High-Temperature Tensile Properties in the Curved Continuous Casting of M2 High-Speed Steel

Abstract

The industrial production of high-speed steel via continuous casting has been impeded by considerable technical obstacles, due to its high carbon content and fast cooling speed, which predispose it to severe segregation and poor high-temperature plasticity; thus, industrial continuous casting of high-speed steel is virtually nonexistent. In 2022, a curved continuous casting process was successfully applied in the production of M2 high-speed steel; in our previous study, it was found that the carbides were finer and better distributed in the billets by curved continuous casting than those in the billets by ingot casting. The change in carbides in the billets is significant in subsequent processes for M2 high-speed steel produced by curved continuous casting. Therefore, it is necessary to investigate the high-temperature tensile properties of M2 high-speed steel produced by curved continuous casting. In this paper, high-temperature tensile tests were conducted using a GLEEBLE-3500 simulator (DSI, located in New York State, USA) at different temperatures and holding times with a certain strain rate to obtain the tensile strength and reduction of area, and then the morphology of carbides near the fracture surface was observed. The results showed that the tensile strength and reduction of area increased with the increase in temperature at 850 °C to 950 °C, and there existed a temperature range between 950 °C and 1120 °C with good thermoplasticity and a reduction of area from 45% to 50%. In addition, a sharp drop in thermoplasticity below 5% occurred at 1180 °C, which is due to the significant growth of carbides. The zero-strength temperature and plastic temperature were 1220 °C and 1200 °C, respectively. In addition, with the increase in holding time at 1150 °C, the reduction of area increased from 34% to 54%, and the tensile strength decreased from 92 MPa to 70 MPa and then increased to 82 MPa. The best solution for carbides in M2 high-speed steel produced by curved continuous casting occurred when the range of the P_HJ value was about 28.0 to 30.5. With the increase in P_HJ value, the shape of carbides gradually changed from fibrous to short rod-like and blocky during high-temperature diffusion.

1. Introduction

High-speed steel is a class of high-carbon alloys which has high hardness, good wear resistance, excellent hot hardness, and a relatively good match of strength and plasticity; thus, it has attracted enough attention to become the cornerstone material of the modern metal cutting industry [1,2,3,4]. M2 (W6Mo5Cr4V2) high-speed steel is a typical representative of high-speed steel that is the most widely produced and widely used tungsten–molybdenum high-speed steel. The continuous casting process of M2 high-speed steel has not been industrialized for production due to a fast cooling rate for the as-cast billet which can easily lead to leakage, breakage, and billet cracks generated in the process [5,6,7,8,9]. In addition, the fast cooling rate of the as-cast billet gives rise to severe segregation of alloy elements, resulting in the coarsening of eutectic carbides and poor uniform distribution in the as-cast microstructure [9,10,11]. The characteristics and evolution of carbides are considered to be crucial for high-temperature tensile properties.

A few investigations into the high-temperature tensile properties of high-speed steels have been reported in recent decades [12,13,14,15,16,17,18]. Chi et al. [14] studied the high-temperature mechanical properties of M2 high-speed steel and found that the optimum plastic temperature ranges from 950 °C to 1150 °C. On the contrary, the brittle temperature exhibits two zones: one is from 800 °C to 950 °C and the other is from 1175 °C to melting point. Combined with the observations on microstructures, this indicates that phase transformation and carbide dissolution are the main factors influencing high-temperature plasticity. The study reported by Wang et al. [16] discussed the hot ductility behavior of M35 high-speed steel, which reveals that the peak stress decreases linearly with the increase in deformation temperature and decrease in strain rate. Although the fracture mechanism at all high-temperature tensile conditions is ductile fracture, the diameter and the depth of the ductile dimple increase with the increase in the deformation temperature, and it forms an intergranular fracture above 1100 °C, suggesting that the size and distribution of the carbides obviously influence the thermal plastic behavior of M35 high-speed steel. In addition, the hot deformation behavior of M2 high-speed steel containing mischmetal and produced by ingot casting was studied by Qu et al. [19], and it was found that the degree of dynamic recrystallization increases with increasing temperature, resulting in a larger shape and higher volume fraction of carbides. However, the undissolved carbides in the substrate can pin dislocations and grain boundaries, leading to poor deformation. As is well-known for high-speed steels with high carbon and alloy elements, carbides’ dissolution and transformation are associated with the activation energy for deformation which depends on the temperature, holding time, and strain rate during the high-temperature deformation process [19,20,21,22,23,24,25,26,27,28,29]. The high-speed steels used in the abovementioned studies were produced by ingot casting, and, to date, studies on high-temperature tensile and other properties have mainly focused on materials produced by ingot casting. This is due to the fact that M2 high-speed steel belongs to ledeburite steel, which presents persistent difficulties in continuous casting for industrial production.

