Quasi‐in‐situ observations of the sintering behaviour of molybdenum‐alloyed sintered steels

The sintering behaviour of molybdenum pre‐alloyed sintered steels was investigated. A scanning electron microscope with a hot stage attachment was used, enabling quasi‐in‐situ observations of the sintering process. The material MSP 1.5Mo (Fe‐1.5% Mo) sinters in the austenitic phase, while MSP 3.5Mo (Fe‐3.5% Mo) sinters completely in the ferritic phase, due to the increased molybdenum mass content. The significantly higher self‐diffusion coefficient of iron in the ferritic phase leads to the accelerated sintering of MSP 3.5Mo, compared to MSP 1.5Mo. The effect on the sintering behaviour by adding the alloying elements chromium and phosphorus was also studied. While phosphorus accelerates sintering processes, chromium decreases the sintering rate. A pronounced shrinkage during non‐isothermal heating, due to cooperative particle movement, was detected for both sintered steels.     

Improvement in toughness of Fe-Cr-Mn-C steels by thermal-mechanical treatments

High-Cr (about 10 wt pct) Fe-Cr-Mn-C microcomposite lath martensite-austenite structural steels have been developed in order to achieve high strength and high toughness for applications in corrosive environments. Processing by controlled hot rolling and air cooling produces a finegrained alloy with excellent toughness. The alloys are air hardenable, and the microstructure consists of lath martensite packets with retained austenite around the laths. The laths contain fine intralath autotempered carbides. The mechanical properties of the steel so produced are found to be superior to those treated by conventional methods of single or cyclic austenitization treatment. Optical metallography, transmission electron microscopy and scanning electron microscopy (SEM) have been used to characterize the effect of various process variables on the mechanical properties. R. RAMESH and N.J. KIM, formerly with the Department of Materials Science and Mineral Engineering, University of California at Berkeley     

Simulation of intercritical annealing in low‐alloy TRIP steels

A coupled thermodynamic/kinetic calculation of austenite formation during intercritical annealing of low‐alloy TRIP steels is presented. The simulation was performed with the use of Dictra computational kinetics software, which employs a procedure for the numerical solution of the coupled diffusion equations involved, as well as mobility databases for the retrieval of the appropriate kinetic data. Calculated results are compared with available experimental data, in order to evaluate the model. Simulation results, regarding the amount and composition of austenite, the rate of transformation and the effect of annealing temperature and heating conditions, are presented and discussed. It is concluded that the simulation can assist the design of the intercritical annealing in these steels.     

The bake hardening behaviour of electro‐galvanized cold rolled CMnSi and CMnAlSi TRIP steels

Cold rolled, intercritically annealed TRIP steels will very likely be used as electrogalvanized sheet steel for passenger safety‐related automotive body parts. This implies that their properties during and after manufacturing, i.e. in the deformed and paint‐baked state, must be known to evaluate their actual in‐service properties. The ageing behaviour of electrogalvanized TRIP steels and the effect of controlled ageing of the complex microstructure by paint baking on the mechanical properties was therefore investigated. Electroplating tests showed that the H uptake during pretreatment and electrogalvanization is limited and that the paintability of the electroplated sheet steel is unaffected even after deformation induced transformation of the austenite. In addition, a strong bake hardening effect was found in the case of the CMnAlSi TRIP steel. Internal friction measurements indicated that a damping peak for interstitial C could be obtained but that the free C content was less than 1 ppm prior to paint baking. A BH mechanism for TRIP steels is proposed.     

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.     

Low‐alloy TRIP steels: a correlation between mechanical properties and the retained austenite stability

In recent years the technology of low‐alloy TRIP steels has considerably advanced. The mechanical properties are characterised by a combination of high yield strength and high uniform elongation as well as enhanced formability. In the present work an effort to correlate mechanical properties with the retained austenite stability was made. Two low‐alloy TRIP steels were investigated. The first of them represents a typical composition of the low‐alloy TRIP steels, while the other one contains aluminum as alloying element. The influence of the heat treatment on the mechanical properties and especially on the amount and stability of the retained austenite was determined. The retained austenite stability was measured with a single specimen technique, in which a tensile specimen was used to determine the M^σ_S temperature with a loading‐unloading procedure. The results showed that there is a strong influence of the stability of the retained austenite on the mechanical properties. Increased stability combined with a high amount of retained austenite, exhibited an increase in both, yield strength and uniform elongation while increased amount of retained austenite with low stability did not show the same good combination of mechanical properties. The results clearly indicate that in order to get the maximum TRIP effect, a good combination of austenite stability and amount is required.     

