# Influence of Boron on Initial Austenite Grain Size and Hot Deformation Behavior of Boron Microalloyed Steels

^{*}

## Abstract

**:**

^{−1}) were conducted. It was found that the initial austenite grain size increases with increasing temperature and boron content. The flow stress decreased with increasing boron content at lower strain rates. The flow stress constitutive equation of hot deformation was developed for the experimental steels; results showed that boron addition has the trend to reduce the hot deformation activation energy. The characteristic points of the flow curves were analyzed. Results revealed that the peak and critical stress decreased in response to an increase of boron content. The work-hardening behavior of both steels was investigated, and it was found that boron addition can decrease the work-hardening rate when strained at lower strain rates. On the contrary, peak and critical strains increased as boron content increased, indicating that boron has the ability to delay the onset of dynamic recrystallization.

## 1. Introduction

## 2. Results and Discussion

#### 2.1. Influence of Boron Content on Initial Austenite Grain

**Figure 1.**Optical micrographs of initial austenite grain at different austenitizing temperatures: (

**a**) B1, 1050 °C; (

**b**) B1, 1100 °C; (

**c**) B1, 1150 °C; (

**d**) B2, 1050 °C; (

**e**) B2, 1100 °C; (

**f**) B2, 1150 °C; (

**g**) B3, 1050 °C; (

**h**) B3, 1100 °C; (

**i**) B3, 1150 °C.

_{2}O

_{3}. This leads to the formation of complex precipitates, such as [MnS + BN], in which a spherical MnS precipitate is surrounded by polycrystalline aggregates of hexagonal BN. This coarsened the boron-containing precipitates. The size of AlN precipitates is much smaller than BN precipitates. The content of AlN reduces via boron addition. Consequently, additional boron can coarsen initial austenite grain size.

Steels | 1050 °C | 1100 °C | 1150 °C |
---|---|---|---|

B1 | 54 | 59 | 83 |

B2 | 56 | 62 | 105 |

B3 | 60 | 68 | 138 |

#### 2.2. Influence of Boron Content on Hot Deformation Behavior

#### 2.2.1. Flow Stress Curves

^{−}

^{1}. When the strain rate decreases to 1 s

^{−}

^{1}, DRV is only observed to occur at a deformation temperature of 900 °C. DRX occurs for all deformation temperatures at a strain rate of 0.1 s

^{−}

^{1}.

^{−1}). The expressed data proves to be identical with Kim’s [16] research. The reasoning for this phenomenon is that boron atoms have insufficient time for reorganization, precluding them from having any significant effect on softening under high strain rate. Additionally, the results exhibit that boron atoms hinder dislocation motion at high strain rates, and pinning effect decreases when boron reaches a distinct concentration limit.

#### 2.2.2. Constitutive Analysis

^{−1}), R is the universal gas constant (8.31 J·mol

^{−1}·K

^{−1}), T is the absolute temperature (K), Q is the activation energy of hot deformation (kJ/mol), A, α, β, n

_{1}and n are the material constants, and σ is the flow stress (MPa). Characteristic stresses such as peak stress, steady state stress, or stress corresponding to a specific strain can be used in these equations [25]. The constitutive equations of the three steels are established using peak stress. The value of α can be calculated from α = β/n

_{1}.

_{p}< 0.8) and high (ασ

_{p}> 1.2) stress levels, taking natural logarithms on both side of Equation (2) produces:

_{p}and $\mathrm{ln}\dot{\epsilon}$ as well as between σ

_{p}and $\mathrm{ln}\dot{\epsilon}$ at different deformation conditions, as shown in Figure 6a,b, respectively. The mean value of n

_{1}and β at different temperature can be computed as 6.7717 and 0.0607, so α = 0.0090 for steel B1.

_{p})] (Figure 7a), the average value of n can be estimated to be 5.4195 for steel B1.

_{p})] and T

^{−1}, which is a set of parallel lines. This means that the activation energy for hot deformation is the same, irrespective of the strain rate. Thus, the activation energy for steel B1 can be calculated according to Equation (8); it is 316.86 kJ/mol.

_{p})]

^{n}

_{p})]

^{n}and Z is presented in Figure 8, from which A can be readily computed as 1.501 × 10

^{12}. The parameters in constitutive equations of all tested steels are shown in Table 2, and the constitutive equations of the experimental steels are shown as follows:

_{(B1)}= 1.501 × 10

^{12}[sinh(0.0090σ

_{p})]

^{5.4195}

_{(B2)}= 4.495 × 10

^{11}[sinh(0.0078σ

_{p})]

^{4.8760}

_{(B3)}= 6.009 × 10

^{10}[sinh(0.0089σ

_{p})]

^{4.6014}

Steels | β | n_{1} | α | n | Q (kJ/mol) | A |
---|---|---|---|---|---|---|

B1 | 0.0607 | 6.7717 | 0.0090 | 5.4195 | 316.86 | 1.501 × 10^{12} |

B2 | 0.0465 | 6.0033 | 0.0078 | 4.8760 | 286.45 | 4.495 × 10^{11} |

B3 | 0.0506 | 5.6940 | 0.0089 | 4.6014 | 277.91 | 6.009 × 10^{10} |

#### 2.2.3. Critical DRX Parameters

_{p}), critical stress (σ

_{c}), peak strain (ε

_{p}) and critical strain (ε

_{c}) were analyzed according to the approach of Poliak and Jonas [28,29,30], an approach that is based on changes in the strain hardening rate (θ) as a function of the flow stress. In the θ-σ plot, the points at which the plot crosses the zero from above represent the peak stress, and the initiation of DRX appears as a distinct minimum in the (−(dθ/dσ))-σ plot. Figure 10a shows the θ-σ curve for steel B2 at strain rate 0.1 s

^{−1}and different temperatures. As can be seen, peak stress of steel B2 clearly increases with decreasing temperature; similarly, critical stress also clearly increases with decreasing temperature, as seen from Figure 10b. Consequently, using θ-σ and (−(dθ/dσ))-σ curves, the peak and the critical stress and strains of texted steels can be determined for all ranges of analyzed deformation and temperature conditions.

