scholarly journals Modeling of Precipitation Hardening during Coiling of Nb–Mo Steels

Metals ◽  
2018 ◽  
Vol 8 (10) ◽  
pp. 758 ◽  
Author(s):  
Jean-Yves Maetz ◽  
Matthias Militzer ◽  
Yu Chen ◽  
Jer-Ren Yang ◽  
Nam Goo ◽  
...  
Keyword(s):  
Electron Backscatter Diffraction ◽  
Bainitic Ferrite ◽  
High Strength Steels ◽  
High Strength ◽  
Carbon Steels ◽  
Age Hardening ◽  
Precipitation Strengthening ◽  
Low Carbon ◽  
Coiling Temperature ◽  
Hsla Steels

Nb–Mo low-alloyed steels are promising advanced high strength steels (AHSS) because of the highly dislocated bainitic ferrite microstructure conferring an excellent combination of strength and toughness. In this study, the potential of precipitation strengthening during coiling for hot-strip Nb–Mo-bearing low-carbon steels has been investigated using hot-torsion and aging tests to simulate the hot-rolling process including coiling. The obtained microstructures were characterized using electron backscatter diffraction (EBSD), highlighting the effects of Nb and Mo additions on formation and tempering of the bainitic ferrite microstructures. Further, the evolution of nanometer-sized precipitates was quantified with high-resolution transmission electron microscopy (HR-TEM). The resulting age hardening kinetics have been modelled by combining a phenomenological precipitation strengthening model with a tempering model. Analysis of the model suggests a narrower coiling temperature window to maximize the precipitation strengthening potential in bainite/ferrite high strength low-alloyed (HSLA) steels than that for conventional HSLA steels with polygonal ferrite/pearlite microstructures.

Development of Low Carbon Microalloyed Ultra High Strength Steels

2005 ◽  
Vol 500-501 ◽  
pp. 551-558 ◽  
Author(s):  
A. Ghosh ◽  
Brajendra Mishra ◽  
Subrata Chatterjee
Keyword(s):  
Room Temperature ◽  
High Strength Steels ◽  
High Strength ◽  
Low Carbon ◽  
Hsla Steels ◽  
Finish Rolling ◽  
Ultra High Strength Steels ◽  
Rolling Temperatures ◽  
Carbon Microalloyed ◽  
Carbon Concentrations

In the present study HSLA steels of varying carbon concentrations, alloyed with Mn, Ni, Cr, Mo, Cu and micro-alloyed with Nb and Ti were subjected to different finish rolling temperatures from 850oC to 750oC in steps of 50oC. The microstructure of the steel predominantly shows martensite. Fine twins, strain induced precipitates in the martensite lath along with e-Cu precipitates are observed in the microstructure. With an increase in carbon content the strength value increases from 1200MPa UTS to 1700MPa UTS with a negligible reduction in elongation. Impact toughness values of 20-26 joules at room temperature and −40oC were obtained in sub-size samples.

A New Approach Using EBSD to Quantitatively Distinguish Complex Transformation Products Along the HAZ in X80 Linepipe Steel

2014 ◽  
Author(s):  
Jennifer M. Reichert ◽  
Matthias Militzer ◽  
Warren J. Poole ◽  
Laurie Collins
Keyword(s):  
Orientation Relationship ◽  
Transformation Temperature ◽  
Retained Austenite ◽  
Electron Backscatter Diffraction ◽  
High Strength ◽  
Transformation Products ◽  
Carbon Steels ◽  
Low Carbon ◽  
Structure Property ◽  
Linepipe Steel

