Hot Deformation Behavior and Processing Maps of Nitrogen-Containing 2Cr13 Corrosion-Resistant Plastic Die Steel

Abstract

To investigate the hot deformation behavior of nitrogen-containing 2Cr13 (2Cr13N) corrosion-resistant plastic mold steel, uniaxial compression tests were conducted at temperatures ranging from 850 to 1200 °C and strain rates between 0.01 and 10 s⁻¹. The results indicate that the flow stress exhibits pronounced peak characteristics under conditions of low strain rate and high temperature, with peak stress decreasing as deformation temperature increases and strain rate decreases. Using the Arrhenius model, a hot deformation equation was established, and activation energy for deformation was 454.85 kJ/mol. The processing diagram was constructed based on the dynamic material model (DMM) theory. The optimal hot working window was at 1050–1150 °C with a strain rate less than 0.05 s⁻¹ and at 1150–1200 °C with a strain rate greater than 2 s⁻¹, with excellent efficiency of power dissipation (η > 0.32) and lower values of Kernel Average misorientation (KAM) (1.2386 and 1.3095, respectively).

1. Introduction

2Cr13 is an Fe-Cr-C-based ternary martensitic stainless steel, with a chromium content of 12–14 wt% and a carbon content of 0.16–0.25 wt% [1], which is widely utilized in the manufacturing of industrial components, including turbine blades, bearings, corrosion-resistant plastic molds, knives and scissors intended for food contact, and medical equipment, etc. [2,3,4]. However, the precipitation of chromium carbides after tempering heat treatment had an adverse effect on the corrosion performance of martensite in 2Cr13 [5,6]. Meanwhile, Fe-Cr-C martensitic stainless steels have the problem of poor matching between strength, plasticity, and toughness [7]. During hot processing, issues like coarse grains and cracking are likely to occur [8,9]. To address these problems, scholars from various countries have carried out research work in many aspects, such as process optimization and composition improvement. Prieto et al. [10] examined the effects of cryogenic treatment on the microstructure and mechanical properties of AISI 420 martensitic stainless steel. Following cryogenic treatment at −196 °C, the material exhibited a hardness exceeding 580 HV and an impact absorption energy greater than 34 J. In comparison to the conventional quenching and tempering (Q-T) process, this treatment resulted in an approximate 5% increase in hardness and a 10% improvement in toughness. Mola et al. [11,12] discovered that the “quenching-distribution (Q&P)” heat treatment process enables Cr13 plastic die steel to retain a certain amount of residual austenite while inhibiting the precipitation of M₂₃C₆ carbides, thereby enhancing its pitting corrosion resistance.

In the realm of compositional enhancement, Groeditz, a German company, has developed the 1.2083 and 1.2316 series steels by modifying the content of alloying elements based on 2Cr13 steel. These steels exhibit superior wear resistance, corrosion resistance, and deformation resistance. Additionally, Liu employed Ce-microalloyed 2Cr13 steel, achieving a 54.55% increase in the transverse impact value at −40 °C compared to non-Ce-treated steel [13].

In recent years, the role of the nitrogen element in enhancing the microstructure and properties of die steel has increasingly gained attention [14,15,16]. The incorporation of nitrogen increases the quantity of undissolved M(C, N)-type carbonitrides during quenching, thereby inhibiting grain boundary migration, refining austenite grains, and achieving grain refinement strengthening. Additionally, it enhances the solid solubility of nitrogen in the matrix and modifies the composition and distribution of precipitated phases through interactions between nitrogen atoms and carbide-forming elements, leading to solid solution strengthening and precipitation strengthening. These effects significantly improve the strength, toughness, and thermal fatigue resistance of die steel, thereby extending its service life [17,18]. Currently, research on nitrogen-containing die steel primarily focuses on the precise control of nitrogen content, the evolution and control methods of liquid carbon (nitrogen) compounds in nitrogen-containing die steel, heat treatment processes, and corrosion resistance [16,17,18,19,20]. Fe-Cr-C martensitic stainless steel exhibits substantial resistance to high-temperature deformation but is prone to cracking during forging. Scholars worldwide have conducted relevant studies [1,8,21]; however, there are limited reports on the high-temperature deformation characteristics of 2Cr13N steel.

