Materials Science Forum Vol. 941

Abstract: Recently, due to the requirements of lightweight and safety, the grade of 980MPa high-strength steel has the demand of high hole expansibility and high yield strength. Due to the large difference of hardness between the soft ferrite and hard martensite, the traditional DP980Y dual phase steel has poor hole expansibility. In order to improve the hole expansibility of DP980Y dual phase steel, the best way is to modify the microstructure into a single-phase to eliminate the large difference of hardness. In this paper, the steel of nearly full bainite microstructure with small amount of ferrite and M/A constituents was studied. Compared to the DP980Y dual phase steel, it was found that this modified steel with a single-phase microstructure has the same grade of 980MPa of tensile strength, but can achieve the demand of higher yield strength and hole-expansion ratio. This study shows reducing the amount of ferrite can increase the homogeneity of matrix with the single phase to improve the hole expansibility. In addition, the use of lower bainite transformation temperature and lower carbon content has the higher hole-expansion ratio due to the less amount of M/A constituents.

Abstract: High strength and sufficient toughness are key requirements for modern high-performance structural steels. In an attempt to develop a suitable estimation of impact toughness transition temperatures for as-quenched steels, we investigated the determiners of low-temperature toughness with a group of thermomechanically rolled direct-quenched steels with varying martensite contents. These were produced by altering chemical composition, finish rolling temperature and reduction below the non-recrystallization temperature, i.e. austenite pancaking, and characterised in terms of microstructural constituents, grain size distributions, texture and fractography. Provided the finish rolling temperature is high enough to avoid the formation of granular bainite on subsequent cooling, high levels of austenite pancaking yield the best combinations of low-temperature toughness and strength by effectively refining the size of the coarsest grains and randomizing the texture. While absolutely no direct correlation is found within as-quenched steels between the impact toughness transition temperatures and yield strength alone, T_28J and T₅₀ do closely follow a dynamic reference toughness, i.e. the opening stress intensity factor defined by yield strength and the size of the coarsest grains in the effective grain size distribution. This parameter reflects the transition temperatures – the lower the temperature, the lower the reference toughness needed to cause a local brittle fracture. Finally, we show that the impact toughness transition temperatures T_28J and T₅₀ of as-quenched steels can be accurately estimated, irrespective of the test specimen orientation, by utilizing just the dynamic reference toughness and the fraction of {100} cleavage planes within ± 15° of the specimen notch plane.

Abstract: The research purpose is to upgrade the mathematical modelling and computer simulation of quenching of steel. Based on theoretical analyze of physical processes which exist in quenching systems the mathematical model for steel quenching is established and computer software is developed. The mathematical model of steel quenching is focused on physical phenomena such as heat transfer, phase transformations, mechanical properties and generation of stresses and distortions. Physical properties that were included in the model, such as heat conductivity coefficient, heat capacity and surface heat transfer coefficient were obtained by the inversion method based on Jominy test results. The numerical procedure is based on finite volume method. By the developed algorithm, 3D situation problems such as the quenching of complex cylinders, cones, spheres, etc., can be simulated. The established model of steel quenching can be successfully applied in the practical usage of quenching.

Abstract: The effect of shot blasting on the bendability of two tempered trial ultrahigh-strength steels has been studied by comparing the bending behaviour of otherwise identical plates with and without shot blasting. The yield strength of the studied 10 mm thick trial steel was 700 MPa and 7.5 mm trial steel 1100 MPa. The local microstructures below the different surfaces were characterized using laser scanning confocal microscopy and FESEM. Microhardness profiles and surface roughness (R_a and R_z) were measured and bendability was determined using three-point brake press bending. Shot blasting was found to decrease the bendability of both steels. For the 700 MPa steel shot blasting increased the surface roughness metric R_a substantially from 7.4 μm with the normal scale surface to 12.6 μm, and for the 1100 MPa steel from 2.8 μm to 6.8 μm. For the 700 MPa steel with the bend axis parallel to rolling direction, the minimum usable punch radius for defect-free bends with normal scale surfaces was 13 mm (1.3 x thickness) and for the 1100 MPa steel, 14 mm (1.9 x thickness), while for shot blasted surfaces corresponding values were respectively 20 mm (2.0 x thickness) and 18 mm (2.4 x thickness). All of these values are excellent considering the strength levels involved. Shot blasting increased the subsurface hardness and introduced work hardening of surface layers. Possible explanations for the observed effects of shot blasting on the bendability are discussed.

