Effect of Cold Deformation and Multiphase Treatment Conditions on Low‐Carbon,Low‐Silicon Multiphase Steel

This paper presents multiphase (MP) treatments of a low‐C, low‐Si cold rolled steel. Despite the much lower content of Si compared to a typical TRIP steel, up to about 8 pct of retained austenite (γ_r) with 1.2 % carbon content can be obtained. Increasing prior cold deformation (i.e. decrease of parent austenite grain size) accelerates the transformation to bainite resulting in a decrease of the volume fraction of residual austenite (γ_r + martensite). Tensile strength of MP steel intercritically annealed at high temperature increases with higher cold reduction degree due to the smaller grain size of the present phases. On the contrary, the ductility and strength‐ductility balance deteriorate because the banded structure becomes more pronounced and the γ_r volume fraction diminishes. Decreasing intercritical annealing temperature results in an increasing γ_r fraction and a uniform distribution of second phases. Hence, the ductility and strength‐ductility balance are improved. Crystallographic preferred orientation is evident in the ferrite and martensite and its extent increases with higher cold deformation.     

含铝TRIP1000钢的断裂机制

通过在两相区温度和贝氏体区温度等温处理制备强度达1000MPa、伸长率为24.3%的含铝低硅TRIP 钢,采用扫描电镜进行原位拉伸试验和能谱分析,研究其断裂机制.结果表明,在拉应力作用下,铁素体内部位错沿滑移带运动到晶粒边界引起位错塞积产生应力集中,同时铁素体与周围硬质相之间的结合力相对较弱,由于应力的作用而萌生微孔....     

Effect of Isothermal Bainite Treatment on Microstructure and Mechanical Properties of Low‐Carbon TRIP Seamless Steel Tube

In this work, a low‐carbon transformation‐induced‐plasticity (TRIP) seamless steel tube (Fe–0.15C–1.34Si–1.45Mn–0.029Nb–0.024Ti, in wt%), having potential in application of hydroforming process, has been successfully manufactured by using piercing, cold‐drawing, and two‐stage heat‐treatment process. The optimal heat‐treatment conditions, inter‐critical annealing (IA), and isothermal bainite treatment (IBT) were firstly obtained to maximize the volume fraction and stability of the retained austenite (RA). The effects of temperature and holding time IBT on the microstructures of the TRIP steel tube were studied via optical microscopy (OM), scanning microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffractometer (XRD). The mechanical properties in the axial direction and hydroformability were also evaluated by conventional tensile test and flaring test, respectively. Two‐stage heat‐treatment was finally performed to achieve the required mechanical properties for the hydroformed tube. The results shows that the RA volume fraction increased at first and then decreased with the increase of IBT holding time and IBT temperature for a particular set of IA temperature and IA holding time. It was also demonstrated that high tensile strength of 618 MPa, total elongation of 35.5%, n‐value of 0.23, and better hydroformability could be successfully produced in this TRIP steel tube at IA temperature of 800°C, holding for 10 min, and IBT of 410°C for 4 min holding time.     

冷轧TRIP800钢的组织性能和形变行为

 采用Gleeble 3800热模拟机、MTS拉伸实验机、LEO1450携带EBSD系统的扫描电镜与高倍电镜等对TRIP800进行了组织性能考察,并对其形变过程裂纹走向与断裂机理进行了观察分析。结果表明,实验钢退火加热温度为840 ℃,保温120 s后获得最优力学性能。其残余奥氏体晶粒主要分布在晶界处,且尺寸在3 μm以下。原位拉伸时,当试样裂纹尖端遇到残余奥氏体时,应力集中促使马氏体转变,裂纹尖端被钝化,产生TRIP效应。最后,试样断口中部为剪切断裂,边部为塑性断裂。     

