Analysis of change in microstructure and properties during high temperature processing of ultralow C and high Nb microalloyed steel

Abstract

To further improve the strength and toughness, the advanced thermomechanical controlled processing has been applied in the development of an ultralow C and high Nb bearing steel. In the present investigation, the effects of processing parameters, consisting of the coiling and starting temperatures in non-recrystallisation region, on the final microstructure and mechanical properties of this steel have been studied by tensile, Charpy impact tests, optical microscopy and transmission electron microscopy. Results indicate that the acicular ferrite dominated microstructure can be greatly refined in grain size with decreasing the starting temperature of finishing rolling. However, for high Nb steels, the too low starting temperature would promote the formation of high temperature transformation products and consequently make against the improvement of mechanical properties. In addition, the optimum temperature window of finishing rolling is found to be also related to alloying levels of austenite stabilising elements. At the high starting temperature of finishing rolling, the precipitation strength contribution increases with increasing coiling temperature. However, the increase in strain accumulation associated with low temperature processing greatly reduces the sensitivity of the precipitation strength contribution to coiling temperature.     

Control of ductile to brittle transition temperature in ferritic pressure vessel steel

Abstract

Controlled amounts of cold work are shown to cause a minimum in the ductile to brittle transition temperature (DBTT) in a ferritic steel at a critical level of ～1·5%. Mechanical property assessments show that the hardness values exhibit the same trend. A theory is advanced for explanation of these effects, based on work hardening and Cottrell–Bilby locking models. Consideration is given to an alternative Ashby–Embury model, but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour. Although independent of fracture surface type, the degree of plastic deformation shows some dependency on the grain boundary character. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. Discussion focuses on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.     

Microstructure and properties of low manganese and niobium containing HIC pipeline steel

The paper describes the concept of using low manganese content in pipeline steels for hydrogen-induced cracking (HIC) applications. The microstructure of thermomechanically processed pipeline steel primarily consisted of polygonal ferrite and low fraction of pearlite. The cleanliness of the steel was evident as was the absence of centerline segregation. The microstructure contained high dislocation density, sub-boundaries and dislocation substructures. Fine-scale precipitation of niobium carbides occurred on parallel array of dislocations and on random dislocations that followed [0 0 1]_NbC//[0 0 1]_α-Fe relationship with the ferrite matrix.     

Effect of cold work and aging on mechanical properties of a copper bearing microalloyed HSLA-100 (GPT) steel

Investigations have been carried out on the effect of cold work and subsequent aging on mechanical properties of a Cu-bearing HSLA-100 steel microalloyed with Nb and Ti. Aging at 400°C after various degrees of cold work (25–70 pct) exhibits multiple hardness peaks. The treatments cause significant improvement in hardness and tensile strength, but at the cost of impact strength. Cold work also causes deterioration in ductility, which again improves on subsequent aging. The C70A treatment involving 70 pct deformation exhibits maximum response to age hardening giving a hardness of 465 VHN and a UTS of 1344 MPa, but with low values of ductility (5 pct) and impact energy (24 J). C50A treatment involving 50 pct cold work and aging results in an optimum combination of mechanical properties. This treatment in the second hardness peak stage yields a hardness of 373 VHN, UTS of 1186 MPa together with a ductility value of 11 pct and impact energy of 109 J. Scanning electron microscopic studies of fracture surfaces reveal that the impact fracture occurs by formation of dimples and nucleation and growth of voids and cracks. Fracture in tensile specimens is caused by formation of voids and cracks at high density striations. Formation of voids and cracks is also assisted by the presence of precipitated carbide particles.     

Copper bearing microalloyed ultrahigh strength steel on a pilot scale: Microstructure and properties

In the present study, copper bearing low carbon microalloyed ultrahigh strength steel has been produced on a pilot scale. Transformation of the aforesaid steel during continuous cooling has been evaluated. The steel sample has been thermomechanically processed followed by either air cooling or water quenching. Variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. A mixture of granular bainite, bainitic ferrite and precipitation of nano-sized (Ti, Nb)C particles is the characteristic microstructural feature of air cooled steel. On the other hand, predominantly lath martensitic structure along with the similar type of microalloying precipitates of air cooled steels and Cu precipitates are obtained in case of water quenched steel. The best combination of strength (1364-1403 MPa) and ductility (11-14%) has been achieved for the selected range of finish rolling temperature of water quenched steel.     