In this paper, the hot deformation behavior of the 2Cr13N was studied on various temperatures and strain rates. The constitutive model was built. The hot processing map was established based on the Prasad instability criterion and analyzed to obtain a better deformation parameters range.

2. Materials and Methods

The test materials were supplied by Shanxi Taigang Stainless Steel Co., Ltd. (Taiyuan, China). Initially, a 500 kg vacuum induction furnace was employed to produce a φ280 mm ingot. Subsequently, the steel ingot was heated to 1250 °C and maintained at this temperature for 1.5 h before being hot rolled into a 100 mm × 300 mm × L mm sample through seven passes using a φ550 hot rolling mill. Following the rolling process, the materials were air-cooled to room temperature and then reheated to 790 °C for an annealing treatment lasting 8 h. Specimens were extracted from the cross-section of the rolled piece, located one-quarter diagonally from the corner, and machined into hot compression samples with dimensions of ϕ 10 mm × 15 mm. The chemical composition of the materials was analyzed using an ARL 4460 Metals Analyzer (Thermo Electron Corporation, Waltham, MA, USA)and an ON836 oxygen-nitrogen analyzer(LeCo, St. Joseph, MI, USA), in compliance with the standards specified in GB/T 11170-2008 [22] and GB/T 20124-2006 [23]. The detailed compositional results are presented in

Table 1. Chemical composition of nitrogen-containing 2Cr13 die steel (wt%).

C	Si	Mn	P	S	Cr	V	N	Fe
0.18	0.59	0.65	0.009	0.005	13.37	0.10	0.085	Bal.

.

The nitrogen-containing 2Cr13 die steel used in this paper was quenched at 1030 °C and tempered at 600 °C. After this heat treatment, its typical mechanical property values are as follows: the proof strength at 0.2% plastic strain (Rp0.2) is 970 MPa, the tensile strength (Rm) is 1100 MPa, the elongation (A) is 15.5%, the reduction in area (Z) is 47%, and the room-temperature impact energy (AKV₂) is 18.4 J.

The high-temperature compression test was conducted in accordance with the ASTM E209 standard [24], and a unidirectional compression test was performed using a Gleeble 3800 thermo-mechanical simulator(Dynamic Systems Inc., Poestenkill, NY, USA). The experimental conditions included deformation temperatures of 850, 900, 950, 1000, 1050, 1100, 1150, and 1200 °C, strain rates of 0.01, 0.1, 1, 5, and 10 s⁻¹, and true strain controlled at 0.7.

The microstructure was characterized by a combination of metallographic analysis and EBSD (Electron Backscatter Diffraction) [25,26]. The metallographic characterization was conducted in accordance with the GB/T 13298-2015 standard [26], and the EBSD characterization was performed in line with the ISO 24173 standard [25].

During the test, the sample was initially heated to 1250 °C at a rate of 5 °C/s, held for 5 min to ensure uniform temperature distribution, and subsequently cooled to the corresponding deformation temperature at 20 °C/s. Compression tests were then conducted sequentially at each specified strain rate. After completing the hot compression, the samples were immediately rapidly cooled to room temperature by water quenching. Subsequently, the samples were longitudinally sectioned using an electric spark wire cutting machine (Chengdu Zhengchuan Precision Machinery & Electromechanical Equipment Co., Ltd., Pidu, Chengdu, China) and polished with 200#, 600#, 1000#, and 1500# sandpaper, followed by lint cloth polishing. The samples were then etched with an FeCl₃-HCl solution (25 mL analytically pure HCl + 25 g FeCl₃ + 100 mL H₂O) and their microstructures were examined under a DMI8C optical microscope (Leica Microsystems Inc., Weztlar, Germany). EBSD specimens were prepared using electrolytic polishing with a voltage of 20 V, a current of 1 A and an electrolyte of 5% perchloric acid alcohol solution. EBSD scans were performed on a Zeiss Supra 55 field emission scanning electron microscope (SEM, Carl Zeiss AG, Oberkochen, Germany) with accelerating voltages of 20 kV, and EBSD scan steps of 0.2 μm. Finally, the EBSD data were analyzed using the AZtecCrystal software (Version 6.1).