Effect of Ti Additions on Micro‐Alloyed Nb TRIP Steel

In the present study, the influence of Ti‐additions on the mechanical properties of Nb‐microalloyed TRIP steel is investigated. Ti micro‐alloying additions to multi‐phase Nb TRIP steel result in a substantial increase of the yield strength and a reduction of strain hardening. The increase of the yield strength can be attributed mainly to grain refinement with a relatively small contribution of precipitation hardening. Based on general principles and well‐known models of alloying strengthening, metallurgical reasons for the observed mechanical behavior of the steel can be formulated. The contribution of precipitation hardening is relatively small as Ti‐addition result in the formation of coarse (Nb,Ti)(C,N) particles. The addition of Ti to a Nb‐microalloyed TRIP steel leads to a pronounced enhancement of precipitation kinetics of (Nb,Ti)(C,N), thereby increasing their phase fraction. The precipitates coarsen and tend to form groups of aggregates of particles rather than single isolated particles with increasing intercritical annealing time. In addition, Ti‐addition to Nb‐microalloyed TRIP steel has a direct influence on the chemical composition of the precipitates, which become Ti‐rich.     

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.     

1000MPa级TRIP钢的组织性能及强化机理

鉴于热镀锌TRIP钢在成分和工艺上的特殊性,试验采用低Si含P以及V微合金化的成分设计思路,并进行镀锌连续退火工艺模拟,分析其微观组织和性能。探讨了残余奥氏体稳定性的影响因素及强化机理。结果表明,添加P和采取较高的冷轧变形量均有利于提高残余奥氏体量及其稳定性。在残余奥氏体的形变诱导马氏体相变、贝氏体和V析出的强化作用以及铁素体晶粒细化的共同作用下,抗拉强度和强塑积分别达到1 035 MPa和25 875 MPa·%。     

The Effect of Size and Shape of Austenite Grains on the Mechanical Properties of a Low‐Alloyed TRIP Steel

The effects of size and shape of austenite grains on the extraordinary hardening of steels with transformation induced plasticity (TRIP) have been studied. The deformation and transformation of austenite was followed by interrupted ex situ bending tests using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). A finite element model (FEM) was used to relate the EBSD based results obtained in the bending experiments to the hardening behavior obtained from tensile experiments. The results are interpreted using a simple rule of mixture for stress partitioning and a short fiber reinforced composite model. It is found that both, the martensite transformation rate and the flow stress difference between austenite and martensite significantly influence the hardening rate. At the initial stage of deformation mainly larger grains deform, however, they do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. A composite model was used to investigate the effect of grain shape on load partitioning. The results of the composite model show that higher stresses develop in more elongated grains. These grains tend to transform earlier as it is confirmed by the EBSD observations.     

Low‐alloyed TRIP‐Steels with Optimized Strength,Forming and Welding Properties

To obtain the superior strength‐ductility‐balance of TRIP‐grades, a special chemical composition in combination with well adapted processing parameters are a prerequisite. Despite of their excellent formability performance in terms of drawability as characterized by high n‐ and elongation values, compared to mild steels TRIP‐grades are challenging in the press and the body shops. The high strength level in combination with the high work hardening of TRIP‐grades result in higher levels of spring back compared to mild steels and higher press forces are required. Furthermore, a higher sensitivity to failure for sharp bending radii and a deterioration of the formability of punched edges is reported for TRIP‐grades. While spring back can only be minimized by advanced forming processes supported by new simulation techniques with improved ability to predict spring back, the sensitivity to failure under special forming conditions can be influenced by optimizing microstructural features. Contrary to the forming behaviour, which is influenced significantly by the microstructure, the weldability is mainly governed by the chemical composition and the surface condition of the material. The high carbon content of TRIP‐grades compared to mild steels results in a higher hardening potential after welding. Additionally, a fracture behaviour untypical for mild steels after destructive testing of spot welds is sometimes observed for TRIP‐grades, which is assessed critically by some OEMs. In this work, after a discussion of the processing conditions, possibilities are demonstrated to improve the forming behaviour by an optimization of the microstructure and the spot weldability by adapting the chemical composition of low‐alloyed TRIP grades. First very promising results for TRIP‐grades with a minimum tensile strength level of 700 MPa are discussed.     

