Showing papers on "Pearlite published in 2004"

Development of High Strength Hot-rolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides

Abstract: A ferritic steel precipitation-strengthened by manometer-sized carbides was developed to obtain a high strength hot-rolled sheet steel having tensile strength of 780 MPa grade with excellent stretch flange formability. Manganese in a content of 1.5% and molybdenum in a content of 0.2 % were added to 0.04 % carbon Ti-bearing steel in order to lower austenite-ferrite transformation temperature for fine carbides and to retard generating of pearlite and large cementites, respectively. Tensile strength of hot-rolled sheet steel increased with titanium content and it was achieved to 800 MPa in a 0.09 % Ti steel. Microstructure of the 0.09 %Ti steel was ferrite without pearlite and large cementites. Fine carbides of 3 nm in diameter were observed in rows in the ferrite matrix of the 0.09 % Ti steel with transmission electron microscope. The characteristic arrangement of the nanometer-sized carbides indicates that the carbides were formed at austenite-ferrite interfaces during transformation. By energy dispersive X-ray spectroscopy, the carbides were found to contain molybdenum in the same atomic concentration as titanium. Crystal structure of the nanometer-sized carbides was determined to be NaCI-type by X-ray diffractometry. The calculated amount of precipitation-strengthening by the carbides was approximately 300 MPa. This is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. Based on the results obtained in the laboratory investigation, mill trial was carried out. The developed hot-rolled high strength sheet steel exhibited excellent stretch flange formability.

On the Modeling and Analysis of Machining Performance in Micro-Endmilling, Part I: Surface Generation

Abstract: This paper examines the surface generation process in the micro-endmilling of both single-phase and multiphase workpiece materials. We used 508 μm dia endmills with edge radii of 2 and 5 μm to machine slots in ferrite, pearlite, and two ductile iron materials at feed rates ranging from 0.25 to 3.0 mm/flute. A surface generation model to predict the surface roughness for the slot floor centerline is then developed based on the minimum chip thickness concept. The minimum chip thickness values were found through finite element simulations for the ferrite and pearlite materials. The model is shown to accurately predict the surface roughness for single-phase materials, viz., ferrite and pearlite. Two phenomena were found to combine to generate an optimal feed rate for the surface generation of single-phase materials: (i) the geometric effect of the tool and process geometry and (ii) the minimum chip thickness effect. The surface roughness measurements for the ductile iron workpieces indicate that the micromilling surface generation process for multiphase workpiece materials is also affected by the interrupted chip-formation process as the cutting edge moves between phases resulting in burrs at the phase boundaries and the associated increases in surface roughness.

On the Modeling and Analysis of Machining Performance in Micro-Endmilling, Part II: Cutting Force Prediction

Abstract: In Part II of this paper, a cutting force model for the micro-endmilling process is developed. This model incorporates the minimum chip thickness concept in order to predict the effects of the cutter edge radius on the cutting forces. A new chip thickness computation algorithm is developed to include the minimum chip thickness effect. A slip-line plasticity force model is used to predict the force when the chip thickness is greater than the minimum chip thickness, and an elastic deformation force model is employed when the chip thickness is less than the minimum chip thickness. Orthogonal, microstructure-level finite element simulations are used to calibrate the parameters of the force models for the primary metallurgical phases, ferrite and pearlite, of multiphase ductile iron workpieces. The model is able to predict the magnitudes of the forces for both the ferrite and pearlite workpieces as well as for the ductile iron workpieces within 20%. @DOI: 10.1115/1.1813471#

Ductile-Brittle Transition Temperature of Ultrafine Ferrite/Cementite Microstructure in a Low Carbon Steel Controlled by Effective Grain Size

