Effect of manganese content on hot deformation behaviour of Fe–(20/27)Mn–4Al–0.3C non-magnetic steels

The influence of Mn content on flow behaviour and constitutive equations of Fe–(20/27)Mn–4Al–0.3C austenitic steels was investigated by uniaxial hot compression tests. The recrystallisation fraction was also measured using electron backscattered diffraction. Although dynamic recrystallisation is slightly delayed by the increased Mn content, the peak stress still has a pronounced decline especially during the deformation at relatively low temperature. The reason should be ascribed to the enhanced stacking fault energy, by which workhardening rate before the onset of dynamic recrystallisation is diminished significantly with the reinforced cross-slip process. The hot deformation activation energy is decreased under the condition of higher Mn addition, while the strain rate sensitivity of flow stress almost remains unchanged.     

Fe-8Mn-3Al-0.2C轻质高强钢的热变形行为

为了探究Fe-8Mn-3Al-0.2C轻质高强钢的热变形行为,在变形温度为1 123~1 423 K,应变速率0.01,0.1,1,10 s-1,真应变为0.6的条件下利用Gleeble-1500热模拟实验机进行热压缩模拟实验,通过实验机记录温度、真应力与真应变的关系,观察组织形貌演变规律.结果表明:流变应力曲线分为3个阶段,即加工硬化、动态软化及稳定流变应力;当变形温度升高和应变速率下降时,峰值应力及其所对应的临界应变减小,说明更容易发生动态再结晶;在变形初期ε0.1时,流变应力曲线出现应变增加而应力几乎保持不变的类屈服平台;压缩后的组织为奥氏体/铁素体双相组织,动态再结晶先在铁素体内部发生,随后由奥氏体承担;随着变形温度的升高和应变速率的下降,晶粒尺寸细化并趋于均匀,说明动态再结晶完成的更充分;本实验钢在本文处理工艺及0.6真应变下的最佳热加工工艺参数区间为1 250~1 400 K,应变速率为0.03~0.3 s~(-1);受合金元素影响,实验用钢的表观应力指数和热变形激活能分别为4.588 9和250.6 k J/mol,本构方程为ε·=6.20×10~9[sinh(0.009σ)]~(4.588 9)exp(-(250 601)/(8.314T)).     

Constitutive modeling for hot deformation behavior of T24 ferritic steel

Hot compression tests of T24 ferritic steel were carried out using Gleeble-3500 thermo mechanical simulator in the temperature range of 1323-1473 K with the strain rate of 0.01-10 s⁻¹ and the height reduction of 60%. The flow behavior of T24 ferritic steel was characterized based on analysis of the true stress-strain curves. Constitutive equations incorporating the effects of temperature, strain rate and strain have been developed to model the hot deformation behavior of T24 ferritic steel. Material constants α, n, ln A and activation energy Q in the constitutive equations were calculated as a function of strain. The flow stress values of T24 ferritic steel predicted by the proposed constitutive equations show a good agreement with experimental results, which indicated that the developed constitutive equations could give an accurate and precise prediction for the flow stress of T24 ferritic steel.     

Dynamic recovery: sufficient mechanism in the hot deformation of Al (<99.99)

A recent OIM study of the substructure in hot compressed Al has observed an increase in the fraction of boundaries both of 15–20° and above 20° as strain rises from 0.9 to 1.5. This was interpreted as evidence of continuous dynamic recrystallization being the mechanism for the steady state deformation. However, when the original grain boundaries and transition boundaries between deformation bands are discounted, the fraction of 15–20° boundaries is reduced to less than 20% and would be much lower if subboundaries less than 0.5° visible in TEM were taken into account. The present authors argue that dynamic recovery maintains the subgrains of constant size, low misorientation and equiaxed to produce a steady state and can permit a limited number of discrete segments with higher misorientation notably as temperature falls. Moreover, continuous dynamic recrystallization is not appropriate terminology because it is far from reaching the completion observed in other instances of continuous recrystallization.     

Constitutive relationship and hot deformation behavior of Armco-type pure iron for a wide range of temperature

The hot deformation behavior and constitutive relationship of Armco-type pure iron were investigated using isothermal compression tests with a wide range of temperature and strain rate ranging from 923 to 1523 K, and 0.1 to 10 s⁻¹, respectively. When deformed with a single phase, the flow stress of Armco-type pure iron increases accompanied by the increase of strain rate and the decrease of deformation temperature. Instability phenomenon of Armco-type pure iron appears when deformed with dual phase. γ-Fe undergoes completed discontinuous dynamic recrystallization (dDRX) at all hot deformation conditions. α-Fe undergoes uncompleted dDRX process at high temperature and low strain rate, however, dynamic recovery (DRV) process is the main restoration process for α-Fe at low temperature and high strain rate. The modified Arrhenius-type constitutive equation considering strain compensation is used to describe the flow stress of γ-Fe and α-Fe. From correlation coefficient (R), root mean square error (RMSE) and average absolute relative error (AARE), the predictability of the constitutive equation for the two phases of Armco-type pure iron was evaluated.     