残余奥氏体中的碳元素在TRIP钢变形前后的分布

利用TEM和EPMA对TRIP钢中残余奥氏体形貌以及碳元素的分配进行了研究,发现TRIP钢中的残余奥氏体以多种形态分布,且碳在残余奥氏体中的浓度显著高于其他两相中的浓度,此时残余奥氏体可以通过EPMA中的贫硅区表示;变形之后的残余奥氏体将会发生相变,通过TEM发现残余奥氏体在受到应力作用而发生相变之后转变为细小的立方马氏体,且由于碳原子来不及扩散,马氏体中的含碳量和奥氏体中的含碳量基本相同。     

Analysis of the Strain‐Induced Martensitic Transformation of Retained Austenite in Cold Rolled Micro‐Alloyed TRIP Steel

The stability of retained austenite and the kinetics of the strain‐induced martensitic transformation in micro‐alloyed TRIP‐aided steel were obtained from interrupted tensile tests and saturation magnetization measurements. Tensile tests with single specimens and at variable temperature were carried out to determine the influence of the micro‐alloying on the M_s^σ temperature of the retained austenite. Although model calculations show that the addition of the micro‐alloying elements influences a number of stabilizing factors, the results indicate that the stability of retained austenite in the micro‐alloyed TRIP‐aided steels is not significantly influenced by the micro‐alloying. The kinetics of the strain‐induced martensitic transformation was also not significantly influenced by addition of the micro‐alloying elements. The addition of micro‐alloying elements slows down the autocatalytic propagation of the strain‐induced martensite due to the increase of the yield strength of retained austenite. The lower uniform elongation of micro‐alloyed TRIP‐aided steel is very likely due to the presence of numerous precipitates in the microstructure and the pronounced ferrite grain size refinement.     

A Novel Heat Treatment for Excavator Dipper Teeth Manufactured from Low‐Carbon Low‐Alloy Steel

This research examined the metallurgy of production of forged excavator teeth manufactured from low‐carbon low alloy steel. A novel partial forging‐remnant‐heat hardening technology was proposed. The critical temperature of isothermal annealing for 40 Cr steel was determined experimentally to be 650°C. Production dipper teeth were produced using this novel technology. Their properties were compared with dipper teeth produced by the alternative routes. The novel technology provided optimum microstructure, good mechanical properties, and a lower economical cost, congruent with the low carbon economy.     

Microstructure and Mechanical Properties of Hot Rolled Low Silicon TRIP Steel Containing Phosphorus and Vanadium

 The microstructure characteristics and mechanical properties of a low-silicon TRIP steel containing phosphorus and vanadium at different finish rolling temperatures were studied by laboratory hot rolling experiments. Different ratios of multiphase microstructure (ferrite, granular bainite and retained austenite) are obtained. With a decrease in finish rolling temperature, the volume fractions of ferrite and retained austenite increase. EBSD analysis reveals that most of the ferrite grains are fine, and decreasing of finish rolling temperature leads to an increase in low angle boundaries. Under the joint effects of fine grain strengthening, dislocation strengthening and precipitation strengthening, higher strength is obtained. When the finish rolling temperature is decreased to 800 ℃, the steel has excellent mechanical properties: Rp02=470 MPa; Rm=960 MPa; Rp02/Rm=049; A50=197%; n=025.     

In‐Situ Study of the Microstructure Development in a CrMnNi TRIP Steel During Plastic Deformation

The microstructure development in CrMnNi TRIP steel during the onset of the plastic deformation was investigated with the aid of in‐situ X‐ray diffraction experiments. The analysis of the shift and broadening of the X‐ray diffraction lines allowed the elastic and the plastic components of the lattice deformation to be separated from each other. This separation made possible to follow the formation of the microstructure features like stacking faults, deformation bands and local lattice rotations that were afterwards confirmed by X‐ray diffraction with high resolution, scanning electron microscopy and transmission electron microscopy.     

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.