy.At 800 • C and 900 • C, both 1# and 2# exhibited a similar oxidation weight, which was significantly lower than that of 22MnB5.The weight gain was different between 1# and 2# at low excitation temperatures due to the slight weight change and the limited testing accuracy.For the long-term (>15 min) oxidation behavior, the oxidation resistance of Cr-Si hot stamping steel (1# and 2#) exclusively improved when oxidated below 1000 • C. The oxidation resistance of Cr-Si hot stamping steels is not as good as that of 22MnB5 when oxidated above 1000 • C.
It is worth noting that some Cr-Si hot stamping steel data deviated significantly from the fit line in Figure 2. Thus, ln K p,t was fitted in segments at 800-1000 • C and 1000-1200 • C, respectively.However, the slope of the piecewise fitting line and origin data trend of 22MnB5 was slightly different from that of 1# and 2#.This trend suggests a different oxidation mechanism for coating-free Cr-Si hot stamping steel at 800-1000 • C and 1000-1200 • C, which needs further analysis and discussion.

Effect of Cr and Si on Oxide Surface Morphology
Figures 3-5 display the macroscopic surface morphology of three types of hot stamping steel after being oxidized at different temperatures.At 1200 • C, the surfaces of the oxide scale in the three types of steel were smooth and shiny.At 1100 • C, the oxide scale of 22MnB5 showed apparent local blisters 3-5 mm in size when the oxidation time increased, while 1# and 2# only showed a few wrinkles.At 1000 • C, the oxide surfaces of 1# and 2# for 7.5 min and 15 min were flat and started to break after 30 min oxidization; the oxide layer of 22MnB5 remained undamaged.When oxidized at 900 • C for 7.5 min and 15 min, the oxide layers peeled off during cooling for 1#and 2#, which exposed a yellow surface and some speckled oxides.When the oxidation time exceeded 30 min, flat oxide layers were observed in 1# and 2# and did not peel off during cooling, whereas the corresponding oxide layers of 22MnB5 were delaminated.For 7.5-15 min of oxidization at 800 • C, the surfaces of 1# and 2# exhibited minor yellowish-brown spots, and the surface of 22MnB5 was evenly covered by a reddish-brown oxide layer.As the oxidation time increased, the spotted oxide layer on 1# and 2# expanded to the entire surface, and the color also changed into reddish-brown, while 22MnB5 showed slight delamination.After observing this macro morphology, it is clear that the surface flatness and consistency of coating-free Cr-Si hot stamping steel is superior to that of 22MnB5 under the same oxidation conditions.Additionally, blistering and local oxide shedding are significantly improved in coating-free Cr-Si hot stamping steel.This suggests that the addition of Cr and Si to hot stamping steel results in the stronger bonding of the oxide layer to the substrate.
Metals 2023, 13, x FOR PEER REVIEW 5 of 13 macro morphology, it is clear that the surface flatness and consistency of coating-free Cr-Si hot stamping steel is superior to that of 22MnB5 under the same oxidation conditions.Additionally, blistering and local oxide shedding are significantly improved in coatingfree Cr-Si hot stamping steel.This suggests that the addition of Cr and Si to hot stamping steel results in the stronger bonding of the oxide layer to the substrate.

Effect of Cr and Si on Oxide Structure
Figure 7 displays the cross-sectional SEM of the oxide layer for the three types of steel after oxidation for 30 min.22MnB5 has the typical oxide layer structure of steel, with only the Fe-O layer.As the oxidation temperature increases, an Mn-rich layer (with a small amount of Si) appears at the substrate interface.At temperatures of 1000-1200 • C, steel undergoes rapid CO 2 production due to excess solid solution gas [28], and the oxidation of C leads to a large gap between the enriched oxide layer of Mn and Fe-O.This phenomenon is related to the fact that the oxide layer of 22MnB5 is prone to blistering and shedding at high temperatures.When oxidized at 1100 • C, the oxide layer of 22MnB5 is separated from the substrate, and a new layer of the oxide structure is formed between the existing oxide layer and the substrate at a sufficiently high-temperature residence time.This double-layer oxide structure is related to blistering time during the oxidation process and, therefore, cannot be observed consistently over the entire oxidized surface.