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10?nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.     

Low carbon high manganese bainitic steel

Abstract

The present study concerns the mechanical properties of low carbon (0·05 wt-%) high Mn bainitic steel. The continuous cooling transformation diagram exhibited bainitic transformation without any prior diffusive transformation of austenite even for a cooling rate as low as 0·5°C/s. The bainitic steels have shown continuous elongation behaviour with attractive combination of strength (>1200 MPa) and elongation (>14%). The bainitic microstructure obtained after annealing treatment has yielded excellent combination of strength, uniform elongation, yield ratio and static toughness value.     

Influence of Mo content on microstructure and mechanical properties of high strength pipeline steel

The effect the Mo content on the microstructure and mechanical properties of high strength pipeline steel X80 was investigated in the present study. Optical microscopy, scanning electron microscopy and transmission electron microscopy were used to investigate the microstructure of the steel. It was concluded that, the proportion of acicular ferrite increased as the amount of Mo content increased, which resulted in the enhancement of yield and tensile strength; on the other hand, the yield ratio decreased and the toughness deteriorated as the quantity of M–A increased.     

Cooling process and mechanical properties design of hot-rolled low carbon high strength microalloyed steel for automotive wheel usage

For the purpose of developing Nb–V–Ti microalloyed, hot rolled, high strength automotive steel for usage in heavy-duty truck wheel-discs and wheel-rims, appropriate cooling processes were designed, and microstructures and comprehensive mechanical properties (tension, bending, hole-expansion, and Charpy impact) of the tested steels at two cooling schedules were studied. The results indicate that the steel consists of 90% 5 μm polygonal ferrite and 10% pearlite when subjected to a cooling rate of 13 °C/s and a coiling temperature of 650 °C. The yield strength, tensile strength, and hole-expansion ratio are 570 MPa, 615 MPa, and 95%, respectively, which meet the requirements of the wheel-disc application. The steel consists of 20% 3 μm polygonal ferrite and 80% bainite (granular bainite and a small amount of acicular ferrite) when subjected to a cooling rate of 30 °C/s and a coiling temperature of 430 °C. The yield strength, tensile strength, and hole-expansion ratio are 600 MPa, 655 MPa, and 66%, respectively, which meet the requirements of the wheel-rim application. Both the ferrite–pearlite steel and ferrite–bainite steel possess excellent bendability and Charpy impact property. The precipitation behavior and dislocation pattern are characterized and discussed.     

Flat friction stir spot welding of low carbon steel by double side adjustable tools

2 mm low carbon steel plates were successfully welded by the flat friction stir spot welding(FSSW) using double side adjustable tools, by which the keyhole formed in the conventional FSSW was eliminated and a flat surface on both the top and bottom sides of the welded joints was obtained. In addition, the hook shape usually generated in the conventional FSSW was eliminated by this technique, and the unbonded interface was parallel to the surface of the sheets. Owing to the enlarged bonded interface width by eliminating the keyhole and the intermixed interface by the adjustable probe, the plug fracture occurred under all the welding conditions in the present study. Due to the suppression of the thickness thinning and elimination of the hook shape, the joint performance was improved in the plug fracture mode. The shear tensile performance was considered to strongly depend on the microstructure in the tip area of the unbonded interface and the maximum shear fracture load of 23.0 kN was achieved in this study.     

Effect of water quench process on mechanical properties of cold rolled dual phase steel microalloyed with niobium

Effect of water quench process on tensile properties of microalloyed dual phase (DP) steel was investigated. The results showed that the tensile strength and yield strength decrease while total elongation increases with decreasing quenching temperature. Different quenching temperatures lead to not only different amounts of martensite, but also different volume fractions of new ferrite. Due to exemption of precipitation strengthening, new ferrite will contribute to not only the increase of ductility, but also the improvement of microalloyed DP steel strength.     