**Figure 10.**Strain harding rate (θ) versus true stress at 0.1 s

^{−1}for steel B2 at different temperatures. (

**a**) θ-σ curves; (

**b**) (−(dθ/dσ))-σ curves.

**Table 3.**The dependence of the characteristic points under different deformation conditions on Z parameter.

Steel B1 | Steel B2 | Steel B3 | |
---|---|---|---|

lnσ_{p}-lnZ | lnσ_{p} = 0.16465 lnZ − 0.09199 | lnσ_{p} = 0.17208 lnZ + 0.15473 | lnσ_{p} = 0.19057 lnZ − 0.40797 |

lnσ_{c}-lnZ | lnσ_{c} = 0.15116 lnZ + 0.18716 | lnσ_{c} = 0.16276 lnZ + 0.30586 | lnσ_{c} = 0.18593 lnZ − 0.16590 |

#### 2.2.4. Analysis of DRX Behaviors

^{−1}). The results at other temperatures and lower strain rates also resemble this trend. Thus, it is inferred that the softening effect exhibited by boron increases its self-diffusivity of iron and expands austenite lattice. In addition, some researchers explained this phenomenon on the basis of the non-equilibrium segregation, as an effect of mobile vacancy-solute atom complexes diffusing through a vacancy gradient towards vacancy sinks (this gradient can be generated via plastic deformation) [13,14,15].

**Figure 12.**Relationship between peak, critical stress and the logarithm of the strain rate (

**a**) peak stress; (

**b**) critical stress.

**Figure 13.**Strain dependence of the work-hardening rate for the experimental steels at 900 °C for strain rates of 0.1 and 1 s

^{−1}.

^{−1}. Therefore, the addition of boron retards the onset of DRX. These obtained results are in agreement with previous research work [16,17,18]. Wang [34] attributed this phenomenon to the fact that boron segregation to austenite grain boundaries retards mobility and recrystallization kinetics during hot working, which retards dynamic recrystallization when solute molecules drag by boron located on austenite grain boundaries. Moreover, the initial austenite grain size has also been shown to have an effect on the DRX behavior. The relationship between critical strain (ε

_{c}) and initial austenite grain size (d

_{0}) was found in the literature proposed by Barnett [35]. The relationship between critical strain (ε

_{c}) and initial austenite grain size (d

_{0}) was proven where the following equation was given: ε

_{c}= 2.2 × 10

^{−4}${\dot{\epsilon}}^{0.24}$ d

_{0}

^{0.43}exp(56500/RT). From this equation, critical strain is found to increase with increasing initial austenite grain size. In this case, as previously mentioned, initial austenite grain size increases with increasing boron content. It is established that, for fine grain size, the onset of DRX is promoted by the rise of the amount of available nucleation sites as the grain boundary area per unit volume is increased [36]. Therefore, the retarding onset of DRX is probably caused by the initial austenite grain size increase due to boron addition. However, it is imperative to note that peak and critical strain has a negligible change with increasing boron content at strain rate 1 s

^{−1}and different temperatures except at 1100 °C. Such behavior is attributed to the faster deformation process that does not allow for additional boron to segregate at austenite grain boundaries. Additionally, the deformation energy per time unit and the amount of available nucleation sites increase by improving strain rate. The driving force is thus significantly enhanced and becomes the primary contributing factor for the occurrence of DRX.

## 3. Experimental Section

^{−1}) were used in hot compression tests and the true strain achieved was 0.8.

Steels | C | Si | Mn | S | P | Al | B | N |
---|---|---|---|---|---|---|---|---|

B1 | 0.18 | 0.184 | 0.619 | 0.025 | 0.016 | 0.051% | 0.002 | 0.004 |

B2 | 0.18 | 0.191 | 0.626 | 0.025 | 0.016 | 0.049% | 0.004 | 0.004 |

B3 | 0.18 | 0.196 | 0.626 | 0.025 | 0.016 | 0.049% | 0.006 | 0.004 |

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**MDPI and ACS Style**

Gao, Y.-l.; Xue, X.-x.; Yang, H. Influence of Boron on Initial Austenite Grain Size and Hot Deformation Behavior of Boron Microalloyed Steels. *Crystals* **2015**, *5*, 592-607.
https://doi.org/10.3390/cryst5040592

**AMA Style**

Gao Y-l, Xue X-x, Yang H. Influence of Boron on Initial Austenite Grain Size and Hot Deformation Behavior of Boron Microalloyed Steels. *Crystals*. 2015; 5(4):592-607.
https://doi.org/10.3390/cryst5040592

**Chicago/Turabian Style**

Gao, Yong-liang, Xiang-xin Xue, and He Yang. 2015. "Influence of Boron on Initial Austenite Grain Size and Hot Deformation Behavior of Boron Microalloyed Steels" *Crystals* 5, no. 4: 592-607.
https://doi.org/10.3390/cryst5040592