相变诱导塑性汽车用钢的发展现状与趋势

论述了相变诱导塑性(TRIP)钢的发展现状及其在汽车工业上的应用,重点讨论了 TRIP 效应的机理及TRIP钢性能的影响因素。介绍了2种采用新型工艺(低温贝氏体转变和淬火-碳分配工艺)的 TRIP 钢,并且通过对TRIP钢研究的最新数据,对比了2种工艺下TRIP钢的高速拉伸性能;最后对汽车用 TRIP 钢的研究方向进行了展望。     

Effect of hot‐rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

TRIP‐assisted multiphase steels have been thoroughly studied in the cold‐rolled and annealed state. The effects of hot‐rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP‐assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low‐carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium‐carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non‐recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low‐carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work‐hardening capabilities. In the case of the medium‐carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low‐carbon steel, the improvement of the work‐hardening behaviour was attributed to an Interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.     

Sheet metal forming behaviour and mechanical properties of TRIP steels

Recently various kinds of high-strength sheet steels have been developed to meet the requirements of the automotive industry such as passive safety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a very good combination of strength and formability so that the applicable region of high-strength steels has been widely enlarged. Multiphase steels have been developed for various purposes because of their ability to tailor properties by adjusting the type, the amount, and the distribution of different phases. Especially new developed triple-phase steels which make use of the TRIP effect (transformation induced plasticity) can further improve formability as well as strength due to the transformation of retained austenite to martensite during the deformation. In this work the transformation behaviour and the mechanical properties of low alloyed TRIP steels were investigated. The influence of the annealing parameters on transformation behaviour and on the amount of retained austenite were determined. In addition the temperature dependence of the mechanical properties and the effect of testing speed on the formability were studied. The investigation was carried out on seven different TRIP steels with different chemical compositions, especially the influence of the microalloying element niobium was considered. For reasons of comparison various mild and high-strength steels were tested parallel to the TRIP steels. It was found that the investigated TRIP steels offer very attractive combinations of elongation and strength values. An interesting temperature dependence of the mechanical properties can be observed, in such a way that the elongation values of the TRIP steels possess a maximum between +50 and +100°C. Due to its effect on grain size and on precipitation behaviour the addition of niobium leads to higher strength values without a strong decrease in ductility. In general, the mechanical properties are strongly affected by the type and the distribution of the different phases. The most important parameters, however, to influence the mechanical behaviour are the amount and the stability of the retained austenite, which are mainly controlled by the heat treatment and the chemical composition.     

Nb含量对TRIP钢组织和性能的影响

利用Gleeble-1500热模拟机对不同Nb含量的TRIP钢的热轧过程进行了热模拟实验.研究了Nb含量对TRIP钢组织和性能的影响.实验结果表明：Nb的加入使得实验钢的组织得到细化,贝氏体量增多;增加Nb含量,残余奥氏体量和残余奥氏体含碳量有所降低,宏观维氏硬度值增加;Nb的质量分数为0.014%时,残余奥氏体量为19.2%、残余奥氏体含碳量为1.422%、宏观维氏硬度为258HV和抗拉强度为851 MPa.     

In‐situ Synchrotron XRD Analysis of the Effect of Static Strain Aging on the Elasto‐plastic Transition in CMnSi TRIP Steel

In situ synchrotron X‐ray diffraction was used to investigate the martensitic transformation kinetics, lattice straining and diffraction peak broadening in cold‐rolled TRIP steel during tensile testing. Direct evidence of stress‐strain partitioning between different phases, dislocation pinning and differences in yielding behaviour of the different phases were clearly observed. The TRIP steel was subjected to a bake‐hardening treatment and a pronounced static strain aging effect was observed. In the present work, the martensitic transformation kinetics and the elastic micro‐strain evolution for both ferrite and retained austenite during the elasto‐plastic transition are reported with an emphasis on bake‐hardening with and without pre‐straining.     

低碳中锰热轧TRIP钢退火工艺及组织演变

研究了第三代高强度高塑性TRIP钢的退火工艺对性能的影响和组织演变规律.热轧后形成的原始马氏体与临界退火时形成的残余奥氏体使TRIP钢具有良好的强度和塑性.结果表明:实验用钢可获得1000MPa以上的抗拉强度和30%以上的断后延伸率,且强塑积>30 Gpa·%;退火温度和保温时间对钢的力学性能具有显著影响,热轧TRIP钢临界退火温度为630℃,保温时间18 h时,实验用钢能获得最佳的综合力学性能.     

Oxidation and Characterization of As‐Cast TRIP Steel Surfaces

This paper presents recent results from a collaborative study between the Department of Ferrous Metallurgy at RWTH‐Aachen University and the Department of Materials Science and Engineering at Carnegie Mellon University on high temperature oxide scale formation of Al and Si containing as cast TRIP steel surfaces under conditions similar to that of continuous casting and hot rolling. A combination of experimental studies consisting of (i) electron microprobe analysis and metallographic studies of the cast steel surface, (ii) direct visualization of the oxide formation through high temperature confocal scanning microscopy and (iii) Tammann furnace oxidation tests were carried out. It was found that internal oxidation of Al and Si takes place along the inter‐dendritic boundaries, where Al and Si were found to have been enriched after casting. The scale formed on the steel surface was a complex mixture of solid fayalite ((FeO_n)₂(SiO₂)), wüstite (FeO_n), magnetite (Fe₃O₄), hematite (Fe₂O₃), and Fe‐particulates and the rate of scale growth appears to have been controlled at high temperatures by the formation of a liquid slag layer that allows for rapid oxygen transport to the steel/scale interface.     

