Recrystallization behavior in deformed austenite of high strength low alloy (HSLA) steels

High temperature deformation experiments were conducted to monitor the recrystallization process in the austenite phase of a vanadium and columbium-treated HSLA steel alloy. The results show that columbium is much more effective than vanadium in retarding recrystallization. As the austenitizing temperature, above 2000°F, or time is increased the austenite recrystallization in the 1500° to 1700°F hot working temperature range is correspondingly increased. If, following the austenitization, the samples are held in the hot working temperature range for up to 5 min before hot working recrystallization occurs at a much faster rate. Several mechanisms are offered to explain the experimental results.     

ULCB钢的奥氏体形变组织研究

采用单道次压缩实验方法,用THERMECHMASTOR-Z热模拟试验机在1 100～850℃、变形速率2 s-1和变形量10%～50%的应变条件下,对900 Mpa级ULCB钢进行应力-应变曲线和奥氏体形变组织的试验.结果表明,在950℃以下的低温变形中不发牛形变再结晶,随着变形量增大,先出现晶内形变带直到晶粒拉长变形.在1 000℃以上的高温变形中,当变形量大于临界变形量时发生形变再结晶.随着变形量增大,奥氏体再结晶晶粒面积百分数依次增加,形变组织为部分或完全再结晶奥氏体,奥氏体晶粒平均截距的大小取决于形变再结晶奥氏体的晶粒尺寸和面积百分数.     

Microstructure/property relations in lath martensitic steels

Many of the high strength,high toughness steels in use or under development are alloy steels with dislocated lath martensitic structures.These microstructures are visually complex,and are difficult to categorize in the detail needed to clarify the structure-property relations.However,substantial progress has been made in recent years.In particular,it is now clear that the martensite block element sets the effective grain size that must be controlled to resist brittleness by cleavage fracture.In previous papers at this conference I have discussed the nature and importance of the block structure.In this study I discuss how the block size can be controlled by thermal treatments to achieve superior properties.     

Effect of deep cryogenic treatment on martensitic lath refinement and nano-twins formation of low carbon bearing steel

The effect of heat treatment and deep cryogenic treatment on microstructural evolution of low carbon martensitic bearing steel was investigated.The experimental results showed that the lath martensite was obtained by quenching and a few twins as substructures formed in some martensitic laths.The rudiment of sub-interfaces of martensitic lath was formed in the highdensity dislocation regions after deep cryogenic treatment;meanwhile,the number of twins increased,especially in the highdensity dislocation regions.This phenomenon is due to the increase in internal stress caused by cryogenic treatment.After tempering,the rudiment of sub-interface further evolved into the martensitic lath boundary,and thus the original martensitic laths were refined.The twins formed by cryogenic treatment did not disappear after tempering.In addition,small quantities of annealing twins formed in tempering process.Martensitic laths morphology and substructures in different stages of the heat and deep cryogenic treatment were observed by tninsmission electron microscopy.     

Deformation and fracture in martensitic carbon steels tempered at low temperatures

This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described. The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering. Dr. George Krauss is currently University Emeritus Professor at the Colorado School of Mines. He received the B.S. in Metallurgical Engineering from Lehigh University in 1955 and the M.S. and Sc.D. degrees in Metallurgy from the Massachusetts Institute of Technology in 1958 and 1961, respectively, after working at the Superior Tube Company as a Development Engineer in 1956. In 1962–63, he was an NSF Postdoctoral Fellow at the Max-Planck-Institut für Eisenforshung (Düsseldorf, Germany). He served at Lehigh University as Assistant Professor, Associate Professor, and Professor of Metallurgy and Materials Science from 1963 to 1975 and, in 1975, joined the faculty of the Colorado School of Mines as the AMAX Professor of Physical Metallurgy. He was the John Henry Moore Professor of Metallurgical and Materials Engineering at the time of his retirement from the Colorado School of Mines in 1997. In 1984, Dr. Krauss was a principal in the establishment of the Advanced Steel Processing and Products Research Center, an NSF industry-university cooperative research center at the Colorado School of Mines, and served as its first director until 1993. He has authored the book Steels: Heat Treatment and Processing Principles, ASM International, 1990, coauthored the book Tool Steels, Fifth Edition, ASM International, 1998, and edited or coedited several conference volumes on topics including tempering of steel, carburizing, zinc-based coatings on steel, and microalloyed forging steels. He has published over 280 papers and lectured widely at technical conferences, universities, corporations, and ASM chapters, including a number of keynote, invited, and honorary lectures. Dr. Krauss has served as the President of the International Federation of Heat Treatment and Surface Modification, 1989–91, and as President of ASM International, 1996–97. He is a Fellow of ASM International and has received the Adolf Martens Medal of the German Society for Heat Treatment and Materials Technology, the Charles S. Barrett Silver Medal of the Rocky Mountain Chapter ASM, the George Brown Gold Medal of the Colorado School of Mines, and several other professional and teaching awards, including the ASM Albert Easton White Distinguished Teacher Award in 1999. He is an Honorary Member of the Iron and Steel Institute of Japan and a Distinguished Member of the Iron and Steel Society of AIME.     