Abstract: To analyze the good toughness of ultrafine ferrite/cementite steels, the concept of effective grain size (d EFF ) is applied to ductile-to-brittle transition temperature, DBTT, for ultrafine ferrite/cementite (Uf-F/C), ferrite/pearlite (F/P), quenched (Q), and quench-and-tempered (QT) microstructures in a low carbon steel. The d EFF is determined to be 8, 20, 100, and 25 μm for Uf-F/C, F/P, Q, and QT, respectively. In F/P and Q, it is in accordance with the ferrite grain size and the prior austenite grain size, respectively. In QT, the d EFF fits the martensite packet size. In Uf-F/C, the ferrite grain size has a bimodal distribution and the larger grain size corresponds to the d EFF , which is the smallest among the four microstructures. In terms of the relationship between d EFF and DBTT, the Uf-F/C, Q, and QT microstructures can be placed into the same group and the F/P to a different one. Furthermore, the Uf-F/C has the highest estimated fracture stress among the four microstructures. These might be the result of the difference in the surface energy of fracture, namely the former is estimated to have a surface energy of 34.6 J/m 2 and latter a surface energy of 7.7 J/m 2 . Thus, the excellent toughness of the ultrafine ferrite/cementite steel can be attributed to the small d EFF and the high surface energy of fracture.

Microstructure of Steels and Cast Irons

Abstract: First Part The history of iron and steel - of swords and ploughshares.- 1 From iron to steel.- 2 Of swords and swordmaking.- 2 The Genesis of Microstructures.- 3 The principal phases in steels.- 4 The basic phase diagrams.- 5 The formation of solidification structures.- 6 Liquid/solid structural transformations.- 7 Grains, grain boundaries and interfaces.- 8 Diffusion.- 9 The decomposition of austenite.- 10 The pearlite transformation.- 11 The martensite transformation.- 12 The bainite transformation.- 13 Precipitation.- 3 Steels and cast irons.- 14 Steel Design.- 15 Solidification macrostructures.- 16 Macro- and microstructures of sintered powder products.- 17 Plain carbon and low alloy steels.- 18 Quench hardening steels.- 19 Stainless steels.- 20 Heat resisting steels and iron-containing superalloys.- 21 Cast irons.- 22 Appendices.- 23 References.- 24 Index.

Strain-hardening due to Dispersed Cementite for Low Carbon Ultrafine-grained Steels

Abstract: Strain(work)-hardening in tensile tests was examined for low carbon steels with various ferrite grain sizes ranged from 0.4 to 16 μm. The steels had microstructures composed of ferrite grains and dispersed cementite particles. They were fabricated through warm caliber bar-rollings with an accumulative area reduction of 93%. Strain-hardening rate at a given strain increased with an increase in volume fraction of cementite particles. The balance of yield strength and uniform elongation for ultrafine-grained structures could be improved by the dispersion of cementite particles. Effects of the cementite dispersion and the ferrite grain size on the strain-hardening rate can be roughly explained by the work-hardening model with GN-dislocation density. Strain-hardening design using dispersed cementites was proved to be effective in controlling ductility of the ultrafine-grained steels.

Relationship between microstructure and strength in eutectoid steels

Abstract: This paper analyzes the influence of the microstructural changes undergone by eutectoid pearlitic steels during continuous cold drawing on their yield strength. To this end, samples from a real manufacturing process (wire drawing) were studied, and consideration was given to the microstructure evolution as the drawing progresses up to the final commercial product which is heavily drawn and has undergone severe straining. It is seen that the pearlite interlamellar spacing (decreasing with cold drawing) influences clearly the improvement of yield strength (increasing with cold drawing, which is the final aim of manufacturing), although a relationship of the Hall–Petch type cannot be fitted in this case, probably because cold drawing produces not only an increasing closeness of the pearlitic packing, but also a microstructural orientation in the material.

Wear resistant cast iron

Abstract: A casting of a white cast iron alloy which includes the following alloy composition, in weight %: chromium: 12-25%; carbon: 1.5-6%; manganese: 2-7%; silicon: up to 1.5%; molybdenum: up to 2; nickel: up to 4%; microalloying elements selected from the group of titanium, zirconium, niobium, boron, vanadium, and tungsten: up to 2% of each of one or more of the elements; and iron: balance. The microstructure of the casting has 15-60 vol % eutectic carbides and primary carbides dispersed in a ferrous matrix of martensite and is at least substantially free of pearlite.

