Warm Rolled Temperature Effect on Microstructure and Mechanical Properties of 18Mn/40Si2CrMo Multilayer Composite Steel

Abstract

In order to obtain a good strength-plastic/toughness match relationship, 18Mn/40Si2CrMo multilayer composite steels were successfully fabricated by a vacuum hot rolling and warm rolling process in this paper. The effects of different warm rolling temperatures (400–600 °C) on the microstructure and mechanical properties of the multilayer composite steel were systematically investigated. The result shows that the warm rolling process reduces thickness of the interfacial diffusion layer, which improves the interfacial bonding strength of multilayer composite steel. With the increase of warm rolling temperature, the total elongation (TEL) increases but ultimate tensile strength (UTS) decreases. The multilayer composite steel with a warm temperature of 500 °C achieves the balance of strength and plastic of which the UTS and TEL are 1.7 GPa and 12.5%, respectively. This is due to the high work-hardening ability of deformation twins of the 18Mn layer and the precipitates nanoscale carbides of the 40Si2CrMo layer to obscure the dislocation movement.

1. Introduction

In the last few decades, with the improvement of global science and technology, in some major engineering areas, the higher requirement is put forward for the properties of metal materials, for example, it has high-strength and high-plasticity [1,2,3]. However, strength and plasticity are usually an inverted relationship, so it is a difficult assignment to achieve both high- strength and high-plasticity.

Recently, multilayer structure offers a novel idea. It refers to the composite of two or more materials by stacking and rolling, can inherit the properties of both materials [4,5,6,7,8]. Herein, metal multilayer materials with heterogeneous layers and bimetallic interfaces have received the attention of many researchers [9,10,11,12,13]. By laminating and rolling different metals, a variety of new materials can be designed to cope with a complex and harsh usage environment. In general, multilayer composite steel usually consists of soft and hard phases, during the deformation process, the stress and strain is redistributed in soft and hard layers, which gives the multilayer composite steel plate excellent mechanical properties; this makes the multilayer composite steel sheet have superior mechanical properties.

Multilayer metal composites have a long history. More than a thousand years ago, the Damascenes used repeated folding to make sharp knives. The knife has a pattern resembling water waves, and its microstructure is a layered material formed by the superposition of laminated ferrite and carburized body [14,15]. Its high strength and high toughness are caused by the carburizing layer and the ferritic layer, respectively. Liu et al. [16] successfully produced multilayer SUS304/Cr17 steels with laminate/network interface by hot rolling and found that annealing treatment can induce alloy element diffusion and enhance interface bonding. Koseki et al. [17] reported that multilayer steel can achieve a good match between strength-elongation and good plastic formability compared with single steel. Wang et al. [18] achieved both strengthening and toughening by hot rolling stainless steel composite plates with mechanical properties, exhibiting an excellent ultimate strength of 543 MPa and fracture elongation of 52%.

In previous studies, most of the research on multilayer materials is limited to hot rolling experiments, but the strength of hot rolling is lower and the performance is poorer, and there is less research in the field of warm rolling. Compared with hot rolling, warm rolling can significantly improve mechanical properties, however, the effect of warm rolling temperatures on the performance of 18Mn/40Si2CrMo multilayer steel is not clear. Herein, 18Mn/40Si2CrMo multilayer composite steel was manufactured by vacuum hot rolling. Subsequently, the effect of different warm rolling temperatures on the interface morphology and element diffusion of multilayer composite steel plate was studied and the evolution of microstructure and toughening mechanism were systematically investigated. It provides a theoretical and experimental basis for the preparation of metal multilayer composite structures, fracture and toughening mechanisms.

2. Materials and Methods

18Mn and 40Si2CrMo were prepared by the vacuum melting method. Subsequently, the fabrication process of multilayer composite steel was as follows: firstly, 18Mn steel and 40Si2CrMo steel sheets with a standard thickness of 0.5 mm were taken as the initiation into the metal plate, and their chemical compositions are shown in

Table 1. The chemical composition of raw 18Mn and 40Si2CrMo [wt.%].