Deformation and fracture in martensitic carbon steels tempered at low temperatures

This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described. The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering. Dr. George Krauss is currently University Emeritus Professor at the Colorado School of Mines. He received the B.S. in Metallurgical Engineering from Lehigh University in 1955 and the M.S. and Sc.D. degrees in Metallurgy from the Massachusetts Institute of Technology in 1958 and 1961, respectively, after working at the Superior Tube Company as a Development Engineer in 1956. In 1962–63, he was an NSF Postdoctoral Fellow at the Max-Planck-Institut für Eisenforshung (Düsseldorf, Germany). He served at Lehigh University as Assistant Professor, Associate Professor, and Professor of Metallurgy and Materials Science from 1963 to 1975 and, in 1975, joined the faculty of the Colorado School of Mines as the AMAX Professor of Physical Metallurgy. He was the John Henry Moore Professor of Metallurgical and Materials Engineering at the time of his retirement from the Colorado School of Mines in 1997. In 1984, Dr. Krauss was a principal in the establishment of the Advanced Steel Processing and Products Research Center, an NSF industry-university cooperative research center at the Colorado School of Mines, and served as its first director until 1993. He has authored the book Steels: Heat Treatment and Processing Principles, ASM International, 1990, coauthored the book Tool Steels, Fifth Edition, ASM International, 1998, and edited or coedited several conference volumes on topics including tempering of steel, carburizing, zinc-based coatings on steel, and microalloyed forging steels. He has published over 280 papers and lectured widely at technical conferences, universities, corporations, and ASM chapters, including a number of keynote, invited, and honorary lectures. Dr. Krauss has served as the President of the International Federation of Heat Treatment and Surface Modification, 1989–91, and as President of ASM International, 1996–97. He is a Fellow of ASM International and has received the Adolf Martens Medal of the German Society for Heat Treatment and Materials Technology, the Charles S. Barrett Silver Medal of the Rocky Mountain Chapter ASM, the George Brown Gold Medal of the Colorado School of Mines, and several other professional and teaching awards, including the ASM Albert Easton White Distinguished Teacher Award in 1999. He is an Honorary Member of the Iron and Steel Institute of Japan and a Distinguished Member of the Iron and Steel Society of AIME.     

Bainite formation in low carbon Cr-Ni steels

A low carbon Cr-Ni steel has been used to investigate the formation of upper bainite. Experimental results indicate that the start temperatures of the three morphologies of upper bainite in this steel,i.e., carbide-free bainite, bainite with carbide between and within ferrite laths, are about 600°, 500δ, and 425 °C, respectively; the habit plane of bainitic ferrite in this steel is close to (1 7 11)_α, which is 13.3 deg away from (0 ll)_α; and the orientation relationship between cementite and ferrite is consistent with Bagaryatskii’s. By means of the superelement approach, a thermodynamic treatment which applies to Fe-C alloys is extended into that suitable for low alloy steels, and calculation shows that the driving force for bainite formation at B_S temperatures is insufficient to compensate for shear strain energy. Formerly Graduate Student, Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China.     