3. Results and Discussion

3.1. True Stress–Strain Curves

Figure 1 illustrates the high-temperature true stress–strain curves of 2Cr13N steel under various deformation conditions. The deformation temperature and rate significantly influence the stress–strain behavior. As the deformation temperature decreases and rate increases, the true stress value of the material rises. In the case of small strain, stress rapidly increases with strain, indicating a typical work-hardening phenomenon. When the strain increases, the stress value tends to stabilize or even slightly decrease due to softening mechanisms such as dynamic recovery or dynamic recrystallization (DRV/DRX) occurring during high-temperature deformation, where softening and hardening processes coexist. Under large strains, the softening behavior becomes dominant. In Figure 1a,b, when the deformation temperature ranges from 1050 to 1200 °C and the strain rates are 0.01 and 0.1 s⁻¹, the stress curve descends to a steady state after reaching its peak, which is indicative of typical DRX behavior. In contrast, the true stress–strain curves with deformation temperatures below 1000 °C in Figure 1a,b, and strain rates exceeding 1 s⁻¹ in Figure 1c–e, maintain a constant value or exhibit slight increases under large deformations, which correspond to typical DRV stress–strain curves.

3.2. Thermal Activation Energy and Constitutive Equation

To establish the constitutive equation for the high-temperature flow stress curve of 2Cr13N corrosion-resistant plastic die steel, the hyperbolic sine Arrhenius high-temperature constitutive model was employed to describe the thermally activated deformation behavior [27,28,29]:

$$\dot{\epsilon }=A{\left[\mathit{sin} \mathit{h}\left(\alpha \sigma \right)\right]}^n\exp \left(-{\displaystyle \frac{Q}{RT}}\right)$$

(1)

where A is the structure factor (s⁻¹), α is the stress parameter (MPa⁻¹), n is the stress exponent, Q is the thermal deformation activation energy (kJ/mol), R is the gas constant, R = 8.314 J/(mol·K), $\dot{\epsilon }$ is strain rate (s⁻¹), σ is the peak stress value of the metallic material (MPa), and T is the thermodynamic temperature (K). Under different stress levels, Equation (1) has the following two expressions forms:

In the case of low stress levels (when ασ < 0.8):

$$\dot{\epsilon }=A_1{\sigma }^{n_1}\exp \left(-{\displaystyle \frac{Q}{RT}}\right)$$

(2)

In the case of high stress levels (when ασ > 1.2):

$$\dot{\epsilon }=A_2\mathrm{e}\mathrm{x}\mathrm{p}\left(\beta \sigma \right)\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\displaystyle \frac{Q}{RT}}\right)$$

(3)

In Equations (2) and (3), n₁ and A₁ are stress exponents and the structural factor (s⁻¹) at low stress levels, A₂ is the structural factor (s⁻¹) at high stress levels, α = β/n₁. The logarithm of both sides of Equations (2) and (3) can be derived as follows:

$$\mathrm{l}\mathrm{n}\dot{\epsilon }=\mathrm{l}\mathrm{n}{A}_1+n_{1}ln\sigma -{\displaystyle \frac{Q}{RT}}$$

(4)

$$\mathrm{l}\mathrm{n}\dot{\epsilon }=\mathrm{l}\mathrm{n}{A}_2+\beta \sigma -{\displaystyle \frac{Q}{RT}}$$

(5)

The peak stresses under various strain rates were substituted into Equations (4) and (5). Scatter plots were then generated using $ln\sigma$ − $ln\dot{\epsilon }$ and $\sigma$ − $ln\dot{\epsilon }$ as coordinates. Linear regression analysis was performed using the least squares method, resulting in Figure 2a,b. In this experiment, the low stress level corresponds to the stress values within the temperature range of 1100~1200 °C in Figure 2a, with an average n₁ value of 7.11, with a coefficient of determination R² of 0.9965 and a standard error of 0.189. The high stress level refers to the two oblique lines at 850 °C and 900 °C in Figure 2b, and the value of β calculated is 0.053, with a coefficient of determination R² of 0.9959 and a standard error of 0.0016. Thus, it can be obtained that α = β/n₁ = 0.053/7.11 = 0.00745.

