Anisotropy of bulk forming limits in hot-rolled steel bars

The anisotropy of bulk formability in hot-rolled AISI 1040 steel bars has been evaluated using upset and bend tests in longitudinal, circumferential, and radial directions. The index of workability is improved approximately 65 pct by aligning the critical tensile stresses parallel to inclusion plane and elongation (longitudinal direction). The lowest workability is attained in the radial direction where tensile stresses are perpendicular to the plane of inclusions. In the circumferential direction, tensile stresses are parallel to the plane of inclusions, though perpendicular to inclusion elongation, leading to intermediate workability. Tensile ductilities in the principal directions are compared with the three workability levels. Tension fracture data agree with the forming limit line in the radial direction. In the other directions, however, the tension test progressively overestimates workability as the workability level increases.     

Hydrogen-assisted ductile fracture in spheroidized 1518 steel

Hydrogen was introduced into smooth and notched tensile specimens of spheroidized 1518 steel by electrocharging under controlled galvanostatic conditions. The role of hydrogen during void nucleation and growth was investigated by identifying two different void nucleation modes. Hydrogen promoted void nucleation at average-sized carbide particles by reducing the critical interfacial strength, σ_C, from 1200 to 1000 MPa (using a dislocation model). The stress-induced hydrogen segregation to the particle interfaces during deformation was estimated to explain the hydrogen-reduced σ_C. Void growth in both longitudinal and lateral directions was enhanced by internal pressurization of hydrogen. In order to better quantify such hydrogen-enhanced void growth, the internal hydrogen pressure inside a void was calculated on the basis of thermodynamics. The final void coalescence stage was analyzed by assuming that the void nucleation rate follows a normal distribution.     

Evaluation of the fracture toughness of hot-rolled low-alloy Ti-V plate steel

The effects of varying finish rolling temperature (FRT) and cooling rate on the mechanical properties of hot rolled plates of an experimental low-alloy Ti-V steel were studied. Fracture toughness was evaluated for various types of specimens at slow and high deformation rates. However, the transition temperatures determined by the various tests do not always correlate. Therefore, it is recommended that fracture toughness be evaluated by both static and dynamic testing. Following a low cooling rate, the best plate properties are obtained at the lowest FRT in the austenite-ferrite range, although occurrence of delaminations at this temperature may be detrimental for specific applications. Higher cooling rates produce higher strength but lower toughness than lower cooling rates in plates with the same FRT.     

Cavity formation from inclusions in ductile fracture of A508 steel

Experiments were performed on A508 class 3 steel in order to determine the conditions for cavity formation from elongated MnS inclusions. Circumferentially notched tensile specimens were employed in order to investigate the effect of negative hydrostatic pressure. The results of finite element calculations and metallographic observations on polished sections were used to evaluate the conditions required for cavity initiation. Tests were performed at different temperatures in both longitudinal and short transverse direction. The results can be explained in terms of a local critical stress independent of temperature. This local stress is tentatively calculated using an extension of Eshelby’s theory for inclusions proposed by Berveiller and Zaoui.     

Effect of prestrain on the ductile fracture behavior of an interstitial-free steel

The effect of tensile prestrain on the ductile fracture behavior of an interstitial-free (IF) steel has been studied using primarily (1) the analysis of void density by optical microscopy, (2) characterization of the size of dimples by scanning electron microscope (SEM) and image analyzer, and (3) estimation of strain hardening behavior of a series of prestrained tensile specimens, loaded until fracture. The variation of void density with local plastic strain around the necked region of the specimens indicated the existence of two types of void nucleation pertaining to inclusions and precipitate particles. The critical strain for void nucleation (ε{inn}) for the precipitate particles initially increases and then decreases with the increase in percentage prestrain. This phenomenon has been explained using the strain hardening exponent and nature of dislocation-particle interaction. The nature of variation of the average size of the dimples and that of ε{inn} with prestrain are found to be similar. The dimple size thus bears a proportional relationship with the void, nucleation strain ε{inn} and hence the former can be used to predict (ε{inn}) for IF steel.     

热轧带钢力学性能预测模型及其应用

为实现对热轧带钢的屈服强度、抗拉强度、断裂延伸率等力学性能的预测及控制,利用人工神经网络技术,分别建立了根据生产工艺参数预测力学性能的质量模型,以及根据力学性能要求对生产工艺参数进行优化的逆质量控制模型.利用质量预测模型,分析得出屈服强度随卷取温度的上升而下降的变化规律,进而可以对组织性能进行在线调整,实现在线应用.     

Hydrogen-assisted ductile fracture in spheroidized 1520 steel: Part II. Pure bending

The effects of hydrogen on ductile fracture were investigated in a spheroidized steel similar to AISI 1520, containing negligible amounts of P and S, using notched bend bar tests. The effects of hydrogen were exhibited either in the form of enhanced plastic instability along characteristic slip traces in mode II or of enhanced, strain-controlled, local crack tip processes. The contri-bution of enhanced plastic instability, however, was only apparent under conditions in which flow localization also occurred without hydrogen. The role of plastic instability near the crack tip was found to be dominant in these bend bar tests, whereas it was small or negligible in previous tests in axisymmetric tension. Microstructural effects were rationalized in terms of a critical, local concentration of hydrogen. The intrinsic effect of hydrogen appeared to be the enhancement of strain-controlled fracture processes. Formerly with Carnegie Mellon University.     

