Effect of billet microstructure and deformation on austenite grain growth in forging heating of a medium-carbon microalloyed steel

https://doi.org/10.1016/j.jallcom.2021.159326Get rights and content

Highlights

  • This paper clarifies the effect of billet microstructure and deformation on austenite grain growth in forging heating.

  • This paper provides important guidance for rolling process of medium-carbon microalloyed steel billets for hot forging.

  • This paper provides a new insight into the control and prediction of austenite grain growth.

Abstract

We prepared samples with different ferrite-pearlite characteristics and deformation by heat treatment and Gleeble thermocompression respectively. Then, the different samples are rapidly heated to 1200 °C for 60 s to study the austenite grain growth. The study results show that the ferrite-pearlite characteristics has little effect on austenite grain growth. With the mean austenite grain size (MAGS) of billet microstructure increasing from 14 µm to 56 µm, the MAGS after heating only increases from 47 µm to 56 µm. With the cooling rate of billet samples increasing from 0.1 °C/s to 1.0 °C/s, the MAGS after heating has basically no change. However, the deformation of billet samples has significant effect on the austenite grain growth. With the deformation at 1150 °C increasing from 10% to 50%, the MAGS after heating decreases from 63 µm to 31 µm. In the meanwhile, the mean distance between element segregation bands decreases from 115 µm to 33 µm, and the mean width of element segregation bands decreases from 70 µm to 20 µm. Electron probe microanalysis shows that the segregation region has higher Si, Cr, Mn and V contents, which generate stronger solute drag effect on austenite grain boundaries. Deformation can crush the as-cast grains and make the segregation bands fine and dispersed, further refining the austenite grains after heating.

Introduction

By means of precipitation strengthening of microalloying particles, medium-carbon microalloyed steels can reach the comparable strength of quenched and tempered steels, so as to eliminate the heat treatment after forging and save energy, which is an important development direction of manufacturing industries. However, compared with quenched and tempered steels with sorbite microstructure, the medium-carbon microalloyed steels with ferrite-pearlite microstructure lack toughness and therefore limited in lightweight potential and applied range. It is well known that the final microstructure of steels can be refined by refining the austenite grain before phase transformation in order to get better toughness [1]. Austenite grain growth unavoidably occurs at the high heating temperature of hot die forging, so it is important to study and control the austenite grain growth behavior.

The austenite grain growth is a complex process controlled by many factors, such as heating temperature, heating rate, heating time, chemical composition and so on. With the increase of heating temperature and holding time, the austenite grain usually gets bigger [2], [3], [4]. However, the austenite grain growth can be inhibited by microalloying, which has been widely studied by researchers all over the world. Staśko et al. [5] found that austenite grains could be refined by adding V to a low-alloy steel with high nitrogen content when the heating temperature was below 1050 °C. Maropoulos et al. [6] studied austenite grain growth behavior in a V microalloyed low-carbon steel. With the increase of heating temperature, the V4C3 particles were ripened, and the austenite grain growth was facilitated. Sha et al. [7] found that the particles riched in Ti had strong pinning effect on austenite grain boundaries below 1250 °C in a Nb-V-Ti microalloyed steel. Cuddy et al. [8] studied the austenite grain growth in steels with different Al, V, Ti, Nb contents. With the increase of complete solution temperature of particles, the austenite grain coarsening temperature increased simultaneously. Analogously, Fernández et al. [9] studied the relationship between particle dissolution and abnormal growth of austenite grains. Yu et al. [10] found that grain-boundary internal adsorption of Nb could inhibit the austenite grain growth in a low-carbon steel in the temperature range of 1150–1230 °C. Danon et al. [11] found that the particles containing Nb could induce abnormal austenite grain growth in a Nb microalloyed martensite steel. At present, V-Ti-N microalloying is usually adopted in medium-carbon microalloyed steels [12], [13].