By taking the differential of the natural logarithm on both sides of Equation (1), the relationship under the entire stress level can be obtained.

$$\mathrm{Q}=R{\left\{{\displaystyle \frac{\partial ln\dot{\epsilon }}{\partial ln\left[sin h\left(\alpha \sigma \right)\right]}}\right\}}_T{\left\{{\displaystyle \frac{\partial ln\left[sin h\left[\alpha \sigma \right]\right]}{\partial \left(1/T\right)}}\right\}}_{\dot{\epsilon }}$$

(6)

The relationships between $1/T$ − $\mathrm{l}\mathrm{n}\left[sin h\left(\alpha \sigma \right)\right]$, as well as between $\mathrm{l}\mathrm{n}[sin h(\alpha \sigma \left)\right]$ − $\mathrm{l}\mathrm{n}\dot{\epsilon }$ were plotted for the steel under varying deformation conditions, as shown in Figure 2c,d. According to Figure 2c, the average slope was obtained as 8697.73, with a coefficient of determination R² of 0.996 and a standard error of 189.46, and the slope of the oblique line in Figure 2d was 6.29, the coefficient of determination R² of 0.9907 and a standard error of 0.283. Substituting these values into Equation (6), the thermal deformation activation energy of the nitrogen-containing 2Cr13 corrosion-resistant plastic die steel was calculated as Q = 454.85 kJ/mol.

In the process of high-temperature deformation, the relationship between deformation temperature T and stress σ is often analyzed by introducing the Zener–Hollomon model [30,31], and the flow stress constitutive model with high accuracy can be established through the Z parameter to effectively predict the high-temperature rheological behavior of metal materials. This model is expressed as:

$$\mathrm{Z}=\dot{\mathsf{\epsilon }}\mathrm{e}\mathrm{x}\mathrm{p}(\mathrm{Q}/\mathrm{R}\mathrm{T})$$

(7)

Substituting Equation (1) into (7) and taking the natural logarithm of both sides of the equation gives the following formula:

$$\mathrm{n}\mathrm{Z}=\mathrm{l}\mathrm{n}\dot{\mathsf{\epsilon }}+\mathrm{Q}/\mathrm{R}\mathrm{T}$$

(8)

$$\mathrm{l}\mathrm{n}\mathrm{Z}=\mathrm{l}\mathrm{n}\mathrm{A}+\mathrm{n}\mathrm{l}\mathrm{n}\left[\mathrm{s}\mathrm{i}\mathrm{n} \mathrm{h}\left(\mathsf{\alpha }\mathsf{\sigma }\right)\right]$$

(9)

The following steps were taken to determine the constants involved in these equations, the substitution of Q, R, as well as the corresponding strain rate and deformation temperature T data into Equation (8) to calculate the ln Z values. Then, hyperbolic sine calculations were performed on the stress values obtained under various deformation conditions in the experiment. A scatter plot was drawn with $ln\mathrm{Z}$ and $ln\left[sin h\left(\alpha \sigma \right)\right]$ as the coordinates and then the least squares method for linear regression was used. The results are shown in Figure 3, where the linear correlation coefficient R² is 0.98976. The slope n = 6.30, the intercept is ln A = 40.01, A =2.378 × 1017.

By substituting A, α, Q, and n into Equation (1), the constitutive equation for nitrogen-containing 2Cr13 corrosion-resistant plastic die steel during hot compression under the conditions of 850–1200 °C and 0.01–10 s⁻¹ is derived, as follows:

$$\dot{\epsilon }=2.378\times {10}^{17}{\left[sin h\left(0.00745\sigma \right)\right]}^{6.3}exp\left(-454.85\times {10}^3/RT\right)$$

(10)

Ren et al. [32] reported that the hot deformation activation energy of AISI 420 (corresponding to the Chinese grade 2Cr13) is 363.313 kJ/mol. In comparison, the deformation activation energy of nitrogen-containing 2Cr13 steel increases to 454.85 kJ/mol.