热轧不锈钢板酸洗工艺

为去除热轧不锈钢板表面的氧化层与贫铬层,钝化基体,改善表面质量,在热轧退火后需进行酸洗处理。介绍了热轧不锈钢板表面氧化层和贫铬层的组成与结构特征及其对酸洗过程的影响,并对热轧不锈钢板的酸洗体系设计和酸洗工艺开发进行了阐述。中国热轧不锈钢板普遍采用前处理(机械除鳞、中性盐电解等)→硫酸(电解)预酸洗→硝酸-氢氟酸终酸洗的酸洗工艺,旨在通过前处理和预酸洗去除大部分的氧化层,在终酸洗阶段去除残余氧化层及贫铬层,同时实现基体钝化。目前,热轧不锈钢板的酸洗工艺存在酸洗效果不稳定、贫铬层去除不完全、局部腐蚀较为严重等问题,且会产生不同程度的环境污染问题。应该根据不同钢种不同退火工艺条件下的表面氧化层与贫铬层的性质与分布特征,研究开发相应的酸洗体系和酸洗工艺。热轧不锈钢板的环保型酸洗新工艺具有广阔的发展前景。     

The influence of strength and phosphorus segregation on the ductile fracture mechanism in a Ni-Cr steel

Upper-shelf toughness and its degradation through ageing at 500°C have been investigated in quenched-and-tempered 300 M steel as a function of tempering treatment at 650°C. Material having a 0.2% proof stress in the range 550–1000 MPa was examined. Toughness was assessed using the parameters J_Ic and reduction-in-area, and was related to the ductile fracture mechanism through measurements of the strain required for void nucleation and void growth to coalescence. Changes in matrix rheology, void-nucleating precipitate morphology, bulk chemistry and interfacial chemistry were monitored during tempering and ageing, and the associated fractography was quantitatively assessed. The ductile fracture process in unaged material was dominated by the strain required for void nucleation on carbide precipitates. Nucleation strain increased with tempering time at 650°C causing a rise in ductility. Ageing at 500°C produced a loss of ductility for all temper conditions, and the sole cause of this effect was the segregation of phosphorus to carbide-matrix interfaces, identified by high resolution Auger spectroscopy. Both the strain required for void nucleation at carbides and that for void growth to coalescence were suppressed by ageing, through a reduction in interfacial cohesion consistent with the embrittling effect of segregated phosphorus.     

Improving the controlled cooling of hot-rolled steel coils

The thermostatting of alloy-steel coils after hot rolling in a continuous broad-strip mill is considered. A technological chart of slow controlled cooling of the coils is developed. Measures are proposed for the production of high-quality and highly profitable steel coils without additional capital expenditures or operating costs.     

Strain instability and fracture at the surface of upset cylinders

A refined method has been developed for measurement of the localized surface strains during axial compression of cylinders. The method has been applied to 1045 steel cylinders in various conditions to study fracture under combined stress states. Because the refined measurement method is used, perturbations are observed in the strain path which would otherwise not be apparent. These perturbations precede surface fracture in every case. It is shown that geometric similarities exist between the strain perturbations in upsetting and those in trough formation preceding fracture in stretch forming of sheet material. In addition, metallographic examination of transverse sections of the upset cylinders reveals the presence of subsurface void formations prior to fracture. It is concluded that the surface fractures in upset cylinders involve localized necking of material between inhomogeneities, and it is suggested that an accurate model of this behavior may give some understanding of the influence of material parameters on fracture.     

Hydrogen-assisted ductile fracture in porous iron

The present paper has studied the effect of porosity and hydrogen on the deformation and fracture properties in porous iron using double notched specimen tests and quantitative metallography analyses to characterize void growth. The presence of hydrogen produced the greatest hardening effect in the sample with a porosity of 0.037. It was found that while the effect of hydrogen on the reduction in area was greatest in the specimen with a porosity of 0.037, its effect on the fracture energy did not depend on the porosity. The ductile fracture surface morphology remained the same in all the specimens without and with hydrogen except for the sample with porosity of 0.003 where hydrogen induced some quasi-cleavage fracture. The quantitative metallography analysis of voids in the region near the fracture surface and below the unfractured notch has demonstrated that hydrogen causes the void growth to increase prior to plastic instability. The effect of hydrogen on the void growth was found to be slightly enhanced as the porosity was increased. The microscopic process of hardening induced by hydrogen is presented. The mechanism for hydrogen-assisted void growth is discussed in light of a recently developed model using a dislocation theory.     

Improving the strength characteristics of hot-rolled steel 10KhSND

Central Scientific-Research Institute of Ferrous Metallurgy. Kommunarsk Mining-Metallurgical Institute (KGMI). Kommunarsk Metallurgical Combine. Translated from Metallurg, No. 10, pp. 36–37, October, 1989.     

用于延性断裂韧性测试的载荷分离方法与应用

使用载荷分离理论的延性断裂韧性单试样方法,选取汽轮机转子钢Cr2Ni2MoV对规则化法和S_Pb分离参数法进行了研究.结果表明,在规则化法中,钝化修正时的钝化线方程仅对裂纹扩展较小时的J阻力曲线产生影响.提出了改进S_Pb分离参数法,消除了参考钝裂纹试样的影响;同时采用钝化修正后的初始裂纹长度和试验终止裂纹长度进行标定,可获得合理的裂纹长度预测结果.由改进S_Pb分离参数法得到J阻力曲线在较小裂纹扩展范围内均略高于由规则化法得到的J阻力曲线,而当裂纹产生扩展较大时,两种方法得到的J阻力曲线几乎完全重合;由规则化法得到的条件启裂J积分J_Q值偏于保守.