State-of-the-art linepipe steels are microalloyed low-carbon steels that combine high strength and fracture toughness with good weldability. During welding of pipe sections the heat affected zone (HAZ) experiences rapid thermal cycles resulting in a graded microstructure that can be significantly different from that of the base metal. In particular a variety of bainitic microstructures can form in the HAZ. Depending on the type of bainite mechanical properties may be improved or may lead to poor fracture resistance and be detrimental to the overall HAZ performance. Optical microscopy is not sufficient to differentiate bainitic morphologies which vary with the transformation temperature. The investigated X80 linepipe steel also contains retained austenite at room temperature. Based on the retained austenite it is possible to characterize the orientation relationship (OR) between austenite and the transformation products. It is found that bainite shows an orientation relationship near Kurdjumov-Sachs with the prior austenite. Variant selection is related to the driving force for the bainite reaction and hence depends on the transformation temperature. In the current study Electron BackScatter Diffraction (EBSD) mapping is used to characterize transformation products based on their orientation relationship. This approach offers a quantitative way to determine volume fractions of different types of bainite in complex HAZ microstructures which is necessary to establish structure-property relationships of the HAZ.

scholarly journals Thermal Diffusivity of Traditional and Innovative Sheet Steels

2010 ◽  
Vol 297-301 ◽  
pp. 893-898
Author(s):  
Elena Campagnoli ◽  
Paolo Matteis ◽  
Giovanni M.M. Mortarino ◽  
Giorgio Scavino
Keyword(s):  
Thermal Diffusivity ◽  
Deep Drawing ◽  
Process Model ◽  
Twip Steel ◽  
Welding Process ◽  
High Strength Steels ◽  
High Strength ◽  
Carbon Steels ◽  
Low Carbon ◽  
Low Carbon Steels

The low carbon steels, used for the production of car bodies by deep drawing, are gradually substituted by high strength steels for vehicle weight reduction. The drawn car body components are joined by welding and the welded points undergo a reduction of the local tensile strength. In developing an accurate welding process model, able to optimized process parameters and to predict the final local microstructure, a significant improvement can be given by the knowledge of the welded steels thermal diffusivity at different temperatures. The laser-flash method has been used to compare the thermal diffusivity of two traditional deep drawing steels, two high strength steels already in common usage, i.e. a Dual Phase (DP) steel and a TRansformation Induced Plasticity (TRIP) steel, and one experimental high-Mn austenitic TWIP (Twinning Induced Plasticity) steel. The low carbon steels, at low temperatures, have a thermal diffusivity that is 4-5 times larger than the TWIP steel. Their thermal diffusivity decreases by increasing temperature while the TWIP steel shows an opposite behaviour, albeit with a lesser slope, so that above 700°C the TWIP thermal diffusivity is larger. The different behaviour of the TWIP steel in respect to the ferritic deep drawing steels arises from its non ferro-magnetic austenitic structure. The DP and TRIP steels show intermediate values, their diffusivity being lower than that of the traditional deep drawing steels; this latter fact probably arises from their higher alloy content and more complex microstructure.

Hydrogen effects on fracture of high-strength steels with different micro-alloying

2015 ◽  
Vol 33 (6) ◽  
pp. 515-527 ◽  
Author(s):  
Olga Todoshchenko ◽  
Yuriy Yagodzinskyy ◽  
Valentina Yagodzinska ◽  
Tapio Saukkonen ◽  
Hannu Hänninen
Keyword(s):  
Chemical Composition ◽  
Electron Backscatter Diffraction ◽  
High Strength Steels ◽  
Thermal Desorption Spectroscopy ◽  
High Strength ◽  
Constant Load ◽  
Carbon Steels ◽  
Hydrogen Induced Cracking ◽  
Load Tests ◽  
Backscatter Diffraction

AbstractConstant load tests of high-strength carbon steels with different micro-alloying using strengths in the range of 1000–1400 MPa were performed at ambient temperature under continuous electrochemical hydrogen charging. Hydrogen markedly affects delayed fracture of all the studied steels. Fractography of the studied steels shows that fracture mechanism depends on the chemical composition of the studied steels and hydrogen-induced cracking exhibits intergranular or transgranular character occurring often in the form of hydrogen flakes. The size and chemical composition of non-metallic inclusions are analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Hydrogen-induced cracking initiates at TiN/TiC particles in steels with Ti alloying. Crack paths are studied with electron backscatter diffraction mapping to analyze crack initiation and growth. The thermal desorption spectroscopy method is used to analyze the distribution of hydrogen in the trapping sites. The mechanisms of hydrogen effects on fracture of high-strength steels are discussed.