The use of Equation (10) can provide a theoretical basis for formulating the hot working process of the nitrogen-containing 2Cr13 corrosion-resistant plastic mold steel.

3.3. Hot Processing Maps

Hot processing maps are usually used to determine the safe processing ranges of materials during the high-temperature plastic deformation process, which provides a basis for formulating reasonable hot working processes. The Dynamic Materials Model (DMM) was proposed by Prasad and other scholars in 1984 [33]. Currently, the processing maps based on this model have become an effective means to study the relationships between thermal deformation parameters, microstructure, properties, and the deformation mechanism of materials. Based on this theory, the input energy P during hot processing is primarily converted into the dissipation quantity G, which causes the material temperature to rise, and the dissipation covariance J, which induces changes in the microstructure.

$$P=G+J={\int }_0^{\dot{\epsilon }}\sigma d\dot{\epsilon }+{\int }_0^{\sigma }\dot{\epsilon d\sigma }$$

(11)

The strain rate sensitivity coefficient (m) has a significant influence on the energy distribution of J and G.

$$m={\displaystyle \frac{\partial J}{\partial G}}={\displaystyle \frac{\partial \left(ln\sigma \right)}{\partial \left(ln\dot{\epsilon }\right)}}={\displaystyle \frac{\dot{\epsilon }\partial \sigma }{\sigma \partial \dot{\epsilon }}}$$

(12)

Assuming at a given strain and deformation temperature, the relationship between true stress and strain rate can be expressed through a cubic spline fit:

$$ln\sigma =a+bln\dot{\epsilon }+c{\left(ln\dot{\epsilon }\right)}^2+d{\left(ln\dot{\epsilon }\right)}^3$$

(13)

According to Equations (12) and (13), the value of strain rate sensitivity coefficient (m) can be described as:

$$m=b+2cln\dot{\epsilon }+3d{\left(ln\dot{\epsilon }\right)}^2$$

(14)

The power dissipation factor η represents the energy dissipation characteristics during the deformation process and is defined as the ratio of J in the nonlinear energy dissipation state to J_max in the ideal energy dissipation state. In the actual production process, the region η > 0.3 is usually selected for processing.

$$\eta ={\displaystyle \frac{J}{J_{max}}}={\displaystyle \frac{2m}{m+1}}$$

(15)

Based on the principle of irreversible thermodynamics of large plastic deformation, Prasad proposed an instability criterion $\mathsf{\xi }\left(\dot{\epsilon }\right)$ for hot working processes. By superposing the instability map and the power dissipation map, the hot working diagram of metal materials under high-temperature plastic deformation can be obtained [34,35].

$$\mathsf{\xi }\left(\dot{\epsilon }\right)={\displaystyle \frac{\partial ln\frac{m}{m+1}}{\partial ln\dot{\epsilon }}}+m<0$$

(16)

The stress values of the experimental steel under different deformation conditions acquired during the high-temperature compression test are shown in

Table 2. Flow stress of nitrogen-containing 2Cr13 at different strains, strain rates, and deformation temperatures, MPa.

Strain, ε	$\mathbf{Strain} \mathbf{Rates}, \dot{\mathit{\epsilon }}$/s−1	Temperature, °C
		850	900	950	1000	1050	1100	1150	1200
0.2	0.01	186.6	143	103	83.5	70.6	57.1	47.9	39
	0.1	219	186	146	124	104	84	65.7	51
	1	243.6	209	166	151	133	106	90	69
	5	281	238	202	175	146.5	125	110	92.2
	10	299	268	211	183	172	136	115	100
0.4	0.01	198.7	156	108	93.4	71	55.3	45.3	34.6
	0.1	237	204	157.7	141	113	94.6	70.6	52.4
	1	266.6	226	174	166	146	125	95	70
	5	307	257	219	189	159.8	135	120	100
	10	323.7	288	229	197	187	148	124	108
0.6	0.01	200	157	107	91.5	62.3	50	44	32.3
	0.1	238	209	159	145	110	91.4	66.5	48
	1	273	230	179	164	150	129	91	72
	5	309	260	223	192	164	139	122	103
	10	327.9	294	235	203	193	154	130	113

. The strain rate sensitivity coefficient m under different conditions was determined by fitting a $\mathrm{l}\mathrm{n}\mathsf{\sigma }-\mathrm{l}\mathrm{n}\dot{\epsilon }$ curve using a cubic spline function. Additionally, the power dissipation factor $\eta$ and instability factor $\xi$ were computed based on Equations (15) and (16).