ODS ferritic steels produced by an alternative route (STARS): microstructural characterisation after atomisation,HIPping and heat treatments

The conventional PM ODS Ferritic Steel (FS) processing route includes gas atomisation of steel powder and its mechanical alloying (MA) with Y₂O₃ powder particles to dissolve yttrium and form, during consolidation, a dispersion of oxide nanoparticles (Y–Ti–O) in a nanostructured matrix. This work presents an alternative route to produce ODS steels avoiding MA: STARS (Surface Treatment of gas Atomized powder followed by Reactive Synthesis). STARS FS powders with composition Fe–14Cr–2W–0.3Ti–0.23Y, already containing the nanoparticles precursors, were gas-atomized. Oxygen, Y and Ti contents were tailored to the required values to form Y–Ti–O nanoparticles during processing. Powders were HIPped at 900, 1220 and 1300°C. Specimens HIPped at 900 and 1220°C were heat treated (HT) at temperatures ranging from 1200 to 1320°C. The microstructural evolution with HIP and HT temperatures, including characterisation of nanoparticles and feasibility of achieving complete dissolution of prior particle boundaries (PPBs) were assessed.     

Strain‐Rate‐Dependent Flow Stress and Failure of an Mg‐PSZ Reinforced TRIP Matrix Composite Produced by Spark Plasma Sintering

A composite consisting of 5 vol% MgO‐partially stabilized ZrO₂ particles (Mg‐PSZ) and a TRIP‐steel‐matrix (CrNiMn steel; transformation induced plasticity) was produced through Spark Plasma Sintering. The processed material was tested under compression at various nominal strain rates (4 × 10^?4 s^?1; 10^?3 s^?1; 1 s^?1, 10² s^?1). Both, the pure steel and the composite showed a considerable plasticity and high strength due to the very fine grained steel matrix. The addition of 5 vol% ceramic particles led to a rise in the offset yield strength of 60 MPa till 90 MPa according to the applied strain rate. Up to a strain rate of 1 s^?1, no change in offset yield strength was measured. A strain‐rate of 100 s^?1 leads to a rise in the offset yield strength of approx. 100 MPa. Both, the ceramic and an increase in the strain rate implicate to an early generation of microdeterioration. Limited by the interfacial strength of steel and Mg‐PSZ, failure occurs early at the interfaces, which is shown in a decrease in the work hardening. During the compression, especially at higher strain‐rates, adiabatic heating occurred and counteracted to the martensitic transformation.     

Reinforcing Mechanism of Mg‐PSZ Particles in Highly‐Alloyed TRIP Steel

Dense TRIP‐matrix composites containing 5 vol.% Mg‐PSZ as reinforcing phase were produced by employing the spark plasma sintering technique. A continuous and seamless interface between the ceramic particles and the steel matrix was achieved. Compression tests revealed better mechanical properties of the 5 vol.% Mg‐PSZ‐TRIP steel composites in comparison with both, pure and Al₂O₃ reinforced TRIP steel. The underlying deformation mechanism within the austenitic matrix entailed a pronounced martensite formation. An additional phase transformation was observed within the ZrO₂ particles. The enhanced mechanical properties of the 5 vol.% Mg‐PSZ composite are dedicated to the transformation strengthening of the ceramic particles. Finally a model of the reinforcing mechanism is proposed.     

利用连铸坏余热对60Si2CrA进行退火处理

利用推钢工连铸坯冷床的特点,连铸坯热量散失慢,对热轧弹簧圆金钢60Si2CrAφ13退火热处理,使其硬度降低,满足用户的使用要求。     

Investigation of the conditions for weld metal hydrogen cracking of low carbon offshore steels by the IRC weldability test

Three low carbon structural steels of different plate thickness have been investigated for hydrogen assisted cold cracking by the IRC weldability test at different restraint intensities. At diffusible hydrogen levels of 10–15 N ml/100 g Fe (ISO 3690), cracking decreases at increasing heat inputs due to a drop in restraint stress and hardness as well as an increase in hydrogen diffusion times. Critical heat inputs for crack prevention range from 0.95 to 1.4 kJmm^?1. Higher restraints enforce higher cracking stresses as well as final stresses of uncracked test welds. Higher restraints and lower heat inputs also induce faster stress increase during cooling which, for the steels containing Ni and Cu, shift the location of cracking from the HAZ to the weld metal. The steel without Ni and lower maximum HAZ hardness reveals weld metal cracking only, regardless of welding conditions. It can be concluded that for weld metal cracking, the relation between stress increase- and hydrogen effusion rates but also the relation between weld metal and HAZ microstructure and mechanical properties are responsible.