Prediction of the flow stress of Al6061 at hot deformation conditions

Isothermal compression tests were carried out on Al6061 using a Gleeble-1500 thermal simulator at temperatures ranging from 573 to 723 K and strain rates from 0.5 to 30 s⁻¹. The flow stress of Al6061 was characterized based on an analysis of the true stress-true strain curves. A mathematical mode coupling flow stress with strain, strain rate and temperature for Al6061 has been proposed by using a hyperbolic sinusoidal type equation. The material constant α is 0.01 MPa⁻¹ in the model, whereas other material constants n, lnA and Q are found to be functions of strain. The predicted results from this proposed model are found to be in good agreement with the experimental flow stress curves which can be used to predict the required deformation forces in hot deformation processes.     

Constitutive analysis of Mg–Al–Zn magnesium alloys during hot deformation

The constitutive behaviors of Mg–Al–Zn magnesium alloys during hot deformation were studied over a wide range of Zener–Hollomon parameters by consideration of physically-based material’s parameters. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135 kJ/mol) can be used in the hyperbolic sine law to describe the flow stress of AZ31, AZ61, AZ80, and AZ91 alloys. The apparent hyperbolic sine exponents of 5.18, 5.06, 5.17, and 5.12, respectively for the AZ31, AZ61, AZ80, and AZ91 alloys by consideration of deformation activation energy of 135 kJ/mol were consistent with the considered theoretical exponent of 5. The influence of Al upon the hot flow stress of Mg–Al–Zn alloys was characterized by the proposed approach, which can be considered as a versatile tool in comparative hot working and alloy development studies. It was also shown that while the consideration of the apparent material’s parameters may result in a better fit to experimental data, but the possibility of elucidating the effects of alloying elements on the hot working behavior based on the constitutive equations will be lost.     

形变温度对Fe-20Mn-3Cu-1.3C TWIP钢拉伸变形行为的影响EI北大核心CSCD

运用温控拉伸实验,分析了在-100~200℃范围内变形时形变温度对Fe-20Mn-3Cu-1.3C钢力学性能和形变机理的影响。观察分析了拉伸试样的显微组织,并利用热力学经典模型,估算了温度对孪晶诱发塑性(TWIP)钢层错能的影响。结果表明:随着形变温度的升高,TWIP钢的层错能显著增加,基体中形变孪晶的体积分数逐渐减少,抗拉强度和屈服强度呈下降趋势,而伸长率先升高后降低,塑性变形机制也由孪生为主逐渐转变为以滑移为主。层错能的拟合公式为γSFE=26.73+9.38×10^(-2) T+4.22×10^(-4 )T2-4.47×10^(-7) T^3,与滑移相比,孪生可获得更高的应变硬化率,从而使TWIP钢获得高强度和高塑性。     

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s^?1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.     

Recrystallization of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation. Part I: Dynamic recrystallization

This paper presents an investigation that characterizes the evolution of the dynamically recrystallized structure of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation, as a starting point for studies of the static recrystallization (SRX) and the metadynamic recrystallization (MDRX) behaviors, by hot compression tests which are performed at the temperatures from 1243 K to 1543 K and strain rates from 0.001 s⁻¹ to 0.1 s⁻¹ on Gleeble-3500 thermo-mechanical simulator, and the corresponding flow curves are obtained. A third-order polynomial is then fitted to the work hardening region of each curve. The critical stress for initiation of dynamic recrystallization (DRX) can be calculated by setting the second derivative of the third order polynomial. By regression analysis, the activation energy in whole range of deformation temperature is determined to be Q = 368.45 kJ/mol. The complete DRX grain size (D_drx) of the test steel is a function of Zener-Hollomon parameter (Z) and is independent of the true strain. The relationship of D_drx and Z is found to be described in a form of power law function with an exponent of −0.24.     