Effect of Cr and Si on Oxide Structure
Figure 7 displays the cross-sectional SEM of the oxide layer for the three types of steel after oxidation for 30 min.22MnB5 has the typical oxide layer structure of steel, with only the Fe-O layer.As the oxidation temperature increases, an Mn-rich layer (with a small amount of Si) appears at the substrate interface.At temperatures of 1000-1200 °C, steel undergoes rapid CO2 production due to excess solid solution gas [28], and the oxidation of C leads to a large gap between the enriched oxide layer of Mn and Fe-O.This phenomenon is related to the fact that the oxide layer of 22MnB5 is prone to blistering and shedding at high temperatures.When oxidized at 1100 °C, the oxide layer of 22MnB5 is separated from the substrate, and a new layer of the oxide structure is formed between the existing oxide layer and the substrate at a sufficiently high-temperature residence time.This double-layer oxide structure is related to blistering time during the oxidation process and, therefore, cannot be observed consistently over the entire oxidized surface.Figure 8 shows the element distribution in the 1# steel oxide layer (oxidized at 1200 • C for 30 min).In addition, there is a Si-rich oxide layer between the Fe-O layer and the loose Fe-Cr-Si-O layer.Within the Si-rich oxide layer, some Fe 2 SiO 4 c is distributed at the grain boundaries as a network, and other Fe 2 SiO 4 forms in the grains, which is consistent with Yuan Qing's research [24].Compared with the oxide layer structure formed at ≤1000 • C, the molten Fe-Cr-Si-O layer in 1# is not utilized as a protective layer to retard the oxidation rate.Therefore, at 1100-1200 • C, coating-free Cr-Si hot stamping steel has a greater oxidation weight gain than 22MnB5, and thus a thicker oxide layer is obtained.
O layer has better deformability because gas generates a loose porous structure rather than a laminar gap, as proved by Figure 7.
Figure 8 shows the element distribution in the 1# steel oxide layer (oxidized at 1200 °C for 30 min).In addition, there is a Si-rich oxide layer between the Fe-O layer and the loose Fe-Cr-Si-O layer.Within the Si-rich oxide layer, some Fe2SiO4 c is distributed at the grain boundaries as a network, and other Fe2SiO4 forms in the grains, which is consistent with Yuan Qing's research [24].Compared with the oxide layer structure formed at ≤1000 °C, the molten Fe-Cr-Si-O layer in 1# is not utilized as a protective layer to retard the oxidation rate.Therefore, at 1100-1200 °C, coating-free Cr-Si hot stamping steel has a greater oxidation weight gain than 22MnB5, and thus a thicker oxide layer is obtained.Figure 9 shows the elemental distribution in the oxide cross-section for three types of hot stamping steel oxidized at 1200 °C for 7.5 min.In situ observations indicate that as the temperature rises to 1173 °C, Fe2SiO4 transforms into the liquid phase [29,30].Since Fe2SiO4 reaches its melting point at the oxidation temperature, liquid Fe2SiO4 has a tendency to penetrate the Fe-O layer along the grain boundaries.Figure 9 shows the elemental distribution in the oxide cross-section for three types of hot stamping steel oxidized at 1200 • C for 7.5 min.In situ observations indicate that as the temperature rises to 1173 • C, Fe 2 SiO 4 transforms into the liquid phase [29,30].Since Fe 2 SiO 4 reaches its melting point at the oxidation temperature, liquid Fe 2 SiO 4 has a tendency to penetrate the Fe-O layer along the grain boundaries.

Effect of Cr and Si on the Oxidation Mechanism of Hot Stamping Steel
Since the oxidation weight gain of hot stamping steel after 7.5 min always follows the order of 22MnB5 2# ≈ 1# in the thermogravimetric curves, the short-term (≤7.5 min) oxidation resistance of the coating-free Cr-Si hot stamping steel is better than that of 22MnB5.Due to the selective oxidation order being Si > Cr > Fe, as shown in Figure 10, the oxide clusters of Cr and Si are preferentially formed on the substrate during oxidation in 1# and 2#.Coating-free Cr-Si hot stamping steel quickly generates a Cr-Si-rich Fe-Cr-Si-O layer, which inhibits the diffusion of Fe to the surface and slows down the reaction between Fe and O 2 .Fe ions pass through the Cr-Si oxide layer from the substrate and react with O 2 in the air, resulting in a thinner Fe-O layer.Although the oxides of Cr and Si are enriched at the interface (as shown in Figure 9), the low content of Cr and Si in 22MnB5 is unable to form a coherent barrier for the oxidation rate.