Recrystallization kinetics of microalloyed steels deformed in the intercritical region

A plain carbon and two microalloyed steels were tested under interrupted loading conditions. The base steel contained 0.06 pct C and 1.31 pct Mn, and the other alloys contained single additions of 0.29 pct Mo and 0.04 pct Nb. Double-hit compression tests were performed on cylindrical specimens of the three steels at 820 °C, 780 °C, and 740 °C within the α + γ field. A’softening curve was determined at each temperature by the offset method. In parallel, the progress of ferrite recrystallization was followed on quenched specimens of the three steels by means of quantitative metallography. It was observed that, in the base steel, a recrystallizes more slowly thany. The addition of Mo retards recrystallization and has a greater influence on γ than on α recrystallization. This effect is in agreement with calculations based on the Cahn theory of solute drag. Niobium addition has an even greater effect on the recrystallization of the two phases. In this steel, the recrystallization of ferrite was incomplete at the three intercritical temperatures. Furthermore, the austenite remained completely unrecrystallized up to the maximum time involved in the experiments (1 hour). The metallographic results indicate that the nucleation of recrystallization occurs heterogeneously in the microstructure, the interface between ferrite and austenite being the preferred site for nucleation.     

变形参数对低碳微合金钢奥氏体相变规律的影响

利用Gleeble-3800热/力模拟试验机,研究了1种低碳微合金钢的形变奥氏体连续冷却转变行为以及终轧温度、终轧变形量对相变组织形态的影响。结果表明,实验钢在研究的冷速范围内均可得到一定量的贝氏体组织。随着冷速的增加,冷速达到50℃/s时,贝氏体组织由粒状贝氏体逐渐过渡到板条状贝氏体。在较快的冷速下,随终轧温度的降低或终轧变形量的减小,贝氏体转变开始与结束温度均降低,获得的贝氏体具有不同的组织形态。     

Influence of boron on the decomposition of austenite in low carbon alloyed steels

The influence of boron on the isothermal decomposition of Fe-Ni₆-C_0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C_0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe₂₃(B, C)₆ (FCC structure with a ≈ 10.6?). The grain boundary Fe₂₃(B, C)₆ were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe₂₃(B, C)₆ precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.     

Influence of boron on the decomposition of austenite in low carbon alloyed steels

The influence of boron on the isothermal decomposition of Fe-Ni₆-C_0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C_0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe₂₃(B, C)₆ (FCC structure with a ≈ 10.6Å). The grain boundary Fe₂₃(B, C)₆ were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe₂₃(B, C)₆ precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.     

Grain structure of bainitic and martensitic steels

In contrast to austenitic and ferritic microstructures, in the case of bainite and martensite the identification of that structural unit representing a grain is less straightforward. There is a general agreement in the literature that the γ → α-transformation follows the Kurdjumow-Sachs relationship (KSR). For the complex microstructures resulting, however, the investigations include practically exclusively lath-type structures. These relatively simple structures may be described as follows. Within the original austenite grain there are several lath packets with only a few variants of the KSR occuring within one bainite or martensite packet. Opinions diverge with regard to the relative orientations within a packet and thus the type of grain boundaries occurring. Most authors, however, agree that a micro-structural unit – such as the grain in the ferrite structure – which determines both the yield strength and toughness properties, does not exist in lath-type structures, and that the yield strength mainly depends on the lath dimensions. The toughness properties, especially the transition temperature, are governed by the packet size, and possibly by the width of the co-variant packet.     

Recrystallization of austenite

A laboratory simulation of hot-rolling schedules, which employs a sequence of high-speed compressions performed over a range of temperatures, was used to determine the effects of composition and processing variables on the state of the austenite at several points in the hot-rolling procedure. The type and quantity of carbide and/or nitride forming elements (Nb(Cb), V, Al) in the steels were varied to produce precipitates at various points in the hot-working range of temperatures. Time-temperature-deformation schedules were designed around schedules currently used in the hot-re-duction of plate steels; variations were introduced to determine the effects of processing variables on the development of the austenite structures. At various points in the deformation schedule, specimens were quenched out and changes in the austenite grain structure were examined. Changes in the volume and aspect ratio of the prior-austenite grains were used as measures of the degree of austenite recrystallization. At temperatures above the precipitation-start temperature, light reductions (< 10 pct) caused grain coarsening. Coarsening could be eliminated by increasing the reduction per pass, decreasing the rolling temperatures, or increasing the concentration of carbide or nitrideforming elements to raise the precipitation-start temperature. It is clear from these results that attempts to study structural changes that occur during plate rolling by using a few high-reduction passes may produce erroneous results. Formerly a summer student with U. S. Steel Research Laboratory. This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metallurgy Committee of The Metallurgical Society of AIME.     