Hot-press-forming method with excellent productivity and automotive member

Abstract: PROBLEM TO BE SOLVED: To provide a hot-press-forming method, which can process a high strength steel sheet with a tensile strength of 980 MPa or more and besides brings about an excellent productivity, and to provide an automotive member manufactured by the method. SOLUTION: The hot press forming method with the excellent productivity comprises: preparing a steel sheet which has composition containing, by mass%, C:0.1-0.40%, Si:≤0.5%, Mn:0.2-4%, B:0.0002-0.1%, Ti:0.001-3.0%, P:≤0.1%, S:≤0.05%, Al:0.005-0.1%, N:≤0.01% and the balance Fe with inevitable impurities, and has a microstructure composed of ferrite and pearlite, or ferrite, cementite and pearlite; heating the steel sheet at a heating rate of 1-100°C/s; retaining it at 750-900°C for 10-6000 seconds; forming it in the temperature range of 400-700°C; and then cooling it so as to obtain martensite structure of 95% or more by an area ratio. COPYRIGHT: (C)2005,JPO&NCIPI

The Effect of Nonmetal Inclusions and Microstructure on Local Corrosion of Carbon and Low-alloyed Steels

Abstract: The resistance of carbon and low-alloyed steels to local corrosion is shown to be directly determined by their phase-structure, rather than chemical, composition. A correlation between the presence of various nonmetal inclusions in steels and the local corrosion rate is revealed. Inclusions containing Ca and S are shown to be the most corrosive, manganese sulfide inclusions act less corrosively, and silicates are the least active. The formation of lamellar pearlite in the steel structure also impairs its resistance to local corrosion. By providing metal pure of corrosive nonmetallic inclusions at a melting stage and ensuring the pearlite spheroidizing during the stages of thermal treatment and processing, it is possible to enhance its resistance to total and local corrosion.

A new approach for rapid annealing of medium carbon steels

Abstract: The kinetic effects of high magnetic field on the solid-state phase transformation during cooling in medium carbon steel are investigated. Imposing of a 14-Tesla magnetic field has promoted the proeutectoid ferritic transformation from austenite, and resulted in uniform and refined microstructure of ferrite and pearlite even under rapid cooling. Insight into the rapid magnetic annealing process contributes to the development of new heat-treatment techniques with high productivity.

A model for ferrite/pearlite band formation and prevention in steels

Abstract: A model for predicting the conditions under which ferrite/pearlite band formation occurs, and therefore the conditions in which it can be avoided in steels, has been developed. The model requires as input the alloy composition and microchemical segregation wavelength, and provides in turn the homogenization temperature and time in which the alloy should be held in the austenite region for band elimination. The model was applied to three alloys and predicted with accuracy the conditions under which bands were observed to disappear in different investigations from literature. The conditions under which the model can be applied to any alloy are explored.

Decomposition of retained austenite during coiling process of hot rolled TRIP-aided steels

Abstract: Laboratory simulation of hot rolling and coiling processes and in situ heating observations have been carried out in order to evaluate the decomposition behavior of the residual austenite in hot rolled 0.2C–1.5Mn–2Si transformation induced plasticity (TRIP)-aided steels. Residual austenite was thermally stable up to 350 °C and then decomposed into various phases depending on the coiling temperatures: carbide-free bainitic ferrite, ferrite associated with cementite and pearlite. The difference of these decomposition behaviors will be discussed in terms of the variation of carbon concentration and thermal stability of the residual austenite depending on coiling temperatures and holding times.

Method for manufacturing high strength component, and high strength component

Abstract: PROBLEM TO BE SOLVED: To provide a method for manufacturing a high strength component which has a strength of over 1200 MPa after forming by high temperature and is excellent in a hydrogen embrittlement resistance, and further to provide the high strength component. SOLUTION: The method for manufacturing the high strength component is carried out as follows. A steel plate containing a composition consisting of, by mass, 0.1 to 0.55% C, 0.1 to 3% Mn, ≤1% Si, ≤0.03% S, ≤0.1% P and ≤0.01% N and the balance Fe with inevitable impurities is heated up to a temperature of from the Ac3 point to the melting point. After that, the forming work is started at a temperature higher than the transformation temperatures for ferrite, pearlite, bainite, and martensite structure. After the forming work, the component is quenched by cooling it in a die to produce the high strength component. In a process that shearing work is carried out, when the heating atmosphere has the hydrogen content of ≤6% and the dew point of ≤15°C, the ratio of the length of the broken-out section of the cut surface to the thickness is ≤80%, and the ratio of the shear droop to the thickness is ≤7.0%. COPYRIGHT: (C)2009,JPO&INPIT

High temperature deformation behavior of bulk cementite produced by mechanical alloying and spark plasma sintering