Effect of Cr and Si on the Oxidation Mechanism of Hot Stamping Steel
Since the oxidation weight gain of hot stamping steel after 7.5 min always follows the order of 22MnB5 ≫ 2# ≈ 1# in the thermogravimetric curves, the short-term (≤7.5 min) oxidation resistance of the coating-free Cr-Si hot stamping steel is better than that of 22MnB5.Due to the selective oxidation order being Si > Cr > Fe, as shown in Figure 10, the oxide clusters of Cr and Si are preferentially formed on the substrate during oxidation in 1# and 2#.Coating-free Cr-Si hot stamping steel quickly generates a Cr-Si-rich Fe-Cr-Si-O layer, which inhibits the diffusion of Fe to the surface and slows down the reaction between Fe and O2.Fe ions pass through the Cr-Si oxide layer from the substrate and react with O2 in the air, resulting in a thinner Fe-O layer.Although the oxides of Cr and Si are enriched at the interface (as shown in Figure 9), the low content of Cr and Si in 22MnB5 is unable to form a coherent barrier for the oxidation rate.

Effect of Cr and Si on the Oxidation Mechanism of Hot Stamping Steel
Since the oxidation weight gain of hot stamping steel after 7.5 min always follows the order of 22MnB5 ≫ 2# ≈ 1# in the thermogravimetric curves, the short-term (≤7.5 min) oxidation resistance of the coating-free Cr-Si hot stamping steel is better than that of 22MnB5.Due to the selective oxidation order being Si > Cr > Fe, as shown in Figure 10, the oxide clusters of Cr and Si are preferentially formed on the substrate during oxidation in 1# and 2#.Coating-free Cr-Si hot stamping steel quickly generates a Cr-Si-rich Fe-Cr-Si-O layer, which inhibits the diffusion of Fe to the surface and slows down the reaction between Fe and O2.Fe ions pass through the Cr-Si oxide layer from the substrate and react with O2 in the air, resulting in a thinner Fe-O layer.Although the oxides of Cr and Si are enriched at the interface (as shown in Figure 9), the low content of Cr and Si in 22MnB5 is unable to form a coherent barrier for the oxidation rate.The oxidation resistance of the three types of hot stamping steel was generally constant when oxidized at ≤1000 • C and ≥15 min.However, at ≥1100 • C and above, the coatingfree Cr-Si hot stamping steel exhibited poorer long-term oxidation resistance than 22MnB5.The thermogravimetric curve shows that the oxidation weight gain of the coating-free Cr-Si hot stamping steel for more than 15 min was about 1.5-2 times that of 22MnB5 when oxidized at 1200 • C.This phenomenon depends on the state and structural evolution of the Fe-Cr-Si-O layer under different oxidation conditions (as shown in Figure 11).
strate and the oxide layer.
At 1200 °C, the Fe2SiO4 liquid penetrated along the grain boundary in the oxide layer of the coating-free Cr-Si hot stamping steel, showing a network structure and intragranular precipitation (in Figure 9).The study by Yuan [24,25,31,32] considered that liquid Fe2SiO4 could not inhibit the diffusion of Fe to the oxide layer at 1200 °C.Based on the above factors, the oxidation rates of coating-free Cr-Si hot stamping steel (1# and 2#) are greatly enhanced and much higher than those of 22MnB5.At 1100-1200 • C, the Fe-Cr-Si-O layer no longer inhibited the diffusion of Fe ions as the Fe-Cr-Si-O layer and the Fe 2 SiO 4 melted at 1100-1200 • C. Fe ions are more prone to diffuse in the molten Fe-Cr-Si-O layer than in the solid Fe-Cr-Si-O or Mn-rich layer.On the one hand, gaps between the oxide layer and substrate of 22MnB5 hinder the diffusion of Fe ions.On the other hand, the molten Fe-Cr-Si-O layer of coating-free Cr-Si hot stamping steel has better adherence, which facilitates the diffusion of Fe ions between the substrate and the oxide layer.
At 1200 • C, the Fe 2 SiO 4 liquid penetrated along the grain boundary in the oxide layer of the coating-free Cr-Si hot stamping steel, showing a network structure and intragranular precipitation (in Figure 9).The study by Yuan [24,25,31,32] considered that liquid Fe 2 SiO 4 could not inhibit the diffusion of Fe to the oxide layer at 1200 • C. Based on the above factors, the oxidation rates of coating-free Cr-Si hot stamping steel (1# and 2#) are greatly enhanced and much higher than those of 22MnB5.
When oxidized at 1100-1200 • C, the high oxidation rate obstructs the gas generation between the substrate and the oxide layer, which leaves holes or gaps at the interface.However, due to the different fluidity between the interface of Fe-Mn oxides and Fe-Cr-Si oxides, delamination or blistering can appear in coating-free Cr-Si hot stamping steel, resulting in a porous structure.The higher Si content in 1# leads to a higher percentage of Fe 2 SiO 4 in the Fe-Cr-Si-O layer and, thus, a bigger hole while the network penetrates intensively into the Fe-oxide layer.Liquid Fe 2 SiO 4 promotes the diffusion of Fe into the Fe-Cr-Si-O layer and Fe oxide grain boundaries.Therefore, at 1200 • C, the long-term oxidation resistance of 1# with a higher Si content is inferior to that of 2#.