The hot processing maps of the samples under the conditions of strain ranging from 0.2 to 0.6, temperature between 850 and 1200 °C, and strain rate varying from 0.01 to 10 s⁻¹ were plotted in the $\mathrm{l}\mathrm{n}\dot{\epsilon }-T$ plane, as shown in Figure 4. In this figure, the gray region corresponds to the deformation instability range, while the isolines represent the power dissipation factor $\eta$. It can be seen from Figure 4a–c that the shaded area gradually expands with increasing deformation, signifying an enlargement of the rheological instability range, whereas the suitable processing area diminishes. In the high-temperature and high-strain-rate deformation region (1130–1170 °C, strain rate 5–10 s⁻¹), an instability zone emerges at a deformation amount of 0.2. As the deformation amount increases, this instability zone vanishes entirely after a deformation amount of 0.4. Ren et al. [32] studied the hot deformation behavior of AISI 420 (corresponding to the Chinese grade 2Cr13) and found that a flow instability region also appears with the change in deformation amount in the high-temperature and high-strain-rate region (temperature: 1087 to 1150 °C, strain rate: 3.2 to 10 s⁻¹). This indicates that there exists a critical strain for the occurrence of flow instability at relatively high deformation temperatures. Similar phenomena has been reported by Li et al. [36]. In the low-temperature and low-strain-rate deformation region (850–950 °C, strain rate < 0.2 s⁻¹), an instability zone appears at a deformation amount of 0.2. With further increases in the deformation amount, this instability zone progressively widens.

Figure 4d shows the comparison of power dissipation factors under varying true strain conditions. The $\eta$ value increases with the increase in strain. The higher the DRX volume fraction, the easier it is to obtain small equiaxed grains [37].

Comprehensively considering the processable regions under various strain conditions, the safe processing range for nitrogen-containing 2Cr13 corrosion-resistant plastic mold steel is 1050–1150 °C with a strain rate less than 0.05 s⁻¹, and 1150–1200 °C with a strain rate greater than 2 s⁻¹.

3.4. Microstructure Evolution

The typical microstructure of nitrogen-containing 2Cr13 corrosion-resistant plastic mold steel under various deformation conditions is shown in Figure 5. The observation area for the specimen is selected at the center. Figure 5a reveals that as the deformation condition is 850 °C/0.01 s⁻¹, the original austenite grains elongate perpendicular to the compression direction, while several fine DRX grains coexist. Although Figure 1 indicates that the flow stress curve exhibits a DRV shape under these conditions, DRX still occurs. When the deformation condition is set to 1000 °C/0.01 s⁻¹, numerous fine DRX grains appear at the deformed grain boundaries, as depicted in Figure 5b. At a deformation condition of 1200 °C/0.01 s⁻¹, dynamic recrystallization is fully completed, as shown in Figure 5c. Furthermore, Figure 5d presents the deformation structure at 1200 °C/10 s⁻¹. Due to the increased deformation rate, the nucleation rate of dynamic recrystallization significantly enhances, leading to finer grains with reduced size.

EBSD analysis was performed on the deformed microstructure at room temperature. It can be seen from the Band Contrast (BC) diagram that the room-temperature structure is mainly composed of lath martensite, as shown in Figure 6a,d,g,j. When the deformation conditions are 850 °C/0.01 s⁻¹ and 1000 °C/0.01 s⁻¹, the martensite blocks are regularly arranged along a certain direction, but the packet and block structures cannot be distinguished, as shown in Figure 6b,e. When the deformation conditions are 1200 °C/0.01 s⁻¹ and 1200 °C/10 s⁻¹, the multilevel structure of martensite can be clearly observed, as shown in Figure 6h,e. In the grain boundary (GB) diagram, different orientation angles are calculated based on the ideal Kurdjumov–Sachs (KS) orientation relationship [38]. The black line represents the middle angle GB (MAGB) ranging from 15° to 45°, and the red line represents the high-angle grain boundaries (HAGBs > 45°).