Constitutive modeling and prediction of hot deformation flow stress under dynamic recrystallization conditions

Simple modeling approaches based on the Hollomon equation, the Johnson–Cook equation, and the Arrhenius constitutive equation with strain-dependent material’s constants were used for modeling and prediction of flow stress for the single-peak dynamic recrystallization (DRX) flow curves of a stainless steel alloy. It was shown that the representation of a master normalized stress–normalized strain flow curve by simple constitutive analysis is successful in modeling of high temperature flow curves, in which the coupled effect of temperature and strain rate in the form of the Zener–Hollomon parameter is considered through incorporation of the peak stress and the peak strain into the formula. Moreover, the Johnson–Cook equation failed to appropriately predict the hot flow stress, which was ascribed to its inability in representation of both strain hardening and work softening stages and also to its completely uncoupled nature, i.e. dealing separately with the strain, strain rate, and temperature effects. It was also shown that the change in the microstructure of the material at a given strain for different deformation conditions during high-temperature deformation is responsible for the failure of the conventional strain compensation approach that is based on the Arrhenius equation. Subsequently, a simplified approach was proposed, in which by correct implementation of the hyperbolic sine law, significantly better consistency with the experiments were obtained. Moreover, good prediction abilities were achieved by implementation of a proposed physically-based approach for strain compensation, which accounts for the dependence of Young’s modulus and the self-diffusion coefficient on temperature and sets the theoretical values in Garofalo’s type constitutive equation based on the operating deformation mechanism. It was concluded that for flow stress modeling by the strain compensation techniques, the deformation activation energy should not be considered as a function of strain.     

应变速率对TWIP钢Fe-23Mn-2Al-O.2C力学性能的影响

开展了固溶处理后TWIP钢Fe-23Mn-2Al-0.2C的拉伸实验,研究了应变速率对其拉伸变形行为的影响.结果表明,当应变速率在2.97×10-4-1.49×10-1s-1范围内变化时,钢的屈服强度没有明显变化,随着应变速率增大,抗拉强度稍有降低,延伸率明显减小.当应变速率较低时,其加工硬化速率随着真应变呈现三个阶段...     

Hot deformation behaviour and constitutive modelling of P92 heat resistant steel

Abstract

Hot compression tests are conducted in the present paper to investigate hot deformation behaviour of the newly developed heat resistant steel P92, which is used to fabricate main steam pipes for ultra supercritical power plants. Stress–strain curves at elevated temperatures and different strain rates are obtained. It is found that dynamic recrystallisation happens only when the temperature is above 1100°C and strain rate is below 0·1 s^?1. Otherwise, dynamic recovery is the main softening mechanism. Constitutive modelling with the hyperbolic sine, including an Arrhenius term, is used to predict peak and saturated stresses. Material constants for this model are determined. Results show that the model can be used to predict peak and saturated stresses. However, the model would fail in predicting flow stress with respect to strain; thus, a model containing nine independent parameters and the complete form of Spittel equation are utilised to predict flow stress curves softened by dynamic recrystallisation and dynamic recovery respectively since there are no unified equations. The predicted stress–strain curves are in good agreement with experimental results, which confirmed that the models developed in the present paper are effective and accurate for P92 steel.     

Artificial neural network modelling to predict hot deformation behaviour of as HIPed FGH4169 superalloy

Abstract

The hot deformation behaviour of as HIPed FGH4169 superalloy was studied by single stroke compression test on MMS-200 test machine at the temperatures of 950–1050°C and the strain rates of 0·004–10 s^?1. Based on the experimental results, a back-propagation artificial neural network model and constitutive equation method were established to predict the flow stress of FGH4169 superalloy. The predictability of two different models was compared. The correlation coefficients of experimental and predicted flow stress with the trained BP ANN model and constitutive equation were 0·9995 and 0·9808 respectively. The average root mean square error (RMSE) values of the trained ANN model and constitutive equation are 0·39 and 2·21 MPa respectively. And the average absolute relative error (AARE) values of the trained ANN model and constitutive equation are 1·79 and 7·47% respectively. The results showed that the ANN model is an effective tool to predict the flow stress in comparison with constitutive equation.     

Improved tensile properties of partially recrystallized submicron grained TWIP steel

The effects of cold rolling reduction and annealing temperature on the mechanical properties of twinning induced plasticity (TWIP) steel have been investigated. The results indicated that the strengthening effect of unrecrystallized areas with a high density of nano-scale mechanical twins increased with increasing cold rolling reduction. In addition, the ductility also increased with increasing annealing temperature. Therefore, utilization of large cold rolling reduction and subsequently annealing treatment in the partial recrystallization region was suggested as an effective method to obtain submicron grained TWIP steel with an excellent combination of strength and ductility.     

