FNN model for carbide size of M50 steel during hot deformation

Isothermal compression of M50 steel was carried out on a Gleeble-3500 simulator at the deformation temperatures ranging from 1223 to 1423 K and the strain rates ranging from 10 to 70 s^??1. The relationship between the deformation temperature, strain rate, strain and the carbide size of M50 steel was acquired by simulating the isothermal compression via finite element method, and a fuzzy neural network model for predicting the carbide size during hot deformation was established. The maximum and average difference between the experimental and the predicted carbide size were 9.2 and 4.1% respectively. Applying the present fuzzy neural network model, the effect of the deformation temperature, strain rate and strain on the carbide size of M50 steel during hot deformation was analysed.     

Hot workability analysis and processing parameters optimisation for 20CrMnTiH steel by combining processing map with microstructure

The processing map of 20CrMnTiH steel is developed by using the dynamic material model according to the hot compression experiments, performed on a Gleeble-3500 thermal simulator at the temperature range of 850–1150°C and the strain rate of 0.01–1?s^?1. Hot workability characteristics of 20CrMnTiH steel are analysed based on the developed processing map. The safe deformation regions with higher power dissipation efficiency η exhibit the dynamic recrystallisation (DRX) mechanism and show fine and homogeneous microstructure. The unstable regions with negative instability coefficient ξ occur at both lower temperature with all strain rates and at high temperature with high strain rate at the strain of 0.2. The area of instability gradually decreases with the increasing strain and only appears at lower temperature and higher strain rate when the strain is above 0.2. The unstable regions indicate the flow localisation by microstructure analysis. Combining with the developed processing map with DRX behaviour, the optimal values of hot processing parameters for 20CrMnTiH steel are obtained to achieve good hot workability and small grains sizes at the process parameters ranged at 1036–1070°C/0.1–1?s^?1 and 918–985°C/0.01–0.014?s^?1.     

20CrMnTiH钢的热变形物理模型及三维加工图

随着精密成形技术的发展,对热锻工艺的要求越来越严格,采用建立材料的物理模型及热加工图这一方法来优化最佳工艺条件,为实现产品的质量精确控制提供了科学保障。通过Gleeble-3800热模拟试验机对20Cr Mn Ti H钢在变形温度为850~1 150℃,应变速率为0.01~10 s~(-1)条件下进行等温热压缩试验,研究了20Cr Mn Ti H钢的热压缩变形特性,采用Zener-Hollomon参数法建立了20Cr Mn Ti H钢高温塑性变形的物理模型;并以热压缩试验为基础,绘制了20Cr Mn Ti H钢的三维热加工图并进行分析,确定了该钢的最佳热成形工艺参数。通过流变曲线可以看出,20Cr Mn Ti H钢在热成形过程中发生了明显的动态回复与动态再结晶,流变应力随应变速率的增加而增加,随变形温度的升高而降低;由热加工图分析得到了该钢在试验参数范围内较优的热加工工艺参数,加工温度为900~1 025℃,应变速率为0.01~0.2 s~(-1)。     

Dynamic Recrystallization Behavior of GCr15SiMn Bearing Steel during Hot Deformation

The hot deformation behavior of GCr15SiMn steel was studied through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator. The initiation and evolution of dynamic recrystallization （DRX） were investigated with microstructural analysis and then the process variables were derived from flow curves. In the present deformation conditions, the curves of strain hardening exponent （n） and the true strain （e） at the deformation temperature of 1423 K and strain rates of 0.1, 1 and 10 s^-1 exhibit single peak and single valley. According to Zener-Hollomon and Ludwik equation, the experimental data have been regressed by using linear method. An expression of Z parameter and hot deformation equation of the tested steel were established. Moreover, the Q values of GCrlSSiMn and GCr15 steels were compared. In order to determine the recrystallization fraction under different con ditions, the volume fraction of DRX as a function of process variables, such as strain rate （ε）, temperature （T）, and strain （ε）, was established. Itwas found that the calculated results agreed with the mierostructure of the steel at any deformation conditions.     

Dynamic Recrystallization Behavior and Microstructure Evolution of Bridge Weathering Steel in Austenite Region

The austenite dynamic recrystallization (DRX) behavior and microstructure evolution of a bridge weathering steel was systematically investigated at a deformation temperature range of 800–1100°C and strain rate of 0.1–10 s^?1 by using hot compression test and optical microscopy. The stress exponent and hot deformation energy were obtained by regression method to determine thermal deformation constitutive equation. The curve of stress versus strain is used, combined with high order polynomial fitting, to accurately determine the critical value of DRX. The relationships between critical strain, critical stress, and Z parameter of the bridge weathering steel were obtained by regression method. Moreover, the influence factors of DRX kinetics of the bridge weathering steel were studied in the light of the experimental results. It is shown that the strain rate has a more significant effect on the rate of DRX than that of the deformation temperature, and there is almost 0.85 orders of magnitude increment in the rate of DRX as the strain rate increases an order of magnitude. The dynamically recrystallized grain size can be decreased with decreasing the deformation temperature and increasing the strain rate during the austenite deformation.     

