Flow Stress Of Steel Research Articles

Quantification of multiple softening processes occurring during multi-stage thermoforming of high-strength steel

During the multi-stage thermoforming of steel, work hardening and the stress softening processes initiated by its dynamic recovery (DRV) and dynamic recrystallisation (DRX) are mainly considered for predicting the flow stress. However, the stress softening components describing the static recovery (SRV), static recrystallisation (SRX), and metadynamic recrystallisation (MDRX) processes that occur during post-deformation annealing have not been reflected in the majority of the existing plasticity models, leading to poor prediction accuracy. To accurately predict the flow stress generated during multi-stage thermoforming, modification has been proposed to the Sellars-Tegart-Garofalo model to account for the evolution of all possible recrystallisation processes and the resultant softening effects in the present work. An integrated response of the flow stress caused by DRV-initiated softening and work hardening was characterised using the Estrin-Mecking dislocation model. Another softening process occurring during thermoforming was quantified by a modified strain-independent DRX kinetic equation. In order to quantify all softening processes occurring during multi-stage thermoforming, comprehensive relationships between the described dynamic and static processes were discussed, and the softening fraction was introduced as a global internal variable for quantifying softening equations. In addition, variations of these physical parameters with the interruption stress and holding time were discussed, and the dependency of the residual softening fraction on the former DRX process was expressed as a logarithmic function. A unified constitutive model capable of predicting the comprehensive contribution of multiple softening processes and the resultant macro-flow stress was constructed by updating internal variables. The flow stress values predicted by the unified model were consistent with the results of compression experiments involving complex loading and isothermal histories. This study proposes an efficient approach to elucidating the relationship between the flow stress of steels and the softening effect of microstructural evolution.

Analysis of rate dependency on roll force calculation during hot strip rolling based on Karman equation

Due to the development of thin slab hot rolling technology, hot rolling thin strip at a higher speed is inevitable. As a result of high-speed rolling, thin slab is deformed at a wide range of strain rate inside the rolling zone. Because the flow stress of steel is strongly dependent on strain rate at elevated temperature, it is imperative to consider its variation when calculating roll force and roll pressure. By substituting time with speed and length, strain rate variation is obtained. A strain rate–dependent flow stress curve for non-oriented silicon steel is implemented into Karman equation to calculate rolling pressure distribution. It is revealed that the rolling force can be effectively reduced by decreasing the radius of work roll. It is further revealed that the appearance of strip/roll surface sticking is more likely at the exit of rolling zone than the neutral point, because strain rate reaches zero and the flow stress drops at the exit. Combined with Influence Function Method for elastic deformation of roll surface, the proposed model can predict roll force with a good accuracy compared with industrial data.

Predicting Flow Stress of Four Types of Microalloyed Forging Steel at High Temperature by Artificial Neural Network Model

Artificial neural network (ANN) gets a lot of applications in predicting flow stress of steels at high temperature. However, few studies have been devoted to simultaneously predict flow stress of several steels by ANN. The purpose of this paper is to determine the effect of ANN on simultaneously predicting flow stress of several steels. Based on the results of previous compression experiments of four types of microalloyed forging steel, using the mass percentage of major chemical composition of the steels, such as as C, Mn, Si and V, and deformation temperature, strain rate and strain as input variables, a three-layers back propagation neural network was established as the constitutive model for them. Standard statistical methods were employed to quantitatively measure the accuracy of predicted results by the model. The calculated correlation coefficient and the average relative error absolute value between the predicted values by the model and experimental values were 0.9982 and 2.4181%, respectively. In addition, the relative error between the two kinds of values was calculated, and for more than 89% samples, the relative error was within ± 5%. The results show that the developed constitutive model can predict the flow stress of the four types of microalloyed forging steel accurately and simultaneously.