Element	C	Mn	Al	Si	Cr	Mo	Fe
18Mn	0.62	17.48	1.46	≤0.1	≤0.1	≤0.05	80.19
40Si2CrMo	0.4	0.2	0.022	2.0	1.0	1.01	95.368

. To remove oxide layers and impurities on the surface, sandpaper was used and cleaned with acetone, respectively, to obtain a clean metal surface, which facilitates the improvement of interfacial bond strength between the constituent layers. Secondly, stack 18Mn steel and 40Si2CrMo steel in an alternate A-B-A-B-A… arrangement in an unsealed carbon steel box, a total of 100 layers, as shown in Figure 1a. Thirdly, the carbon steel box is sealed by tungsten inert gas (TIG) welding and then a round hole with a diameter of 5 mm is drilled on the surface of the billet box, which is connected with a carbon steel pipe by welding. After the vacuum degree of the billet is reduced to 2.0 × 10⁻² Pa by a triplex diffusion pump, the carbon steel pipe was pressed through hydraulic pliers. Finally, the billet was heated at 1200 °C, soaked for 60 min and then rapidly hot rolled (HR) with a reduction of 92% (HR-92), as shown in Figure 1b,c. Subsequently, the hot-rolled multilayer steels were warm rolled (WR) with five rolling temperatures of 400 °C (W-400), 450 °C (W-450), 500 °C (W-500), 550 °C (W-550), 600 °C (W-600), respectively, the total rolling reduction ratio is 95.2%. The diameter of working roll is 350 mm, the circumferential rate is 50 rpm, the strain rate of each hot rolling pass is 0.2, and the strain rate of each warm rolling pass is 0.1. In order to avoid excessive temperature reduction, the furnace is heated for 10 min for each two rolling passes.

JSM-7100F scanning electron microscope (SEM), and its electron backscattered diffraction (EBSD) technique, Tecnai G2 F30 S-TWIN transmission electron microscopy (TEM), JXA-8530F electron probe microanalyzer (EPMA) were used to analyze interface bonding, element distribution, and microstructure evolution. Smartlab 9 KW X-ray diffraction (XRD) was used to analyze phase structure and dislocation density. HMV-G-XY-S microhardness tester was used to display the work-hardening behavior of the warm-rolled multilayer steel with different rolling temperatures. The AGS-50kNX universal testing machine was used to measure the tensile and bending properties of multilayer steels; the sample size for testing is shown in Figure 1d; five samples of each state were tested. The strain rate of tensile test was 1mm/min, and a YYJ-10/10-L electron extensometer was used to measure the elongation. The Vickers’ hardness (HMV-G-FA) was estimated with HV0.2 and hold pressure for 15 s.

3. Results

3.1. Microstructure

Figure 2 shows the EBSD analysis of the hot-rolled multilayer composite steel (HR-92). As shown in Figure 2a, the 18Mn layer has a large grain size and forms a small portion of twins, while the 40Si2CrMo layer has a smaller grain size and the average grain size is 20 ± 5 μm and 4 ± 2 μm, respectively. As shown in Figure 2b, the green areas are an 18Mn layer, which is face centered cubic (fcc) structure. The yellow areas are the 40Si2CrMo layer, which is body centered cubic (bcc) structure.

Figure 3 shows the EPMA surface scanning maps of multilayer 18Mn/40Si2CrMo steel. As can be seen from the figure, when the warm rolling temperatures are low, the interface shape is curved and local necking was found at lower temperatures (400–450 °C), because of different hardening rates of 18Mn and 40Si2CrMo leading to unstable behavior [19]. As the warm rolling temperature increases, the interface between the multilayer composite steel becomes flat, so the plastic deformation capacity can be improved by increasing the warm rolling temperatures.

Figure 4 shows the Mn, Si, Cr and Mo elemental distributions near the interface of the multilayer composite steel. According to Arrhenius equation [20]:

$$\mathrm{D}={\mathrm{D}}_0{\mathrm{e}}^{\left(-\frac{\mathrm{Q}}{\mathrm{RT}}\right)}$$

(1)

${\mathrm{D}}_0$ is the diffusion constant, Q is the activation energy per mole of an atom, R is the gas constant, and T is thermodynamic temperature. Diffusion distance (X) is related to diffusion coefficient (D) and diffusion time (t),

$$\mathrm{X}=2\surd \mathrm{Dt}$$

(2)

So, the diffusion coefficient is proportional to temperature, diffusion distance is proportional to the diffusion coefficient, and diffusion time.

$${\mathrm{D}}_{\mathrm{Mn}}=1.49\times {10}^{-4}{\mathrm{e}}^{-\frac{66.4}{\mathrm{RT}}}$$

(3)

$${\mathrm{D}}_{\mathrm{Cr}}=1.8\times {10}^4{\mathrm{e}}^{-\frac{97}{\mathrm{RT}}}$$

(4)

$${\mathrm{D}}_{\mathrm{Mo}}=6.84\times {10}^{-2}{\mathrm{e}}^{-\frac{59}{\mathrm{RT}}}$$

(5)

$${\mathrm{D}}_{\mathrm{Si}}=7\times {10}^{-6}{\mathrm{e}}^{-\frac{243}{\mathrm{RT}}}$$

(6)

The calculation results are shown in

Table 2. Interfacial element diffusion distances of 18Mn/40Si2CrMo multilayer composite steel with different warm rolling temperatures.