Abstract: 9Ni steel is a low temperature serving ferrite steel, providing high strength and excellent low temperature toughness, which could serve well at-196°C. Therefore 9Ni steel is widely used in storage tanks and transport ships for liquefied natural gas (LNG). Nevertheless there are some challenges in the industrial application, such as hot cracking, cold cracking, magnetic arc blow, etc.. In this paper, the study on the welding technology of 9Ni steel developed by Baosteel is carried out. Firstly the weldability is analyzed through welding thermal simulation using Gleeble 3500 system, Y-groove cracking test, maximum hardness in weld heat-affected zone test. The results prove that 9Ni steel could be welded without preheating. The welding consumables have also significant influence on the performance of the welded joints. In this paper the characteristics of different types of welding consumables, including ferrite base type, austenitic stainless steel type, Ni-base alloy type and Fe-Ni base alloy type are analyzed, then the selecting principle for welding consumables is proposed. Furthermore welding process experiments are undertaken using various welding procedures such as SMAW, GTAW, FCAW and SAW. The results indicate that heat input and interpass temperature should be controlled to ensure a sound weld joint. Finally fracture toughness at-196°C of 9Ni steel and its joint is studied using CTOD test. In conclusion, 9Ni steel developed by Baosteel has good weldability and can meet the requirements of industrial application.

Abstract: Double annealing of low carbon medium Mn steel was studied. The second intercritical annealing was done at 650°C within a range of holding time: 3min to 30h. Tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Furthermore, a model, based on the mixture law combined with the considerations of equivalent increment of work in each microstructural constituent during mechanical loading, was proposed. The individual mechanical behavior of each considered microstructural constituent was described with the approaches existing in the literature. The complete model shows a very good agreement with the experimental stress-strain curves and predicts well the optimum strength-ductility balance after 2h holding.

Abstract: A steel containing Fe-0.2C-2Mn-Si-Al was annealed using intercritical Q&P cycles. Quenching temperature and partitioning time at 440°C was varied. Analysis of microstructure evolution during such Q&P treatments was performed using different characterization tools: FEG-SEM, EBSD, dilatometer and saturation magnetization measurements. Especially, phenomena taking place during partitioning were investigated. The microstructure was analyzed at the end of soaking, before and after partitioning and at the end of the annealing cycle. It was found that bainite transformation happens during partitioning and has a significant effect on the final retained austenite fraction. Quenching temperature has an important effect on both martensitic transformation during cooling and subsequent bainite transformation during partitioning.

Abstract: The use of duplex stainless steels (DSSs) is steadily increasing. For many uses where joining is needed, gas tungsten arc welding (GTAW) is one of the most important joining methods for DSSs. Since hydrogen embrittlement (HE) occasionally occurs in DSSs, understanding the relationship between the extent of HE and the welding condition is crucial to prevent HE. In this research, the effect of the heat input of GTAW process on the microstructure and the extent of HE in a UNS S31260 (JIS SUS329J4L) has been investigated. For this purpose, three samples have been prepared with diffrent velosity. All the samples have been cathodically hydrogen-charged, and then subjected to tensile test at a strain rate followed by fractography observation. Thermal desorption spectroscopy (TDS) has been carried out on the samples welded at low and average velosities. The results showed that tensile properties of the welded specimens were lower than those of base metal due to coarsening of the matrix ferrite grains and loss in the fraction balance of ferrite and austenite phases in the weld metal zone, where fracture took place.

Abstract: Due to increasingly stringent regulations governing fuel economy and emissions, new technological developments towards automobile efficiency are in play including lightweighting by reducing weight of the structural components. The historical development of steel grades for autos has resulted in strength increases, including in recent advanced high strength steels (AHSS), but it has come with commensurate decreases in ductility and formability. NanoSteel 3^rd Generation AHSS overcomes the trade-off in ductility due to novel structural changes during cold deformation through a complex Nanophase Refinement and Strengthening (NR&S) mechanism leading to material strengthening. During stamping, the ability of a steel blank to be formed into complex parts is paramount and it has to retain sufficient ductility for energy absorption during a subsequent crash event. In this paper, the specific characteristics of the NR&S mechanism in two NanoSteel grades will be detailed including structure and property changes during stamping analyzed by utilizing destructive and non-destructive approaches to predict localized yield strength changes in the final stampings.

Abstract: A cold forging process of Mo-alloyed sintered steel was simulated by finite element method (FEM) analysis considering density change in the process. Moreover, the effect of sintering time on the behavior of the densification and the plastic deformation of it in the cold-forging process was also investigated. Using the true stress-true strain diagram obtained by the compression test with a sintered specimen, the modified true stress-true strain diagram was derived for large plastic deformation analysis with the porous material model. The result of FEM analysis for the cold compression process of the sintered specimen revealed that the analysis can simulate the shape of the excessive metal part and density change of it. Also, it was found that local deformation becomes large and thus the excessive metal part extends with increasing sintering time although the difference in the true stress-true strain diagrams is negligible.