Retained austenite and tempered martensite embrittlement in medium carbon steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Somein situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M₃C filM_s upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe₃C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The sit11Ation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which o°Curs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can o°Cur in the same steels, depending on the tempering conditions.     

An acoustic emission study of martensitic transformation of retained austenite in intercritically annealed HSLA steels

The martensitic transformation of retained austenite particles in an intercritically annealed high-strength low-alloy steel has been studied using the acoustic emission technique. It was found that the stability of austenite particles was mainly dependent on the particle size and to a lesser extent on the enrichment of austenite by carbon and manganese. We have demonstrated that a substantial fraction of the retained austenite particles in the as-annealed condition already contains martensite embryos which can initiate martensitic transformation during cooling below 300 K. The martensite embryos are believed to form from a suitable group of dislocations. Those austenite particles which do not contain such dislocations will not transform by supercooling alone. The particle size stabilization effect is viewed as a decreasing probability of finding dislocations in a particle with decreasing particle volume. Plastic deformation can bring about the transformation by introducing dislocations in the austenite particles. The transformation of the retained austenite particles is shown to enhance the ductility of the HSLA steel.     

Retained austenite and mechanical properties in bainite transformed low alloy steels

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austenite. In this work, intercritical annealing followed by bainite transformation resulted in the retention of austenite with sufficient stability for transformation plasticity interactions. The effect of retained austenite on mechanical properties was studied in two low-alloy steels. Bainite transformation was carried out in the range of 400 to 500°C. The strength properties (yield strength and ultimate tensile strength) were more sensitive to bainite isothermal transformation temperature than holding time. Maximum strength properties were obtained for the lower transformation temperatures. On the other hand, high uniform and total elongation values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation time. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount of retained austenite. The same was true for total elongation and ultimate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processing with respect to austenite stabilisation may lead to a new class of triple-phase high-strength high-formability low-alloy steels.     

The influence of niobium supersaturation in austenite on the static recrystallization behavior of low carbon microalloyed steels

This work describes the effect of Nb supersaturation in austenite, as it applies to the strain-induced precipitation potential of Nb(CN), on the suppression of the static recrystallization of austenite during an isothermal holding period following deformation. Four low carbon steels, microalloyed with Nb, were used in this investigation. Three of the steels had variations in Nb levels at constant C and N concentrations. Two steels had different N levels at constant C and Nb concentrations. The results from the isothermal deformation experiments and the subsequent measurement of the solution behavior of Nb in austenite show that the recrystallization-stop temperature (T _RXN) increases with increasing Nb supersaturation in austenite. Quantitative transmission electron microscopy analysis revealed that the volume fraction of Nb(CN) at austenite grain boundaries or subgrain boundaries was 1.5 to 2 times larger than Nb(CN) volume fractions found within the grain interiors. This high, localized volume fraction of Nb(CN) subsequently led to high values for the precipitate pinning force (F _PIN). These values forF _PIN were much higher than what would have been predicted from equilibrium thermodynamics describing the solution behavior of Nb in austenite.     

Bainite transformation in deformed austenite

In this work the effect of deformation of metastable austenite on its subsequent isothermal decomposition in bainite regions was investigated in three chromium steels with varying carbon contents. Four different types of treatments were used and the effect of carbon content was studied. Results of the kinetic measurements and of structural changes showed that the most important feature of these treatments was formation of narrow ferrite strips along the slip bands in austenite, which preceded formation of bainite of the usual morphology. A marked dependency of this ferrite strips formation on temperature suggests that the over-all transformation would strongly depend on deformation and recovery processes in austenite and on their mutual relation. A new qualitative model of the bainite transformation in deformed austenite is presented.     

Bainite transformation in deformed austenite

In this work the effect of deformation of metastable austenite on its subsequent isothermal decomposition in bainite regions was investigated in three chromium steels with varying carbon contents. Four different types of treatments were used and the effect of carbon content was studied. Results of the kinetic measurements and of structural changes showed that the most important feature of these treatments was formation of narrow ferrite strips along the slip bands in austenite, which preceded formation of bainite of the usual morphology. A marked dependency of this ferrite strips formation on temperature suggests that the over-all transformation would strongly depend on deformation and recovery processes in austenite and on their mutual relation. A new qualitative model of the bainite transformation in deformed austenite is presented.