Distance (μm)	HR-92	W-400	W-450	W-500	W-550	W-600
Mn	4.4	2.1	2.4	2.5	2.5	2.5
Si	2.8	2.2	1.9	2.1	1.8	2.6
Cr	4.4	2.4	2.6	2.6	2.6	3
Mo	3.2	1.6	2	1.8	2.2	2.4

. In the same temperature range, ${\mathrm{D}}_{\mathrm{Cr}}$ > ${\mathrm{D}}_{\mathrm{Mn}}$ > ${\mathrm{D}}_{\mathrm{Mo}}$ > ${\mathrm{D}}_{\mathrm{Si}}$, the diffusion driving force caused by concentration difference is small, and mutual diffusion will occur in the rolling process [17].

Figure 5 shows the TEM and SEM microstructure of the W-500 sample. 18Mn layer and 40Si2CrMo layer produce strain incompatibility at the interface, resulting in a wavy interface. As rolling changes the state of strain, the 18Mn layer expands more than the 40Si2CrMo layer in the rolling direction, resulting in shear stresses at the interface and local shear bands, as shown in Figure 5a. Al₂O₃ are formed at the interface with a diameter between 200–300 nm, and the interfacial oxide can improve the interfacial bonding strength [21], Figure 5b. In the 18Mn layer, grain refinement, dislocation proliferation and twinning are evident, a large number of nanoscale grain boundaries and high-density dislocation are formed. As shown in Figure 5c, the width of the twins is mostly around 5–30 nm, nano-scale twin achieve reinforcement by the introducing high-density surfaces. In the 40Si2CrMo layer, the precipitated nanoscale carbides can pin the dislocation motion, thus improving the performance.

3.2. Tensile Properties and Hardness

Figure 6 shows the hardness distributions of multilayer steels with different warm rolling temperatures, compared with the HR-92, the hardness of the warm-rolled plate will be greatly improved. Li et al. [22] found that warm rolling deformation can significantly increase the dislocation density of austenitic steel, the free carbon atoms interact with high-density dislocations, and then dynamic strain aging occurs in the 18Mn layer, which improves the hardness. On the other hand, the formation of deformation twins in the 18Mn layer will further hinder the movement of dislocations and accelerate the plugging of dislocations. The dislocation density in the 40Si2CrMo layer increases as the warm rolling temperature decreases, and the further precipitation of fine carburized particles will also play a “pegging role” in the movement of dislocations, triggering the process hardening phenomenon. As the temperature rises, the hardness value gradually decreases, and the difference between the two layers decreases significantly, which increases the ratio of movable dislocation to immovable dislocation so that the hardness decreases [23].

Figure 7 shows the engineering stress–strain curves of multilayer composite steel with different warm rolling temperatures; the tensile properties are listed in

Table 3. Tensile properties of 18Mn/40Si2CrMo multilayer composite steel with different warm rolling temperatures.

Sample	UTS (MPa)	YS (MPa)	TEL (%)
HR	1028	1226	25.3
W-400	1879	1978	4.6
W-450	1760	1859	10.5
W-500	1670	1780	12.2
W-550	1496	1625	12.4
W-600	1340	1515	12.8

. It is found that warm rolling can improve the yield strength (YS) and ultimate tensile strength (UTS) of HR-92. With the decrease of warm rolling temperature, the YS and TS increase continuously, while the total elongation (TEL) after fracture decreases; when the rolling temperature drops to 400 °C, the tensile strength is close to 2 GPa, but the elongation is too low to meet the requirements of most products. In conclusion, good comprehensive mechanical properties can be obtained at the temperature of 450–550 °C.

Figure 8 shows the normal fracture morphologies of multilayer 18Mn/40Si2CrMo sheets of steel with different warm rolling temperatures, the black box is enlarged below. All of them show obvious interfacial delamination, which indicates the increased plastic deformation capacity. Meanwhile, the fracture morphologies show dimples of two different sizes, with the increase of warm rolling temperature, the width of the transition layer at the interface increases gradually, this could be caused by the diffusion of elements at the interface [16]. The dimples of the 18Mn steel layer are large and deep, and the number is increasing, showing that the 18Mn layer has better plastic deformation ability than the 40Si2CrMo layer.