Effect of Coiling Temperature on Kinetics of Austenite Formation in Cold Rolled Advanced High Strength Steels

2012 ◽  
Vol 706-709 ◽  
pp. 2112-2117 ◽  
Author(s):  
R.R. Mohanty ◽  
O.A. Girina
Keyword(s):  
High Strength Steels ◽  
High Strength ◽  
Continuous Heating ◽  
Austenite Formation ◽  
Continuous Annealing ◽  
Hard Material ◽  
Low Carbon ◽  
Coiling Temperature ◽  
Hot Rolled ◽  
Kinetics Of

A systematic experimental investigation was conducted using lab processed low carbon 0.08C-2.0Mn-Cr-Mo steel microalloyed with Ti/Nb to evaluate the influence of initial hot-rolled microstructures on the kinetics of austenite formation and decomposition after cold-rolling and subsequent annealing. Coiling temperature as a major hot rolling parameter was used to obtain different types of hot-rolled microstructures. Dilatometer and continuous annealing simulator were employed for austenite formation studies and annealing simulations, respectively. It was found that the coiling temperature affects the processes occurring during heat treatment in continuous annealing lines of full hard material: ferrite recrystallization, austenite formation during continuous heating and austenite decomposition during cooling. A decrease in coiling temperature accelerates the recrystallization of ferrite and nucleation of austenite, which results in formation of refined ferrite-martensite structure. The effect of initial hot rolled structure on final mechanical properties after continuous annealing was also investigated.

CHARACTERIZATION OF BAINITIC MICROSTRUCUTRES IN LOW CARBON HSLA STEELS

2008 ◽  
Vol 22 (31n32) ◽  
pp. 5965-5970 ◽  
Author(s):  
JU SEOK KANG ◽  
CHAN GYUNG PARK
Keyword(s):  
Acicular Ferrite ◽  
Focused Ion Beam ◽  
Ion Beam ◽  
Bainitic Ferrite ◽  
Carbon Steels ◽  
Granular Bainite ◽  
Low Carbon ◽  
High Angle ◽  
Rate Condition ◽  
Hsla Steels

The austenite phase of low carbon steels can be transformed to various bainitic microstructures such as granular bainite, acicular ferrite and bainitic ferrite during continuous cooling process. In the present study site-specific transmission electron microscope (TEM) specimens were prepared by using focused ion beam (FIB) to identify the bainitic microstructure in low carbon high strength low alloy (HSLA) steels clearly. Granular bainite was composed of fine subgrains and 2nd phase constituents like M/A or pearlite located at grain and/or subgrain boundaries. Acicular ferrite was identified as an aggregate of randomly orientated needle-shaped grains. The high angle relations among acicular ferrite grains were thought to be caused by intra-granular nucleation, which could be occur under the high cooling rate condition. Bainitic ferrite revealed uniform and parallel lath structure within the packet. In some case, however, the parallel lathes showed high angle relations due to packet overlapping during grow of bainitic ferrite, resulting in high toughness properties in bainitic ferrite based steels.

Low-Carbon Steels

2015 ◽  
pp. 233-275
Keyword(s):  
High Strength ◽  
Carbon Steels ◽  
Interstitial Free ◽  
Low Carbon ◽  
Low Carbon Steels ◽  
Trip Steels ◽  
Hsla Steels ◽  
Twip Steels ◽  
Cold Rolled ◽  
Hot Rolled

This chapter discusses various alloying and processing approaches to increase the strength of low-carbon steels. It describes hot-rolled low-carbon steels, cold-rolled and annealed low-carbon steels, interstitial-free or ultra-low carbon steels, high-strength, low-alloy (HSLA) steels, dual-phase (DP) steels, transformation-induced plasticity (TRIP) steels, and martensitic low-carbon steels. It also discusses twinning-induced plasticity (TWIP) steels along with quenched and partitioned (Q&P) steels.