Microstructural research on hot strips of low carbon steel produced by a compact strip production line under different thermal histories

Coupons with the same composition and thickness (4.0 mm nominal gauge) obtained from hot strips of low carbon steel underwent a series of investigations to analyze the microstructural characteristics and mechanisms responsible for their differences in mechanical properties. Two different industrial technologies were adopted, although the strips used in this research were produced on the same Compact Strip Production (CSP) line. One of the strips was produced with a routine γ→ CSP thermal history, but the other with a γ→→γ* conventional thermal history. The only difference between them was that one technology had a →γ* thermal history. Different specimens of both types of strips were prepared for metallographic observation, tensile tests, electron back-scattered diffraction tests and positron annihilation technique tests. Experimental results showed that the differences in mechanical properties could be ascribed to dissimilarities not only in the grain size and textural components but also in dislocation density.     

Effect of heat input on microstructure and mechanical properties of dissimilar joints between super duplex stainless steel and high strength low alloy steel

In the present study, microstructure and mechanical properties of UNS S32750 super duplex stainless steel (SDSS)/API X-65 high strength low alloy steel (HSLA) dissimilar joint were investigated. For this purpose, gas tungsten arc welding (GTAW) was used in two different heat inputs: 0.506 and 0.86 kJ/mm. The microstructures investigation with optical microscope, scanning electron microscope and X-ray diffraction showed that an increase in heat input led to a decrease in ferrite percentage, and that detrimental phases were not present. It also indicated that in heat affected zone of HSLA base metal in low heat input, bainite and ferrite phases were created; but in high heat input, perlite and ferrite phases were created. The results of impact tests revealed that the specimen with low heat input exhibited brittle fracture and that with high heat input had a higher strength than the base metals.     

Equal-channel angular sheet extrusion of interstitial-free (IF) steel: Microstructural evolution and mechanical properties

Interstitial-free steel (IF-steel) sheets were processed at room temperature using a continuous severe plastic deformation (SPD) technique called equal-channel angular sheet extrusion (ECASE). After processing, the microstructural evolution and mechanical properties have been systematically investigated. To be able to directly compare the results with those from the same material processed using discontinuous equal channel angular extrusion, the sheets were ECASE processed up to eight passes. The microstructural investigations revealed that the processed sheets exhibited a dislocation cell and/or subgrain structures with mostly low angle grain boundaries. The grains after processing have relatively high dislocation density and intense micro-shear band formation. The electron backscattering diffraction (EBSD) examination showed that the processed microstructure is not fully homogeneous along the sheet thickness due probably to the corner angle of 120° in the ECASE die. It was also observed that the strengths of the processed sheets increase with the number of ECASE passes, and after eight passes following route-A and route-C, the yield strengths reach 463 MPa and 459 MPa, respectively, which is almost 2.5 times higher than that of the initial material. However, the tensile ductility considerably dropped after the ECASE. The limited ductility was attributed to the early plastic instability in the tensile samples due to the inhomogeneous microstructure. The specimen orientation with respect to the ECASE direction did not have a considerable effect on the stress-strain response. Appropriate low temperature annealing of ECASE-processed IF-steel resulted in a good strength-ductility balance.     

Copper precipitation and its impact on mechanical properties in a low carbon microalloyed steel processed by a three-step heat treatment

The structure–mechanical property relationship, with particular focus on effect of tempering process on the microstructural evolution and mechanical properties was investigated in a low carbon Cu-bearing steel that was processed in three-steps, namely, intercritical annealing, intercritical tempering, and tempering heat treatment. The objective of adopting three steps was to elucidate the nature and evolution of microstructural constituents that contributed to high strength–ductility combination in the studied steel. The three-step processing led to a microstructure primarily comprising of ferrite, retained austenite, and small amount of bainite/martensite. The mechanical properties obtained were: yield strength > 720 MPa, tensile strength > 920 MPa, uniform elongation > 20%, total elongation > 30%, and low yield ratio of 0.78. The tempering step led to a significant increase in both yield and tensile strength and decrease in yield ratio, without reducing ductility, a behavior attributed to the precipitation of copper in retained austenite and ferrite. The precipitation of copper enhanced the stability of retained austenite and work hardening rate, leading to a high volume fraction of retained austenite (∼29%), with consequent increase in elongation and significant increase in yield and tensile strength during tempering.     

Effect of Nb-Mo additions on precipitation behaviour in V microalloyed TRIP-assisted steels

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.     

