Hot Workability of 300M Steel Investigated by In Situ and Ex Situ Compression Tests

Abstract

In this work, hot compression experiments of 300M steel were performed at 900-1150 • C and 0.01-10 s −1 . The relation of flow stress and microstructure evolution was analyzed. The intriguing finding was that at a lower strain rate (0.01 s −1 ), the flow stress curves were single-peaked, while at a higher strain rate (10 s −1 ), no peak occurred. Metallographic observation results revealed the phenomenon was because dynamic recrystallization was more complete at a lower strain rate. In situ compression tests were carried out to compare with the results by ex situ compression tests. Hot working maps representing the influences of strains, strain rates, and temperatures were established. It was found that the power dissipation coefficient was not only related to the recrystallized grain size but was also related to the volume fraction of recrystallized grains. The optimal hot working parameters were suggested. This work provides comprehensive understanding of the hot workability of 300M steel in thermal compression.

Modelling the Deformation, Recrystallization, and Microstructure-Related Properties in Metals

“…The exceptional properties of various metallic polycrystalline systems, such as high strength, excessive hardness, great ductility at room temperature, superior energy absorption capacity, and good corrosion resistance, make them outstanding candidates for a wide variety of applications where one or another of the mentioned qualities, or the combination of several, is of crucial importance. The presented contributions [1][2][3][4][5][6][7][8][9] evidently demonstrate that the properties of metallic materials are microstructure-dependent and, therefore, the thermomechanical processing (TMP) of the polycrystalline aggregates should be strictly controlled to guarantee the attainment of the desired suite of qualities. Given this, the assessment of microstructure evolution in metallic systems is of extraordinary importance.…”

“…In the most general case, the TMP of metals involves a sequence of deformation and annealing processes (or the combination of both into one technological step) that lead to morphological changes in the polycrystalline aggregate, the evolution of crystallographic texture, or phase transformation. All these aspects of microstructure development are partially discussed in the presented contributions [1][2][3][4][5][6][7][8][9]. In the first manuscript [1], the deformation flow in a single-phase alloy with a face-centered crystal structure is analyzed by means of a finite element (FEM) approach and computationally efficient flow-line model (FLM), which enabled the effective simulation of cold rolling in terms of the evolution of crystallographic texture.…”

A Flow Stress Model of 300M Steel for Isothermal Tension

“…Specifically, for 300M steel, a dislocation–based constitutive model considering dynamic, meta–dynamic, and static recrystallization in both single and multiple pass compression has been established by our group [ 9 , 10 , 11 ]. The softening behavior of 300M steel between passes was investigated by Liu et al, and models quantifying the meta–dynamic [ 12 ] and static recrystallization softening [ 13 ] were built.…”

Advances on the deformation mechanism and forgingtechnology of 300M high-strength steel heavy componentsin the whole forging process

“…1 Typical components and forging process. (a) Typical heavy components [23] 得了广泛的关注 [8~12] 。在高强钢大型构件锻造成形机理方面，刘凯等人 [13,14] 研 究了 300M 高强钢动态再结晶动力学行为，发现材料在不同变形参数下可能会发 生一轮动态再结晶现象或多轮动态再结晶现象， 并基于此建立了材料的动态再结 晶动力学模型。Qi 等人 [15] 对单道次热压缩变形的应力应变曲线进行了温升和摩 擦修正，建立了 300M 高强钢本构模型。Liu 等人 [16] 研究了 300M 高强钢微观组 织遗传性的特点，并提出了消除微观组织遗传性的热处理制度。Liu 等人 [17] 基于 双道次压缩曲线研究了 300M 高强钢的亚动态再结晶动力学行为，发现变形温度、 应变速率和道次间保温时间均会显著影响亚动态再结晶动力学过程。 Liu 等人 [18] 研究了 300M 高强钢在不同应变下冷却过程的微观组织演化规律，发现材料变形 过程中是否发生动态再结晶对随后的相变过程具有重要影响。 在大型构件锻造工 艺方面，Lu 等人 [19] 基于渐进式结构优化概念，提出了一种用于预锻件设计的拓 扑优化方法。在该方法中，基于静水压力提出了新的工件边界消元及加元准则， 并通过编程方法将其与 Defrom 2D 软件包集成。Jia 等人 [20] 研究了不同坯料高径 比及凸模形状尺寸等对锻件成形性能的影响，并通过模拟实现工艺参数的优化。 He 等人 [21] 针对大型支撑梁的成形，在半开式模具中进行了多段变速等温加载实 验及模拟研究。采用该方法可以显著缩短锻造周期，获得的锻件晶粒尺寸细小均 匀，锻件中无裂纹。陈春等人 [22] 利用 Deform 软件对飞机起落架的锻造成形过程 [24,27] [24] 。在热变形过程中，材料的软化机 制主要是动态回复和动态再结晶机制 [28] 。动态再结晶机制通常包括非连续动态 A c c e p t e d https://engine.scichina.com/doi/10.1360/TB-2021-1122 再结晶和连续动态再结晶 [29] 。非连续动态再结晶机制包括形核和长大过程，当 应变达到临界应变时， 非连续动态再结晶机制被触发， 发生动态再结晶形核现象。 随着变形的持续发生，新生的动态再结晶核心会发生长大现象 [30] 。而连续动态 再结晶机制的发生主要是通过增加小角度晶界的取向偏角逐渐形成大角度晶 界 [31] 。通常认为，非连续动态再结晶机制发生于中低层错能材料中，而连续动 态再结晶机制发生于高层错能材料中 [32] 。 300M 高强钢属于低层错能材料， 然而， Guo 等人 [33] 基于热压缩实验及微观组织观察发现其动态再结晶机制类型并不唯 一，而是与应变速率密切相关。在低应变速率下，300M 高强钢主要发生非连续 动态再结晶，此时热压缩流动曲线存在明显的峰值应力，其平均动态再结晶晶粒 尺寸会随着应变速率的增加而减小； 在高应变速率下， 主要发生连续动态再结晶， 此时其热压缩流动曲线没有峰值应力， 平均动态再结晶晶粒尺寸的大小与应变速 率无关。 而动态再结晶机制由非连续动态再结晶转向连续动态再结晶的平均临界 应变速率为 1.8 s -1 。此外，Xiong 等人 [34] 也指出 300M 高强钢热变形过程中动态 再结晶机制的类型与变形条件密切相关。除了应变速率，Zhao 等人 [35] 发现变形 温度对 300M 高强钢动态再结晶过程有显著影响，进而会影响材料的宏观流动行 为及微观组织演化。 变形温度越高， 材料热变形过程中的动态再结晶进展越充分。 这是因为动态再结晶过程是一个热激活过程，在高温下原子扩散更剧烈，位错的 移动速度更快，加剧了动态再结晶的发生。此外，Zhao 等人 [36] 将保温过程与变 形过程联合研究， 发现材料变形前的晶粒尺寸能够影响随后变形过程中的动态再 结晶程度，变形前细小的晶粒尺寸能够促进材料动态再结晶的发生。上述讨论均 为压应力状态下 300M 高强钢的动态再结晶机制，而不同应力状态对材料的宏微 观变化也有显著的影响。相比于拉应力状态下 300M 高强钢的动态再结晶程 度 [37,38] ，其在压应力状态下动态再结晶进展更加充分 [39] 。为了更好地确定材料 在变形不同阶段所发生的主要软化机制，Li 等人 [40] 机制 [24] ；(b)多道次变形的本构模型建模流程…”