Low Temperature Precipitation in Nb-Added Si- and Si-Free Bainitic Steels

2012 ◽  
Vol 706-709 ◽  
pp. 2384-2389 ◽  
Author(s):  
Ian Zuazo ◽  
Sebastian Cobo
Keyword(s):  
High Strength Steels ◽  
High Strength ◽  
Atom Probe ◽  
Carbon Steels ◽  
Peak Hardness ◽  
Low Carbon ◽  
Steel Strips ◽  
Bainitic Steels ◽  
Transmission Electron ◽  
Precipitation Phenomena

Precipitation strengthening by fine Nb-rich particles represents an important element on the design of low carbon high strength steels. This is typically obtained on steel strips by thermal holding at temperatures above 600°C following the austenite to ferrite transformation. These conditions are beneficial to obtain a large precipitation of small Nb-rich precipitates. On the other hand, precipitation at lower temperatures, in a phase already hard, such as bainite, has been scarcely studied. In this work, the precipitation phenomena occurring during isothermal treatments following the austenite to bainite transformation at 450°C are investigated. For this purpose, two Nb-alloyed low carbon steels with and without silicon are studied and the evolution of the microstructure is determined by the use of transmission electron microscopy and followed by hardness measurements. The presence of a hardness peak is not detected until long isothermal times (150h). Preliminary atom probe tomography (APT) characterization provides insight on the possible presence of fine NbC precipitates at the peak hardness treatment. A comparison with a Nb-free alloy indicates a significant hardening effect of niobium on the bainitic structure.

Precipitation Strengthening in a Low Carbon Nb-Microalloyed Steel

2005 ◽  
Vol 500-501 ◽  
pp. 481-488 ◽  
Author(s):  
D.Q. Bai ◽  
F. Hamad ◽  
J. Asante ◽  
S. Hansen
Keyword(s):  
Mechanical Properties ◽  
Microalloyed Steel ◽  
High Strength Steels ◽  
Aging Treatment ◽  
High Strength ◽  
Precipitation Strengthening ◽  
Microalloyed Steels ◽  
Low Carbon ◽  
Nb Microalloyed Steel ◽  
Carbon Microalloyed

Among modern weldable high strength steels, low carbon microalloyed steels have been widely used for linepipe, construction, and automobile industries. One of the major technical components to successfully produce these steels is to effectively use precipitation strengthening. In the present paper, the effect of an aging treatment on the microstructure and mechanical properties of a low carbon Nb-microalloyed steel is analyzed.

Phase Field Modelling of Austenite Formation in Low Carbon Steels

2011 ◽  
Vol 172-174 ◽  
pp. 1050-1059 ◽  
Author(s):  
Matthias Militzer ◽  
Hamid Azizi-Alizamini
Keyword(s):  
Phase Field ◽  
Intercritical Annealing ◽  
High Strength Steels ◽  
High Strength ◽  
Carbon Steels ◽  
Austenite Formation ◽  
Low Carbon ◽  
Advanced High Strength Steels ◽  
Robust Processing ◽  
Multi Phase

There is renewed interest in the investigation of austenite formation due to the development and increased use of advanced high strength steels for automotive applications. Intercritical annealing is an essential processing step for cold rolled and coated steel products with multi-phase microstructures. During intercritical annealing the initial ferrite-pearlite microstructure transforms partially to austenite. Models for the austenite formation are critical to predict the austenite fraction as a function of the thermal cycle thereby facilitating the design and control of robust processing paths. Modelling the austenite formation is challenging because of the morphological complexity of this transformation. Phase field models are a powerful tool to describe the evolution of microstructures with complex morphologies, e.g. formation of finger-type features during austenite formation. The present paper gives an overview of model approaches for the austenite formation. Phase field simulations are presented for two scenarios: (i) austenite formation from a fully pearlitic structure with a lamellar arrangement of carbide aggregates and (ii) austenite formation from ferrite-pearlite microstructures. Simulation results are compared with experimental observations for pearlitic steels. The challenges are delineated for the development of austenite formation models with predictive capabilities.

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