Conclusions
This paper conducted high-temperature oxidation experiments on traditional hot stamping steel and two types of new coating-free Cr-Si hot stamping steels with different Cr and Si contents.The thermogravimetric results and the oxide scale microstructure were analyzed, and the mechanism of oxidation resistance at different conditions was discussed.The conclusions are as follows: 1.
According to the thermogravimetric curves, new coating-free Cr-Si hot stamping steel has a higher oxidation activation energy, indicating that Cr and Si elements improve the oxidation resistance of hot stamping steel in a wide temperature range.However, the thermogravimetric fitting results show that the oxidation mechanisms of Cr-Si hot stamping steel are different at 800-1000 • C and 1000-1200 • C. The oxidation resistance of Cr-Si hot stamping steel at 800-1000 • C is two times better than 22MnB5 but worse than 22MnB5 at 1000-1200 • C.

2.
The Fe-Cr-Si-O layer formed between the Fe-O layer and the substrate altered the iron oxide structure of conventional hot stamping steel.It is worth noticing that the oxidization temperature significantly affected the structure of the Fe-Cr-Si-O layer in coating-free Cr-Si hot stamping steel.At 800-1000 • C, coating-free Cr-Si hot stamping steel generated a Cr-Si-rich solid Fe-Cr-Si-O layer, while the Fe-Cr-Si-O layer melted and was left with holes or gaps when oxidized over 1100 • C.

3.
Adding Cr and Si elements imparts excellent short-term (≤7.5 min) oxidation resistance to hot stamping steel.Due to the selective oxidation of Si and Cr elements and the rapid formation of the Fe-Cr-Si-O layer, the diffusion of Fe from the substrate to the Fe oxide layer is inhibited, thereby improving the oxidation resistance of hot stamping steel.4.
The long-term (≥15 min) oxidation resistance of coating-free Cr-Si hot stamping steels is inferior to that of 22MnB5 when oxidized at ≥1100 • C. The higher the Si content, the worse the oxidation resistance.The Fe-Cr-Si-O layer melts at the interface between the oxide layer and the substrate, and liquid Fe 2 SiO 4 penetrates the Fe oxide layer, which promotes the diffusion of Fe.Correspondingly, the oxidation efficiency of 22MnB5 is hindered by gaps between the oxide layer and substrate due to local blistering.

Figure 2 .
Figure 2. Hot stamping steel oxidation rate constant and fit curve.

Figure 2 .
Figure 2. Hot stamping steel oxidation rate constant and fit curve.

Figure 4 .
Figure 4. Macroscopic surface morphology after high-temperature oxidation of 2#.Figure 4. Macroscopic surface morphology after high-temperature oxidation of 2#.The microscopic observation and EDS results of spot-like oxides (formed at 900 • C) in 1# and 2# are shown in Figure 6.These local oxide clusters appeared at lower temperatures and shorter oxidation times.The selective preferential oxidation of Cr and Si elements [27] stimulated the formation of SiO 2 and Cr 2 O 3 , as indicated by the local enrichment areas of Cr and Si and the corresponding cluster positions.

Figure 5 .
Figure 5. Macroscopic surface morphology after high-temperature oxidation of 22MnB5.The microscopic observation and EDS results of spot-like oxides (formed at 900 °C) in 1# and 2# are shown in Figure6.These local oxide clusters appeared at lower temperatures and shorter oxidation times.The selective preferential oxidation of Cr and Si elements[27] stimulated the formation of SiO2 and Cr2O3, as indicated by the local enrichment areas of Cr and Si and the corresponding cluster positions.

Figure 5 .
Figure 5. Macroscopic surface morphology after high-temperature oxidation of 22MnB5.The microscopic observation and EDS results of spot-like oxides (formed at 900 °C) in 1# and 2# are shown in Figure6.These local oxide clusters appeared at lower temperatures and shorter oxidation times.The selective preferential oxidation of Cr and Si elements[27] stimulated the formation of SiO2 and Cr2O3, as indicated by the local enrichment areas of Cr and Si and the corresponding cluster positions.