Influence of B and Ti on hot ductility of high Al and high Al,Nb containing TWIP steels

Abstract

The addition of ～0·002%B and ～0·04%Ti as microalloying additions to improve the poor hot ductility and high risk of cracking on continuous casting of high Al containing twinning induced plasticity (TWIP) steels has been examined. Tensile specimens were either cast in situ or heated to 1250°C before cooling at 60 K min^?1 to test temperatures in the range 700–1100°C and strained to failure at 3×10^?3 s^?1. For tensile specimens reheated to 1250°C, the presence of B with sufficient Ti to combine with all the N improved ductility over the temperature range of 700–950°C, the reduction in area (RA) values being >40%. For the higher strength more complex high Al, TWIP steels having Nb present, there was no improvement in ductility with a similar B and Ti addition, when the average cooling rate after melting to the test temperature was 60 K min^?1. Reducing the cooling rate to 12 K min^?1 resulted in the RA values being close to the minimum required to avoid transverse cracking throughout the temperature range 800–1000°C. Using these additions of B and Ti, transverse cracking was found not to be a problem when continuously casting these high Al containing TWIP steels.     

Deformation behaviour of a low carbon high Mn TWIP/TRIP steel

TWIP and TRIP phenomena have been observed in Fe–20Mn–2.5Si–0.3Al–0.06C (wt-%) steel during cold deformation (CD). Mostly austenite, annealing twins and stacking faults are observed in hot rolled solution treated (HRACST) samples. Cold deformation results in γ→?→α′ transformation due to its low stacking fault energy (18?mJ?m^?2). 50CD (50% CD) sample reveals strong Goss, Brass and weak Copper Twin texture components due to slipping and micro-twinning. Maximum ductility of 62% with lower tensile strength is perceived in HRACST sample, whereas, 30CD sample shows excellent tensile strength (1039?MPa) with a lower ductility (23%). Premature failure in 50CD samples is related to the formation of α′-martensite (≈35%) and deformation twins along with a higher strain hardening due to low Al content.     

A modified Zerilli–Armstrong constitutive model to predict hot deformation behavior of 20CrMo alloy steel

True stress and true strain values were obtained from isothermal hot compression tests conducted on a Gleeble thermal simulation machine, in a wide range of temperatures (1173–1373 K) and strain rates (1.5 × 10⁻³–1.5 × 10⁻² s⁻¹). The experimental data were used to develop a modified Zerilli–Armstrong constitutive model. The predicted flow stresses using the developed model were compared with experimental values. A correlation coefficient (R) of 0.989 and an average absolute relative error (AARE) of 7.71% between the measured and calculated flow stresses have been obtained. Comparing with a modified Johnson–Cook model developed in the authors’ previous study, the accuracy, the number of material constants involved and the computational time required of the model were evaluated.     

低周疲劳变形过程中Fe-33Mn-4Si合金钢的微观组织演变EI北大核心CSCD

借助X射线衍射和电子背散射衍射,研究低周疲劳变形过程中Fe-33Mn-4Si合金钢的微观组织演变及其对力学行为的影响。结果表明:实验用钢的原始微观组织由奥氏体和热诱发ε马氏体两相组成。原始组织通过影响变形过程中ε马氏体相变来影响实验用钢的低周疲劳变形行为。在变形初期(100周次内),随循环周次增加,ε马氏体含量迅速增加并且马氏体不同变体之间频繁相互交叉作用,使实验用钢的平均峰值应力和循环加工硬化程度快速增加;随后至疲劳断裂,ε马氏体成为变形微观组织中主要组成相,ε马氏体含量和马氏体不同变体的交叉频次随循环周次的增加而增速放缓,导致平均峰值应力和循环加工硬化程度的增速也明显减缓。     

Influence of B on hot ductility of high Al,TWIP steels

Abstract

The influence of B on the hot ductility of high Al, Ti containing twinning induced plasticity (TWIP) steels has been examined. It was established that provided the B was fully protected by adding sufficient Ti to combine with all the N, then B could segregate to the austenite grain boundaries and improve ductility. This improvement was particularly marked for the temperature range of 700–900°C, the range in which the straightening operation often takes place in continuous casting. Of most importance in the present work has been the detection of B at the boundaries using a secondary ion mass spectrometry technique. The cooling rate from the reheating temperature of 1250°C to the tensile testing temperature range of 700–1200°C was 60 K min^?1, but it is likely that slower cooling rates ≤25 K min^?1, more in keeping with the secondary cooling rate on continuous casting, will give even better ductility. Ti additions in themselves are beneficial to the hot ductility of these steels as precipitation of AlN at the austenite boundaries is avoided, but only if the cooling rate is sufficiently slow to allow the TiN particles to coarsen. However, to ensure freedom from cracking, an addition of B is also required.