The characterization of eutectic carbides in M2 high-speed steels is different between ingot casting and curved continuous casting, according to previous work [30]. The carbide characteristics of M2 high-speed steel billets produced by continuous casting have a significant impact on their thermoplasticity, thereby affecting subsequent hot working processes such as forging and rolling. Hence, investigating the high-temperature thermoplasticity of M2 high-speed steel produced by continuous casting has important theoretical and practical significance for accelerating the continuous casting of M2 high-speed steel for industrial production. This paper analyzes the thermoplasticity of M2 high-speed steel produced by continuous casting during a high-temperature tensile process through the Gleeble-3500 simulator, and the relationship between the high-temperature tensile properties of M2 high speed steel produced by continuous casting and eutectic carbides under different temperatures and holding times is demonstrated.

2. Materials and Experiments

M2 high-speed steel was smelted using an electric arc furnace, a ladle furnace, and a vacuum degassing furnace in turn to attain the AISI standard chemical compositions which are listed in

Table 1. Chemical compositions of the M2 high-speed steel.

C	P	S	Cr	Mo	Al	V	W	Ca	Fe
0.85	0.022	0.003	4.08	4.7	0.0111	1.92	5.65	0.0019	Bal.

. Then, a square billet with a section size of 150 mm × 150 mm was produced by a vertical bending continuous caster fabricated with an arc radius of R9 m using a withdrawal rate of 1.1 m/min. Soon afterwards, the square billet was annealed at a temperature of 880 °C with a holding time of 6 h, followed by the furnace slowly cooling to reduce the residual stress.

In order to investigate the thermoplasticity of the M2 high-speed steel produced by curved continuous casting, a GLEEBLE-3500 simulator (DSI, located in New York State, USA) was used to obtain the stress–strain curves, tensile strength, and reduction of area at different conditions. A high-temperature tensile specimen was cut at the 1/4 thickness position of the square billet and processed with a diameter of 10 mm × 120 mm; the schematic diagrams of the specimen’s position and size are shown in Figure 1a,b. In the high-temperature tensile tests, temperatures from 850 °C to 1220 °C and holding times from 10 min to 30 min were selected and the tests proceeded at a certain strain rate of 5 × 10⁻³/s; the detailed processes are shown in Figure 1c,d. Two replicate specimens were tested for each temperature and holding time. The stretched specimens were cut in half to observe the morphology of the microstructure near the fracture surface. Then, the cross-section of the stretched specimens was cold-mounted in epoxy resin and ground successively with 100#, 240#, 500#, 1000#, 1500#, and 2000# grit SiC paper. After grinding, the specimens were mechanically polished with 1 μm diamond suspension and etched with 8% nitric acid solution (10 mL HNO₃; 90 mL C₂H₅OH) to reveal the morphology of carbides in the deformation zone. In addition, several specimens were cut at the 1/4 thickness position of the square billet to perform carbide dissolution at the temperature and holding time ranges of 850 °C to 1220 °C and 0.5 h to 8 h in a Nabertherm LT24/12/P330 muffle furnace (Nabertherm, Lilienthal, Germany), according to the phase diagram of M2 high-speed steel. Then, specimen preparation was conducted in the same way as above. The observations of the microstructure and carbides in all the specimens were performed using a LEICA DM2700M optical microscope (OM) (LEICA, Wetzlar, Germany) and a ZEISS SIGMA300 field emission scanning electron microscope (SEM) (ZEISS, Heidelberg, Germany). The type of carbides was analyzed and identified through alloy element distribution and atomic ratio obtained by the SEM equipped with an energy-dispersive spectroscope (EDS).