Figure 6 .
Figure 6.Microscopic morphology and elements shown by X-ray EDS maps of the speckled oxide on 1# and 2#.

Figure 6 .
Figure 6.Microscopic morphology and elements shown by X-ray EDS maps of the speckled oxide on 1# and 2#.

Figure 7 .
Figure 7.The sectional micromorphology of the oxide layer in hot stamping steels after 30 min hightemperature oxidation.At the same temperature, the oxide layers of 1# and 2# are similar in morphology, consisting of the Fe-Cr-Si-O layer (adjacent to the substrate) and the Fe-O layer (near the surface).For the structure of Fe-O layers, there are no significant differences between 1#, 2#, and 22MnB5.However, the morphology of Fe-Cr-Si-O layers clearly changes when the temperature increases.The Fe-Cr-Si-O layers are flat and dense under ≤1000 °C of oxidization.When the temperature rises to 1100 °C, a number of holes appear in the Fe-Cr-Si-O layers.At 1200 °C, the holes in the Fe-Cr-Si-O layer begin to merge and expand.When oxidized between 1100 °C and 1200 °C, Fe2SiO4 with a lower melting point leads to the melting of the Fe-Cr-Si-O layer.Compared to the Mn-rich layer in 22MnB5, the Fe-Cr-Si-

Figure 7 .
Figure 7.The sectional micromorphology of the oxide layer in hot stamping steels after 30 min high-temperature oxidation.At the same temperature, the oxide layers of 1# and 2# are similar in morphology, consisting of the Fe-Cr-Si-O layer (adjacent to the substrate) and the Fe-O layer (near the surface).For the structure of Fe-O layers, there are no significant differences between 1#, 2#, and 22MnB5.However, the morphology of Fe-Cr-Si-O layers clearly changes when the temperature increases.The Fe-Cr-Si-O layers are flat and dense under ≤1000 • C of oxidization.When the temperature rises to 1100 • C, a number of holes appear in the Fe-Cr-Si-O layers.At 1200 • C, the holes in the Fe-Cr-Si-O layer begin to merge and expand.When oxidized between 1100 • C and 1200 • C, Fe 2 SiO 4 with a lower melting point leads to the melting of the Fe-Cr-Si-O layer.Compared to the Mn-rich layer in 22MnB5, the Fe-Cr-Si-O layer has better deformability because gas generates a loose porous structure rather than a laminar gap, as proved by Figure7.Figure8shows the element distribution in the 1# steel oxide layer (oxidized at 1200 • C for 30 min).In addition, there is a Si-rich oxide layer between the Fe-O layer and the loose Fe-Cr-Si-O layer.Within the Si-rich oxide layer, some Fe 2 SiO 4 c is distributed at the grain boundaries as a network, and other Fe 2 SiO 4 forms in the grains, which is consistent with Yuan Qing's research[24].Compared with the oxide layer structure formed at ≤1000 • C, the molten Fe-Cr-Si-O layer in 1# is not utilized as a protective layer to retard the oxidation rate.

Metals 2023 , 13 Figure 9 .
Figure 9.The oxide layer of three types of hot stamping steel oxidized at 1200 °C for 7.5 min.

Figure 10 .
Figure 10.Mechanistic diagram of short-term oxidation of hot stamping steel.

Figure 9 .
Figure 9.The oxide layer of three types of hot stamping steel oxidized at 1200 • C for 7.5 min.

Figure 9 .
Figure 9.The oxide layer of three types of hot stamping steel oxidized at 1200 °C for 7.5 min.

Figure 10 .
Figure 10.Mechanistic diagram of short-term oxidation of hot stamping steel.Figure 10.Mechanistic diagram of short-term oxidation of hot stamping steel.

Figure 10 .
Figure 10.Mechanistic diagram of short-term oxidation of hot stamping steel.Figure 10.Mechanistic diagram of short-term oxidation of hot stamping steel.

Figure 11 .
Figure 11.Mechanism of the Fe-Cr-Si-O layer affecting oxidation at different temperatures.Figure 11.Mechanism of the Fe-Cr-Si-O layer affecting oxidation at different temperatures.

Figure 11 .
Figure 11.Mechanism of the Fe-Cr-Si-O layer affecting oxidation at different temperatures.Figure 11.Mechanism of the Fe-Cr-Si-O layer affecting oxidation at different temperatures.