Deformation behavior of MnO13%-Al2O318%-SiO269% inclusion in different steels during hot rolling processes

To investigate the influence of steel properties on the deformation behavior of MnO13%-Al2 O3 18%-SiO2 69% inclusions during hot rolling processes. The deformation of inclusions with rolling temperatures (800, 850, 900, 925, 950 and 1000 °C) in three steels (A, B, C) have been simulated with a finite element method. The equivalent plastic strain distributions and the shapes of inclusions after rolling are obtained by the model. The influences of rolling temperatures, flow stress of steel, reduction, etc., on the voids length and the deformation degree of inclusions are discussed. We note that 900, 925 and 950 °C are the critical temperatures of the inclusion deformations in steel A, B and C, respectively. No voids occur above the critical temperature. Voids form between the matrix and inclusions below the critical temperature. And the lower rolling temperature leads to the longer length of voids and the smaller deformation of inclusions. The mechanism of voids formation is also analyzed. Experiments on the slab deformations using a titanium ball instead of the inclusion inside the slab at 950 °C during hot rolling processes are conducted. The simulated shapes of the inclusions mostly agree with the experimental results.

Investigation on Hardening and Softening Behavior of Steel after Rapid Strain Rate Changes

The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Estimation of the Hall–Petch strengthening coefficient of steels through nanoindentation

A method to estimate the Hall–Petch coefficient k for yield strength and flow stress of steels through nanoindentation experiments is proposed. While determination of kf for flow stress is on the basis of grain boundary strengthening evaluated by sharp indentation, ky for yield strength was computed with pop-in data from spherical indentations. Good agreement between estimated and literature data, obtained from the tensile tests, validates the proposed methodology.

MATHEMATICAL-ARTIFICIAL NEURAL NETWORK HYBRID MODEL TO PREDICT ROLL FORCE DURING HOT ROLLING OF STEEL

Accurate prediction of roll force during hot strip rolling is essential for model based operation of hot strip mills. Traditionally, mathematical models based on theory of plastic deformation have been used for prediction of roll force. In the last decade, data driven models like artificial neural network have been tried for prediction of roll force. Pure mathematical models have accuracy limitations whereas data driven models have difficulty in convergence when applied to industrial conditions. Hybrid models by integrating the traditional mathematical formulations and data driven methods are being developed in different parts of world. This paper discusses the methodology of development of an innovative hybrid mathematical-artificial neural network model. In mathematical model, the most important factor influencing accuracy is flow stress of steel. Coefficients of standard flow stress equation, calculated by parameter estimation technique, have been used in the model. The hybrid model has been trained and validated with input and output data collected from finishing stands of Hot Strip Mill, Bokaro Steel Plant, India. It has been found that the model accuracy has been improved with use of hybrid model, over the traditional mathematical model.

シャルピー衝撃試験と破壊力学靭性試験の延性・脆性破壊遷移温度相関に及ぼす鋼材特性の影響の評価モデルの提案

There have been many correlations between Charpy test results and fracture toughness test results such as CTOD test results.However, these correlations are not based on the theoretical model but quite empirical. Therefore, the current empirical correlations between both test results, that is especially the brittle-to-ductile transition temperatures, would not be necessarily applicable to the structural steels with various properties. In this study, an analytical evaluation model to correlate Charpy transition temperature with CTOD transition temperature is proposed only after applying the following new ideas with aiming at the effect of steel properties on the transition temperature difference. One is the definition of fracture transition temperature where brittle fracture coincides with ductile crack initiation in both Charpy and CTOD testing. Another one is employing "local plastic strain criterion" for ductile cracking and "Weibull stress criterion" for brittle fracture initiation. This proposed evaluation model provides fracture transition temperature difference ΔTi (= Ti Charpy - Ti CTOD) between Charpy transition temperature Ti Charpy and CTOD transition temperature Ti Charpy, and on effect of steel properties quantitatively. It is analyzed that strength level and work hardening as well as toughness and ductility level of steels can influence transition temperature difference ΔTi. A higher strength and a higher ductility of steels decreases ΔTi, on the contrary, a higher work hardening and a higher toughness increases ΔTi. The effects of these mechanical properties on ΔTi can be interpreted by strain rate and temperature effects on flow stress of steels.

An investigation on the effect of carbon and silicon on flow behavior of steel

The effect of carbon and silicon on the occurrence of dynamic recrystallization as well as on the flow stress of steel was investigated. For this purpose, three grades of steels including low carbon, medium carbon and high silicon low carbon steel were examined. Single hit experiments at temperatures of 900–1200 °C and strain rate of 0.01–2 s −1 were performed to determine the phase transformation kinetics. Different socking times were utilized to detect the effect of initial austenite grain size on the kinetics of dynamic recrystallization and flow behavior of the steels. It was found that increasing the carbon content leads to a higher rate of dynamic recrystallization at high temperatures and low strain rates, while silicon causes postponement of the occurrence of dynamic recrystallization. Moreover, increasing the initial austenite grain size postpones the onset of dynamic recrystallization and leads to a higher flow stress at equivalent deformation conditions.