3. Results and Discussion

3.1. High-Temperature Tensile Properties

The reduction of area is one of the key indicators for evaluating the plasticity of materials. The magnitude of the reduction of area directly reflects the excellent thermal plasticity of the material. At the same time, the tensile strength is the critical point for uniform plastic transformation. When the steel yields to a certain value, the ability of the matrix to resist deformation will significantly decrease. The stress–strain curves of the M2 high-speed steel produced by curved continuous casting at different temperatures are shown in Figure 2a, and its tensile strength and cross-sectional shrinkage at different temperatures are shown in Figure 2b. It can be seen that between 850 °C and 950 °C, with the increase in temperature with holding time of 10 min, there was a slight increase in tensile strength and reduction of area. This may be due to the austenite phase transformation occurring within this temperature range and carbides beginning to decompose. Some decomposed carbides enter the matrix in the form of alloy elements and reach their peak at 950 °C. The tensile strength is 210 MPa, and the reduction of area is 50%. Chi et al. [14] believe that this temperature range is the two-phase zone of M2 high-speed steel produced by ingot casting, which is a low-temperature superplasticity zone. Hot working in the temperature range will help to fracture carbides. The tensile strength decreases with increasing temperature in the range of 950 °C to 1120 °C, and the reduction of area is in the plateau zone of 45% to 50%, which is a good hot working range. Within this temperature range, the matrix structure is austenite as a single-phase structure, which undergoes dynamic recrystallization and good thermoplastic properties. When the temperature exceeds 1120 °C, there is a sharp decrease below 5% in the reduction of area. As the temperature continues to rise and reaches 1200 °C, the plasticity is almost zero. Therefore, in the subsequent hot working process, the temperature during forging and rolling should be lower than 1120 °C to avoid excessive core temperature, splitting, severe cracking, and other phenomena. Figure 2c shows the stress–strain curves under different holding times, and the tensile strength and reduction of area under different holding times are shown in Figure 2d. As the holding time increases from 10 min to 30 min, the reduction of area increases from 34% to 54%. In addition, the tensile strength decreases from 92 MPa to 70 MPa and then increases to 82 MPa.

3.2. Carbide Morphology During High-Temperature Tensile Process

The morphology of carbides near the high-temperature tensile fracture of the M2 high-speed steel produced by curved continuous casting is shown in Figure 3. It can be seen that before 950 °C, with the increase in temperature, the fibrous carbides undergo melting and break into small, distributed, short rod-shaped carbides, which will be conducive to the improvement of thermoplasticity. After 950 °C, carbides begin to dissolve, and some solute elements enter the steel matrix. When the temperature reaches 1180 °C, carbides show obvious aggregation and growth. As the temperature increases, the size of carbides also increases. Large-sized carbides are prone to stress concentration in the matrix. Combined with the results of the sudden drop in reduction of area at 1180 °C mentioned earlier, it can be seen that carbides show obvious aggregation and growth at 1180 °C, and exhibit aggregation distribution. The uneven distribution of eutectic carbides causes the deterioration of the hot processing performance and plasticity of the material. Moreover, within this temperature range, the segregation of impurity elements between dendrites can easily lead to a decrease in grain boundary strength, causing some grain boundaries to melt and be pulled apart along the grain boundaries under external forces.

The morphology of carbides near the high-temperature tensile fracture of M2 high-speed steel produced by curved continuous casting is shown in Figure 4. It can be seen that at 1150 °C, with the increase in holding time, the as-cast fibrous eutectic carbides gradually decompose into short rod-like structures, dispersed in the matrix. Combined with Figure 2b,d, it can be seen that at 1150 °C, after holding for 30 min, the primary eutectic carbides are not completely decomposed. With the increase in holding time, the reduction of area increases.

3.3. Carbide Evolution During High-Temperature Diffusion

Figure 5 shows the morphology of carbides during high-temperature diffusion in M2 high-speed steel produced by curved continuous casting, where Figure 5a presents the original-annealed state and Figure 5b–d show the steel after it was separately reheated at 1150 °C × 2 h, 1180 °C × 6 h, and 1220 °C × 2 h. Figure 6 shows the EDS analysis for the carbide types during high-temperature diffusion which are related to the carbides in Figure 5. As the temperature and holding time increased, it was found that the primary eutectic M₂C-like fibrous carbides, as indicated by the red arrow in Figure 5a and confirmed by the EDS analysis in Figure 6a, began to decompose. Meanwhile, the Fe in γ matrix diffused into the interior of the M₂C carbides after their high-temperature decomposition. And then it aggregated at the edges of the M₂C carbides to form M₆C carbides, appearing light gray under SEM observations, circled with green dashed lines in Figure 5b, and identified by the EDS analysis in Figure 6b. With the progress of decomposition, a portion of the M₆C carbides combined with the Fe in the γ matrix transformed into MC carbides which appear dark gray under SEM observations, circled with blue dashed lines in Figure 5c, and confirmed by the EDS analysis in Figure 6c, to generate a more stable mixture. The MC and M₆C carbides gathered to grow and become more angular, while clear boundaries between the MC and M₆C carbides can be observed in Figure 5d. In addition, the shape of carbides gradually changed from fibrous to short rod-like and blocky at different high-temperature diffusion states.