On the other hand, many researchers tried to establish mathematical models for predicting the austenite grain size accurately in the past decades. Lee et al. [14] studied the austenite grain growth behavior in 16 kinds of low-alloy steel and established the prediction model considering the effect of chemical composition. Maalekian et al. [15] studied the austenite grain growth behavior in a microalloyed pipeline steel and proposed an efficient method for establishing austenite grain growth models by in situ measurement and theoretical calculation. Xu et al. [16] studied the austenite grain growth behavior in a hot-rolled dual-phase steel and established the quantitative models considering initial austenite grain size and time exponent to predict the austenite grain size in different heating condition accurately. Similarly, Illescas et al. [17] found that the time exponent decreased with the increase of heating temperature in a low-alloy high strength steel. Pous-Romero et al. [18] studied the austenite grain growth behavior in a nuclear pressure vessel steel and avoided the over-fitting problem by using a Bayesian neural network, so as to get a more accurate activation energy for austenite grain growth. Moon et al. [19] measured the Zener coefficient of cubic TiN particles and established austenite grain growth models in isothermal heating and continuous welding thermal cycle respectively. Banerjee et al. [20] predicted the austenite grain size of a pipeline steel in different welding condition by using the austenite grain growth model combined with the Nb-rich particles dissolution model. Gao et al. [21] predict the size of austenite grains and particles in a Al-V-Ti-N microalloyed steel by using a modified Gladman model considering the combined effects of AlN and TiN particles. In recent years, the phase field theory has been applied to study the solute drag effect and austenite grain growth. Cha et al. [22] established a phase field model to study the solute drag on moving grain boundaries in a binary alloy system. The model includes both solute drag proposed by Cahn [23] and free energy dissipation by Hillert and Sundman [24] at a one-dimensional steady state. Kim et al. [25] established a phase field model for austenite grain growth considering grain boundary segregation in two-dimensional polycrystalline systems. Grönhagen et al. [26] proposed a phase field model to study the dynamics of grain-boundary segregation to a stationary boundary and solute drag on a moving boundary. Based on solute drag theory, Yogo et al. [27] proposed a austenite grain growth model considering segregation effect of each substitutional element. Dépinoy et al. [28] and Fujiyama et al. [29] proposed austenite grain growth models considering both Zener pinning effect and solute drag effect, respectively.

In conclusion, for the control and prediction of austenite grain size, researchers have carried out extensive research on heating process, chemical composition, second phase pinning, solute drag and other aspects. However, for medium-carbon microalloyed steels, the austenite grain growth is closely related to their two-stage production process: hot rolling and hot forging. The billets for hot forging is produced by hot rolling as follows. At first, the continuous casting billets are heated to about 1200 °C, in which the as-cast ferrite-pearlite transforms into coarse austenite grains. Second, the casting billets are hot rolled to bars in the temperature range of 800–1200 °C, in which the as-cast austenite grains are crushed and refined. Third, the bars are cooled to ambient temperature, in which the austenite transforms into ferrite-pearlite. The billets microstructure is controlled by rolling process and may have an effect on austenite grain growth in the rapid induction heating before forging. However, little research has been designed to investigate this. In present study, billet microstructure with different ferrite-pearlite characteristics and deformation are prepared by heat treatment and Gleeble thermocompression respectively. Then, the effect of billet microstructure on austenite grain growth in forging heating is studied. It is believed that the data obtained is important for the control of rolling process and will provide a new insight into the prediction of austenite grain growth.

Section snippets

Material

The steel selected for this study is obtained from commercial production and has the chemical composition (wt%) of 0.47 C, 0.92 Mn, 0.32 Si, 0.21 Cr, 0.006 P, 0.046 S, 0.09 V, 0.022 Ti and 0.015 N. The steel was continuous casted to a 280 mm × 250 mm slab, then deformed to ⌀110 mm bars by rough rolling and continuous finish rolling. The rough rolling temperature is about 1150 °C, while the finish rolling temperature is about 950 °C.

Preparation of samples with different ferrite-pearlite characteristics

Samples with size of ⌀15 × 20 mm were cut at the 1/2 radius of

Effect of ferrite-pearlite characteristics on austenite grain growth

The microstructure with different ferrite-pearlite characteristics is shown in Fig. 2. The proeutectoid ferrite (white colour) first precipitates at the austenite grain boundaries as a second solid phase in the cooling process, then the austenite eutectoid transforms into pearlite (gray colour). Related studies [30], [31] show that the volume fraction of a second solid phase wetting grain boundaries can depend on the transformation temperature. With the cooling rate of billet samples decreasing

Nucleation sites of austenite

The ferrite-pearlite characteristics can affect the austenite nucleation during heating [32], [33], [34]. As shown in Fig. 8, austenite grains can nucleate at the interface of pearlite colonies (position A), ferrite and pearlite (position B), ferrite and cementite in pearlite lamellae (position C). With the refinement of ferrite-pearlite, the size of pearlite colonies decreases simultaneously. The interface of pearlite colonies increases, so does the interface of ferrite and pearlite. As a

Conclusions

The effect of billet microstructure, including ferrite-pearlite characteristics and deformation, on austenite grain growth in forging heating is studied in this paper. The study results show that the ferrite-pearlite characteristics has little effect on austenite grain growth. With the MAGS of billet microstructure increasing from 14 µm to 56 µm, the MAGS after heating increases from 47 µm to 56 µm. With the cooling rate of billet samples increasing from 0.1 °C/s to 1.0 °C/s, the MAGS after

CRediT authorship contribution statement

Fan Zhao: Conceptualization, Methodology, Investigation, Writing - original draft. Hao Hu: Formal analysis, Data curation. Xinhua Liu: Visualization. Zhihao Zhang: Methodology, Supervision, Writing - Review & Editing. Jianxin Xie: Resources, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors appreciate the financial support by the National Natural Science Foundation of China (52090041), the Fundamental Research Funds for the Central Universities (FRF-TP-20-030A1) and the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (P2021-002).

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