Abstract: Using the bulk cementite samples produced by mechanical alloying and spark plasma sintering technique, high temperature deformation behaviors of cementite were studied. To avoid decomposition at high temperature, 5 at.% Mn alloyed cementite ((Fe 0.95 Mn 0.05 ) 3 C) was mostly used. Brittle fracture was observed in the bulk cementite when deformed at a high strain rate at elevated temperatures (studied up to 1273 K). However, compression strain (this true strain larger than 2) was observed at around A 1 temperature (1000 K) at stresses in the range of 100–150 MPa. Specimens were deformed without cracks and without change in hardness. After this superplastic flow, grains are slightly elongated and grain growth was observed. It was demonstrated that the bulk cementite sheets can be deformed cooperatively with low carbon steel sheets and fine layered cementite/low carbon steel laminates could be fabricated by superplastic deformation. As a possible deformation mechanism of lamellae cementite in pearlite, grain boundary sliding was proposed.

Microstructure control and wear resistance of grain boundary allotriomorphic ferrite/granular bainite duplex steel

Abstract: The abrasive wear resistance of a hot-rolled and air-cooled high strength low alloy (HSLA) steel with grain boundary allotriomorphic ferrite/granular bainite (F GBA /Bg) duplex microstructure was investigated by comparison with the granular bainitic steel with the same composition and other two commercial HSLA ferrite/pearlite steels. The results from the wet sand/rubber wheel abrasion test show that the F GBA /Bg duplex steel possesses remarkably higher wear resistance than the two ferrite/pearlite steels, even higher than the granular bainitic steel with a higher hardness. The cross-sectional morphology of the worn surfaces reveals that the higher resistance to abrasive wear is mainly ascribed to the dispersive hard martensite/austenite (M/A) islands. The qualitative analysis on the work hardening characteristics and the microcrack initiation mechanisms indicates that in the F GBA /Bg duplex steel, the existence of the grain boundary allotriomorphic ferrite and the retained austenite in the self-tempered M/A islands improves the accommodated deformation capacity of the subsurface microstructure, and prevents the brittle exfoliation occurring in the granular bainitic steel.

Hot forming method

Abstract: PROBLEM TO BE SOLVED: To provide a hot forming method, which improves formability of a steel plate in drawing more greatly than conventional methods, and which produces a product having an excellent strength after forming SOLUTION: The hot forming method for a steel plate is carried out by using a die 11 having a blank holding part 13 and a flange part 17 of a die 16, which are composed of materials having smaller heat conductivity than steel After heating the steel plate to a temperature ranging from A C3 point to its melting point, while holding the heated steel plate between the blank holder part 13 and the flange part 17 of the die 16, forming of the steel plate is started at a temperature higher than that to cause any of transformation into ferrite, pearlite, bainite, and martensite to draw the steel plate, and rapid cooling is conducted after the drawing COPYRIGHT: (C)2005,JPO&NCIPI

Effects of strong magnetic fields on bainitic transformation

Abstract: Effects of magnetic fields on bainitic transformation temperature, transformation behavior and transformed structures in Fe-based alloys are reviewed, and compared with corresponding effects on ferrite, pearlite and martensite. Bainitic transformation start temperature is increased and the transformation behavior is accelerated by applying magnetic fields. A magnetic field of 10 T is much more effective than the applied stress of 172 MPa for the acceleration of bainitic transformation. No magnetic effect on the transformed structure was observed so far except that the volume fraction of bainite is much larger in magnetic fields.

Dilatometric analysis on phase transformations of intercritical annealing of Fe−Mn−Si and Fe−Mn−Si−Cu low carbon TRIP steels

Abstract: Evaluations of austenite fraction and transformation kinetics upon intercritical annealing of low carbon TRIP steels were attempted using quantitative dilatometric analysis The measured dilation curves were analyzed by taking the carbon distribution between austenite and its decomposed phases into account The amount of austenite formed during intercritical annealing and its carbon content obtained by dilatometric measurement was compared with the values predicted by thermodynamic calculations under the ortho-equilibrium and para-equilibrium conditions The kinetics of the reaustenization process including pearlite dissolution and non-isothermal and isothermal formation of austenite could be quantitatively characterized by means of a modified JMAK (Johnson-Mehl-Avrami-Kolmogrov) equation