Figure 7 shows the decarburized layer thickness during high-temperature diffusion in M2 high-speed steel produced by curved continuous casting, where Figure 7a shows different temperatures at a holding time of 2 h and Figure 7b shows different holding times at a temperature of 1180 °C. It is found that the decarburized layer thickness increases with temperature and holding time. Moreover, at 1180 °C, the decarburized layer thickness reaches a plateau from 3 h to 5 h and then shows significant growth.

In the M2 high-speed steel, the M₂C carbides undergo a two-stage transformation upon prolonged exposure to elevated temperatures: M₂C + Fe(γ) → M₆C + Fe(γ) → MC + M₆C. Figure 8 demonstrates the carbide decomposition during high-temperature diffusion for M2 high-speed steel produced by curved continuous casting. As the temperature and holding time increase, the M₂C carbides decompose and their growth decreases, and the optimal decomposition range for carbides becomes smaller. The refined MC and M₆C carbides in the best dissolution states can improve the high-temperature tensile properties during the hot working process compared to that in conventionally cast ingots. It is known that a connection between the mechanical properties (usually referring to hardness, tensile strength, and yield strength) and microstructural evolution can be considered using a parameter named Hollomon–Jaffe parameter [31,32,33], where the Hollomon–Jaffe parameter (P_HJ) is associated with the temperature and holding time. The Hollomon–Jaffe parameter is calculated using Equation (1):

$$P_{HJ}=T\left(\log t+20.3\right)$$

(1)

where the error indicates standard deviation, and T and t are in K and h. Hence, carbide evolution during high-temperature diffusion in M2 high-speed steel produced by curved continuous casting can be revealed by the temperature–holding time equivalence. The coarsening of precipitates and decrease in their number density are due to a thermal activation process, and expected to be controlled by the elementary process of self-diffusion and solid diffusion in the alloy matrix. It is found that carbide transformation proceeds to completion, resulting in a homogeneous and fine structure, when the range of the P_HJ value is about 28.0 to 30.5. Moreover, the carbides gather and grow at a P_HJ value of more than 30.5. When the P_HJ value is less than 28.0, the carbides do not dissolve and the morphology does not obviously change.

Based on the above results regarding the relationship between the high-temperature tensile properties and the carbides, it can be concluded that the optimal hot working temperature range is between 950 °C and 1120 °C, which has a reduction of area from 40% to 50%. Moreover, from the perspective of decarburized layer thickness and energy conservation, the holding time during the high-temperature diffusion should be less than 5 h. Therefore, the hot working process includes high-temperature diffusion at 1120 °C to 1180 °C, with a holding time of 2 h to 5 h, and then forging or hot rolling at 1000 °C to 1120 °C.

4. Conclusions

The high-temperature tensile properties of M2 high-speed steel produced by curved continuous casting were discussed in this paper. In addition, the carbide characteristics and evolution during high-temperature diffusion under different temperatures and holding times were demonstrated.

(1)

With the increase in temperature with holding time of 10 min, the tensile strength and reduction of area increase at a temperature from 850 °C to 950 °C. Up to a range of 950 °C to 1120 °C, the tensile strength decreases while the reduction of area reaches a plateau area at 45% to 50%. At 1120 °C, a decrease in reduction of area begins to occur, and at 1180 °C, a sudden drop occurs, leading to a reduction of area below 5%. The zero-strength temperature and plastic temperature are 1220 °C and 1200 °C, respectively. With the increase in holding time at 1150 °C, the reduction of area increases from 34% to 54%. In addition, the tensile strength decreases from 92 MPa to 70 MPa and then increases to 82 MPa.

(2)

A range of P_HJ value of about 28.0 to 30.5 exhibits the best solution for carbides. With the increase in P_HJ value, the shape of carbides gradually changes from fibrous to short rod-like and blocky during high-temperature diffusion.

(3)

The hot working process includes high-temperature diffusion at 1120 °C to 1180 °C, with a holding time of 2 h to 5 h, and then forging or hot rolling at 1000 °C to 1120 °C.