In a previous study, Kimura et al. [24,25] prepared ultrafine elongated grain structure (UFEG) with layered distribution by tempforming technology. Multilayer composite steel has many similarities with the fracture mode of UFEG, when subjected to bending loads; significant delamination occurs because of the anisotropic UFEG structure. The local strain state of the crack tip can be changed from triaxial to uniaxial by delamination, causing the cracks blunting. In addition, the delamination causes the crack growth rate to slow down and change from a dynamic state to a low-rate state, resulting in plastic deformation, thus showing high toughness.

During bending deformation, the position close to the indenter is not easy to form cracks due to compressive stress, while the position far away from the indenter is prone to crack initiation and propagation due to tensile stress. The crack perpendicular to the layer direction is easy to deflect at interface. As shown in Figure 9, with the increase in warm rolling temperature, the number of crack propagation retarded and deflected increased significantly, the crack propagation path is increased, as shown by the red arrow, the crack propagation energy is consumed, and the anti-crack propagation ability of multilayer composite steel is improved [26,27]. Figure 10 shows the bending stress–strain curves of multilayer composite steel with different warm rolling temperatures, with the decrease of warm rolling temperature, the growth rate of initial bending stress becomes faster, and the yield point increases, so does the bending strength.

4. Discussion

In this study, it was found that the warm rolling process can obtain multilayer composite steel with excellent performance. Compared with cold rolling, warm rolling can significantly reduce the microstructure damage and optimize the microstructure; the reinforcement mechanisms for each group element layer are discussed further below.

Twin boundary as a special coherent grain boundary not only hinders the movement of dislocation to improve the strength of the 18Mn layer but also serves as a dislocation slip surface to absorb and store dislocation and to improve the plastic deformation ability. Curtze et al. [28] found that there are two main deformation mechanisms in 18Mn steel, twinning strengthening and dislocation slip, which are related to the magnitude of Stacking fault energy (SFE). 18Mn steel is mainly deformed by twinning when the SFE is between 20–40 mJ/m². When the SFE is greater than 45 mJ/m², dislocation slip is the main strengthening mechanism. The calculated SFE value of 18Mn at room temperature is 26.80 mJ/m², so the deformation strengthening mechanism of the 18Mn layer is mainly twinning.

As shown in Figure 5c, a large number of nano-scale twins appear in the 18Mn layer, which will hinder the movement of the dislocations and change the movable dislocation into immobile dislocation [29], strain hardening behavior is produced and the strength is increased. In addition, the stress concentration and applied shear stress on the twin interface will increase with the increase of the number of dislocations plug, and the number of pluggable dislocations within the twin decreases with the thinning of the twinning layer. When the thickness of the laminar is too low to be pluggable, an extremely high applied stress is required to force a single dislocation across the twin boundary [30,31].

There are many diffusively distributed carbide particles in the 40Si2CrMo layer, approximately 30-80 nm in diameter, as shown in Figure 5d. Fine nano-carbides can pin dislocations, produce work hardening and improve strength. The interaction between nano precipitates and dislocations can be divided into two mechanisms. First, when the sliding resistance of the precipitated phase is less than the maximum reaction force of dislocation slip motion, the nano-precipitated phase can be regarded as deformable particles and thus be cut by dislocation. Second, if the resistance of relative dislocation precipitated is greater than or equal to the reaction force of dislocation slip motion, the nanoparticles become immovable particles, resulting in the strengthening of the dislocation bypass mechanism [32]. In fact, the strengthening mechanism of carbide is mainly determined by the ratio of the carbide diameter R to the Burgers vector b [33]. If R/b < 15, it is mainly dislocation cutting mechanism, and if R/b ≥ 15, it is mainly dislocation bypassing mechanism. 40Si2CrMo has R/b =12.5 < 15, so the main strengthening mechanism is dislocation cutting mechanism.

Figure 11 shows the X-ray Diffraction patterns of multilayer composite steel with different warm rolling temperatures. Compared with the diffraction peak of hot-rolled multilayer composite steel plate, no new phase is generated after warm rolling, and the (200) γ diffraction peak disappears, (111) γ diffraction peak and (110) α diffraction peak decrease in intensity. To determine the dislocation densities for multilayer steel with different process parameters, the Scherrer formula can be used to calculate [34], the dislocation density is related to the average microcrystalline size and microstrain, which microcrystalline size and microstrain can be estimated according to the Scherrer formula and Williamson–Hall formula, respectively [20,35], the relevant calculation results are shown in

Table 4. The full width at half maximum, dislocation densities, and dislocation enhancement value for 18Mn/40Si2CrMo multilayer composite steel with different parameters.