Deformation behavior of steels in mushy state

Processing metals in a semisolid or mushy state has emerged as a vital commercial process to produce metal and metal-matrix composite components. The understanding of the basic deformation behavior of materials in the mushy state is critical for better control of various semisolid processes. The aim of the present study is to quantify the flow stress of mushy-state steels under uniaxial deformation conditions. A hot compression tester has been developed to provide the high temperature setting required for testing mushy-state steels. An analytical scheme, which only requires measuring the dimensions of the final solidified specimen, has been developed to eliminate the barreling effects occurring in compression tests. The temperature of specimens has been carefully controlled to correlate the solid phase content. It has been found that the flow stress of steels in mushy states was highly dependent on the solid phase percentage. The relaxation of steel and stress reduction at mushy states is also discussed.

The Prediction of the Flow Stress of an Extra-Low Carbon Steel in the Two-Phase Region Using Continuous Cooling Curves

Continuous cooling curves of an extra-low carbon steel under three cooling rates are measured. The flow stress of the steel is established in compression tests during which the temperature is continuously decreasing. The phase transformation temperatures are determined from the cooling rate curve. The latent heat during phase transformation is calculated. A new variable, related to the volume fraction of transformation, is defined. Experimental results show that the relationship between the softening ratio of the flow stress due to phase transformation and this new variable may be described by a quadratic relationship. Based on this relationship and the continuous cooling curves, the flow stresses in the two-phase region are successfully predicted.

鋼の熱間変形抵抗におよぼす微量合金成分の影響

鋼の熱間変形抵抗におよぼす微量合金成分の影響

Measurement of friction and the flow stress of steels at room and elevated temperatures by ring-compression tests

The ring-compression test is traditionally conducted for identifying interfacial friction conditions during plastic deformation. The purpose of this investigation was to pursue the test for determining the flow stress of the material at high strain rates and elevated temperatures. Theoretical calibration curves were established based on a velocity field capable of describing the bulge phenomena. Experiments were first carried out at room temperature using specimens of a steel with 0.009% carbon under three lubrication conditions teflon film, phosphorate coating, and dry friction. The tests were performed in three ways: (i) a single specimen was compressed in multi-steps; (ii) several specimens were compressed in single steps to different reductions; and (iii) a single specimen was compressed in a single step to the final strain. The results indicate that the interfacial friction factor did not vary significantly with strain, but increased with strain rate when solid lubricants were used. True stress-true strain data obtained from different lubrication conditions and loading types had virtually no differences. At elevated temperatures (900–1050°C) tests were carried out in single steps using samples of a 0.045% carbon steel. Boron nitride was applied for lubrication. Strain rate and temperature did not have significant effects on frictional conditions. Comparison of the current results with those obtained by others indicates that the flow stresses determined from the results of ring-compression tests are reliable.

Effect of carbon and the microstructure on the hardening of steel during cold deformation

1. The carbon content and the shape of cementite in the steel (lamellar or granular) have a substantial effect on the flow stress during cold plastic deformation. The formulas obtained in this work make it possible to determine this value quantitatively. 2. The flow stress of steels with a structure of divorced pearlite is lower than that of steel with lamellar pearlite, the difference increasing with the carbon content of the steel, i.e., with increasing amounts of pearlite in the structure.

Strain-rate and strain-rate-history effects in several metals in torsion

A machine for testing thin-walled tubes in torsion at shear-strain rates up to 25/sec is described. Results of constant and variable-strain-rate tests are presented for 1100-0 aluminum, AISI 1020 steel, and 50-A titanium. Results indicate that 1100-0 aluminum is very slightly strain-rate sensitive, but steel and titanium are noticeably sensitive to both strain rate and strain-rate history. Variable-rate tests show that subsequent dynamic loading on a statically prestrained specimen causes an increase in the flow stress in steel and a decrease in the flow stress in titanium.