Hot Deformation Behavior and Microstructural Evolution of a Nickel-Free Austenitic Steel with High Nitrogen Content

In current study, the effect of microstructure on hot ductility of nickel-free austenitic high nitrogen steel DIN EN 1.4452 was investigated. Phase transformations and precipitation were modeled as well as experimentally determined via microstructural evaluation. Hot tensile and compression tests were used to simulate the hot deformation behavior at temperatures between 1173 K and 1573 K (900 °C and 1300 °C). Hot tensile test determined the high-temperature properties. The effect of temperature on cracking sensibility during hot deformation was investigated using hot compression test. The results showed that better hot ductility is observed at temperatures ranging from 1423 K to 1523 K (1150 °C to 1250 °C). The increase of hot ductility depends on grain refinement due to dynamic recrystallization at this temperature range.     

Dynamic Recrystallization Behavior of Medium Carbon Cr-Ni-Mo-Nb Steel during Hot Deformation

Hot compression deformation behaviors of medium carbon Cr-Ni-Mo-Nb steel were investigated at deformation temperatures ranging from 1223 to 1423 Kand strain rates of 0.1,1and 5s^-1.Dynamic recovery（DRV）and dynamic recrystallization（DRX）were observed during the hot compression deformation.For all of the samples,DRX occurred at deformation temperatures above 1323 Kat different strain rates,while below 1223 K,no DRX was observed.The activation energy of the tested steel was determined as 386.06kJ/mol.The ratio of critical stress to peak stress and the ratio of critical strain to peak strain were 0.835 and 0.37,respectively.Kinetic equations interpreting the DRX behavior of the tested steel were proposed,and the corresponding parameters including the volume fraction and the average grain size were determined.Moreover,the microstructures induced under different deformation conditions were analyzed.     

Mechanical Behavior and Microstructural Change of a High Nitrogen CrMn Austenitic Stainless Steel during Hot Deformation

The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900 to 1200 °C) and strain rate range 0.01 to 10 s⁻¹ was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing maps, flow instability was observed at higher strain rate regions (1 to 10 s⁻¹) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain size on Z value was obtained.     

Grain growth behaviour of an AISI 422 martensitic stainless steel after hot deformation process

The metadynamic softening behaviour and grain size refinement of an AISI 422 martensitic stainless steel in the temperature range of 950–1150°C was investigated by double-hit compression tests. The deformed specimens were held at deformation temperature with delay times of 5–300?s after achieving a strain of 0.3. Based on the experimental results, a model was established for estimation of a softening fraction at different deformation parameters, and the softening fraction was compared with a recrystallised fraction. A major deviation was observed at the beginning of interpass time denoting a significant contribution of recovery to the fractional softening (FS). However, by increasing the time and temperature, the difference between the FS and recrystallised fraction is reduced. The finer grain size was achieved by prior fine pre-austenite grain and lower secondary deformation temperature. The initial grain size of 53?µm decreased down to 32 and 19?µm at the deformation temperatures of 1020 and 940°C, respectively. The austenite grains have considerable growth at a temperature higher than 1020°C, while the grain coarsening is negligible at lower deformation temperatures.     

20CrMnTiH钢唯象本构模型及动态再结晶行为

采用Gleeble-3800热模拟试验机对20CrMnTiH钢进行了等温热压缩试验,研究了该钢在变形温度为850~1 150℃、应变速率为0.01~10 s~(-1)条件下的高温热变形行为,运用数学回归方法和热力学不可逆原理,建立了20CrMnTiH钢应变补偿的唯象本构方程和动态再结晶模型,并对该应变补偿的唯象本构模型进行了有效验证。在真应力-真应变曲线中,变形温度和应变速率对20CrMnTiH钢的流变应力影响显著,表现出正的应变速率敏感性和负的温度敏感性;由本构模型计算得到的流变应力值与试验值两者之间有很好的相关性(R=0.976 64),平均相对误差为5.544 2%;在应变硬化速率与流变应力关系曲线中,利用单一参数法和求解拐点法获得了不同变形条件下动态再结晶的临界应力σ_c和临界应变ε_c值,建立了临界应力、临界应变和Zener-Hollomon参数的数学模型ε≥ε_c=0.007 9 lnZ-0.153 23,且临界应变ε_c随着温度补偿应变速率因子Z的增加而增加。