18Mn/40Si2CrMo		The Full Half Maximum (B)	Dislocation Density (mm−2)	Dislocation Enhancement Value (MPa)
Hot rolling	18Mn	0.209	9.953 × 106	324.8
	40Si2CrMo	0.325	1.492 × 107	397.7
W-400	18Mn	0.366	3.051 × 107	566.8
	40Si2CrMo	0.468	4.781 × 107	712.3
W-450	18Mn	0.359	2.934 × 107	557.6
	40Si2CrMo	0.430	4.035 × 107	654.0
W-500	18Mn	0.351	2.808 × 107	545.5
	40Si2CrMo	0.396	3.425 × 107	602.5
W-550	18Mn	0.311	2.203 × 107	483.2
	40Si2CrMo	0.382	3.190 × 107	581.5
W-600	18Mn	0.277	1.746 × 107	430.2
	40Si2CrMo	0.358	2.797 × 107	544.5

.

$$\mathrm{D}=\frac{\mathrm{k}\mathsf{\gamma }}{\mathrm{Bcos}\mathsf{\theta }}$$

(7)

$$\frac{\mathrm{Bcos}\mathsf{\theta }}{\mathsf{\lambda }}=\frac{2\langle {\mathsf{\epsilon }}_{50}^2\rangle \sin \mathsf{\theta }}{\mathsf{\lambda }}+\frac{\mathrm{K}}{{\mathrm{d}}^{\prime }}$$

(8)

D is the average microcrystal size, K is Schell factor, K = 1, $\mathsf{\gamma }$ is the wavelength of the X-ray, $\mathsf{\gamma }$ = 0.154056 nm. B is full width at half maximum, $\mathsf{\theta }$ is Bragg angle. ${\mathsf{\epsilon }}_{50}^2$ is microstrain, $\mathsf{\lambda }$ is the wavelength of the ${\mathrm{K}}_{\mathsf{\alpha }}$, $\mathsf{\lambda }$ = 0.15418 nm, ${\mathrm{d}}^{\prime }$ is average grain size. The calculation formula of dislocation density is shown in Formula (9).

$$\mathsf{\rho }=\frac{3{\left(2\mathsf{\pi }\right)}^{\frac{1}{2 }}}{\mathrm{Db}}\langle {\mathsf{\epsilon }}_{50}^2\rangle$$

(9)

b is the Burgers vector, b = 2.5 × 10⁻⁷ mm. According to Formula (4), the contribution value of dislocation to material strengthening in 18Mn layer and 40Si2CrMo layer was estimated, respectively.

$${\mathsf{\sigma }}_{\mathrm{dis}}=\mathsf{\alpha }\mathrm{MGb}{\rho }^{\frac{1}{2}}$$

(10)

${\mathsf{\sigma }}_{\mathrm{dis}}$ is dislocation density contribution, $\mathsf{\alpha }$ is constant, $\mathsf{\alpha }$ = 2. M = 2.9 [36], G is shear modulus, G = 7.1 × 10⁴ MPa.

5. Conclusions

• The multilayer composite steel in the hot rolled state has good deformation coordination and the interface transitions to wavy as the warm rolling proceeds. Local necking occurs to varying degrees between both layers, which is due to the difference in hardening rates and flow stresses between 18Mn and 40Si2CrMo, resulting in shear stresses at the interface;
• The diffusion distance of Mn, Si, Cr, and Mo at the interface all increase with the increase of warm rolling temperature, which is mainly due to the increase of diffusion coefficient caused by the increase of temperature;
• As the warm rolling temperature decreases, the YS and UTS increase, TEL decreases. When the warm rolling temperature is 500 °C, the YS of the sample reaches 1670 MPa, YS reaches 1780 MPa, and the TEL is 12.2%, which is an excellent strength-plasticity combination;
• In the bending process, lower warm rolling temperature leads to premature failure with a low number of deflection cracks, while with a higher warm rolling temperature due to good interfacial bonding, multiple cracks are passivated; there are numerous deflection cracks formed, and continuous crack deflection and delamination can absorb a large amount of energy, thus enhancing the bending resistance.