In order to explore the influence of thermal deformation on the microstructure, the GB distribution under different deformation conditions was analyzed. As can be seen from Figure 7, under different deformation conditions, GB distribution contains a high proportion of low-angle GBs (LAGBs < 15°). This is because the room-temperature structure of the nitrogen-containing 2Cr13 die steel is mainly lath martensite, while the GB of the lath martensite is mainly LAGBs. Figure 7e compares the MAGBs under different deformation conditions. The results show that the proportion of MAGBs increases with the increase in strain rate and the decrease in deformation temperature. At low temperatures and low strain rates (850 °C/0.01 s⁻¹, 1000 °C/0.01 s⁻¹), the softening mechanisms are DRV and partial DRX, and incomplete dynamic recrystallization occurs. When the deformation condition is 1200 °C/0.01 s⁻¹, the higher deformation temperature is more conducive to the migration of GBs, thereby promoting the rapid growth of DRX grains. When the strain rate is increased to 10 s⁻¹, a larger strain rate accelerates the deformation process and generates more strain storage energy, which makes the nucleation rate of DRX higher, but the time for DRX grains to grow is reduced.

In order to analyze the local orientation difference angle under different deformation conditions, the KAM maps obtained using EBSD data are shown in Figure 8. According to the comparison of the average local orientation differences in Figure 8e–h, it can be seen that the KAM value decreases with the increase in deformation temperature [38]. A higher KAM value corresponds to higher local orientation differences, with higher dislocation density and stored energy. At the same deformation rate, an increase in temperature leads to a greater extent of DRX, which consumes a substantial number of dislocations. Consequently, the KAM value decreases. At the same deformation temperature, a high strain rate will significantly increase the internal energy storage of the material, resulting in a high DRX nucleation rate, but the deformation time is short due to the increased strain rate, and the dislocation cells fail to fully merge, resulting in a high KAM value. The high strain rate is conducive to grain refinement at the same temperature.

The grain orientation spread (GOS) module of the AZtecCrystal software was used to perform grain orientation extension calculations, and the results are shown in Figure 9. At the same strain rate, as the deformation temperature increased, the recrystallization fraction increased from 5.1% at 850 °C to 87.8% at 1200 °C. At 1200 °C, as the strain rate increased from 0.01 s⁻¹ to 10 s⁻¹, the recrystallization fraction decreased. This is related to the fact that the increase in strain rate leads to a reduction in deformation time, preventing recrystallized grains from fully nucleating and growing.

4. Conclusions

1. The flow stress of nitrogen-containing 2Cr13 plastic die steel decreases with increasing deformation temperature and decreasing strain rate during high-temperature hot compression deformation. When the deformation temperature exceeds 1050 °C and the strain rate is less than 0.1 s⁻¹, the flow stress curve exhibits characteristics of dynamic recrystallization.

2. The thermal deformation activation energy of nitrogen-containing 2Cr13 corrosion-resistant plastic die steel is 454.85 kJ/mol, and the constitutive equation for high-temperature deformation is $\dot{\epsilon }=2.378\times {10}^{17}{\left[sin h\left(0.00745\sigma \right)\right]}^{6.3}exp\left(-454.85\times {10}^3/RT\right)$

3. The hot processing maps are constructed based on the dynamic materials model, and the optimal processing ranges for nitrogen-containing 2Cr13 corrosion-resistant plastic die steel are determined as follows: 1050–1150 °C with a strain rate of less than 0.05 s⁻¹, and 1150–1200 °C with a strain rate greater than 2 s⁻¹.

4. According to the EBSD analysis results, nitrogen-containing 2Cr13 die steel exhibits a relatively high average KAM value under deformation conditions of low strain rate at low temperatures (850 °C/0.01 s⁻¹) and high strain rate at high temperatures (1200 °C/10 s⁻¹). The local orientation difference angle is closely related to the volume fraction of recrystallization.