Role of zirconium in microalloyed steels: a review

Recently there has been a renewed interest in the addition of zirconium to microalloyed steels. It has been used since the early 1920s, but has never been universally employed, as have niobium, titanium or vanadium. The functions of zirconium in steelmaking are associated with a strong chemical affinity, in decreasing order, for oxygen, nitrogen, sulphur and carbon. Historically, the main use of additions of zirconium to steel was for combination preferentially with sulphur, to avoid the formation of manganese sulphide, known to have a deleterious influence of the impact toughness of wrought and welded steel. Modern steelmaking techniques have also raised the possibility that zirconium additions can reduce the austenite grain size and increase dispersion strengthening, due to precipitation of zirconium carbonitrides, or in high nitrogen vanadium–zirconium steels, vanadium nitride. This review gathers information on the compounds of zirconium identified in steels together with crystallographic data and solubility equations. Also brief accounts of the role of sulphides and particles in general on austenite grain size control and toughness are included.     

Stress-induced non-equilibrium grain boundary segregation of phosphorus in a Cr–Mo low alloy steel

Grain boundary segregation of phosphorus under a 40 MPa tensile stress at 520 °C in a 0.025 wt.% P-doped 2.25Cr1Mo steel, which has already been thermally equilibrated, is examined using Auger electron spectroscopy. The segregation of phosphorus during stress-ageing has a non-equilibrium characteristic, i.e. it is non-equilibrium segregation. The segregation level first increases with increasing stress-ageing time until about 0.5 h and then diminishes with further increasing stress-ageing time, leading the boundary concentration of phosphorus to return to its thermal equilibrium value after ageing for about 15 h. Therefore, the critical time for this non-equilibrium grain boundary segregation of phosphorus is about 0.5 h at which the segregation is peaked. At this critical time, the boundary concentration of phosphorus is about 20.5 at.%, which is about 4.5 at.% higher than its thermal equilibrium level. Xu's kinetic model for stress-induced grain boundary segregation [T.D. Xu, Philos. Mag. 83 (2003) 889–899; T.D. Xu, B.-Y. Cheng, Prog. Mater. Sci. 49 (2) (2004) 109–208] is used to analyse the experimental results, demonstrating that the measured data may be well simulated by the model.     

Low cycle fatigue behaviour of low carbon microalloyed steel: microstructural evolution and life assessment

Abstract

In this paper the cyclic stress–strain response, low cycle fatigue (LCF) behaviour, and evolution of dislocation structures under LCF loading in the case of a low carbon microalloyed steel are discussed. The cyclic stress response revealed cyclic softening resulting from the propagation of Lüders bands. The experimental LCF life was compared with the life predicted using Tomkins' model and the modified universal slopes (MUS) equation. While the life predicted by Tomkins' model showed good correlation with the experimental results, the life predicted using the MUS equation grossly overestimated the life. Inclusion induced delaminations under cyclic loading were thought to be responsible for the overestimation by the MUS equation. Low energy dislocation structures, i.e. cells, were observed near the fracture surfaces. Interrupted tests revealed cell formation after 10 cycles at a total strain amplitude of 0·3%.     

Mechanical properties and hot-rolled microstructures of a low carbon bainitic steel with Cu-P alloying

A low carbon bainitic steel with Cu-P alloying was developed. The new steel aims to meet the demand of high strength, high toughness and resistance to chloride ion corrosion for the components used in the environment of sea water and oceanic atmosphere. Mechanical properties of the steel were tested and strengthening and toughening mechanisms were analyzed by comparing hot-rolled microstructures of the low carbon bainitic steels with and without Cu-P alloying. The results show that Cu-P alloying provided strong solution strengthening with weak effect on ductility. The toughness loss caused by Cu-P alloying could be balanced by increasing the amount of martensite/remained austenite (M/A island) at lower finishing temperature. The static recovery process during rolling interval was delayed by the interaction of phosphorous, copper atoms with dislocations, which was favorable to the formation of bainitic plates. Super-fine Nb(C, N) particles precipitated on dislocations had coherency with bainite ferrite at 830 °C finishing temperature. Raising finishing temperature to 880 °C, Nb(C, N) particles were prone to coarsening and losing coherency. It was also found that no accurate lattice match relationship among retained austenite, martensite and bainite in granular bainitic microstructure.