High-strength Low-carbon Steel Research Articles

Study of Nano-Precipitate in High Strength Low Carbon Steel during Tempering by TEM

The behavior of nanoprecipitates of 800Mpa grade high strength low carbon steel during tempering has been studied. Transmission electron microscope (TEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive spectrometry (EDS) were used to systematically analyze the morphology of precipitates and their grain orientation with matrix at different tempering temperatures. Experimental results confirm that the composition of these nanometer sized particles in the matrix was compound carbonitrides containing Ti, V, Mo and other elements. The precipitates of the as-received steel are (Nb,Ti)(C,N) at low tempering temperature, while those at high tempering temperature are composite carbides containing a variety of elements such as Mo, V, Ti and Nb. On the other hand, as tempering temperature increases, precipitates in the steel were slowly growing up and roughening according with the typical Oswald ripening mechanism; a sharp orientation relationship exists between precipitates and matrix.

Achieving ultrafine dual-phase structure with superior mechanical property in friction stir processed plain low carbon steel

Ultrafine dual-phase structure was successfully prepared in a plain low carbon steel via friction stir processing (FSP) at a tool rotation rate of 400rpm and a travel speed of 50mm/min under rapid water cooling. Ultrafine polygonal ferrite grains (~1μm) were distributed around the martensite phase consisting of nanostructured laths with a width of ~200nm. FSP steel exhibited a good combination of ultrahigh yield strength of ~1100MPa and large uniform elongation of ~7%. This work provides an effective strategy to prepare high strength low carbon steels.

Lengthening kinetics of ferrite plates in high-strength low-carbon low alloy steel

The lengthening of ferrite plates in a high-strength low carbon low alloy steel, which contained 1.62 mass% Mn and small amounts of Si, Mo, Ni, Cr, and Nb, was observed in situ under high-temperature confocal laser scanning microscopy. Applying the analysis by Speich and Cohen to the growth of a lath-type plate, it was found that the measured lengthening rates are likely to fall between those calculated assuming para- and NPLE modes from the semiempirical equation proposed by Hillert, while they scattered widely presumably because the plate morphology was not a lenticular disk. In the temperature range of measurement, solute drag due to alloying element segregation at the boundary between plate tip and the matrix may not operate to an appreciable extent. The retardation of lengthening from that calculated under paraequilibrium could occur by intrinsic friction of α/γ boundary.

Effect of coil cooling conditions on microstructural and mechanical properties uniformity of flat hot rolled AHSS

Experimental and computational results of measurement of the temperature field due to cooling of coils of hot-rolled strip from low-carbon high-strength steel are presented. It is shown that in a conventional production process the coils cool nonuniformly. The nonuniformity of the cooling causes inhomogeneity of the properties both over the length of the strip and over its width.

Three-dimensional characterization of bainitic microstructures in low-carbon high-strength low-alloy steel studied by electron backscatter diffraction

Abstract We investigated the microstructural evolution of high strength low alloy steel, Fe–2.0Mn–0.15Si–0.05C (wt.%), by varying the continuous cooling rates from 1 K/s to 50 K/s using three-dimensional electron backscatter diffraction and transmission electron microscopy. Granular bainitic microstructure was prevalent under a slow cooling rate of 1–10 K/s, while lath-type bainite was dominant at a high cooling rate of 50 K/s. The acicular ferrite that was the major microstructure under the intermediate ranges of cooling rates between 10 K/s and 30 K/s was tangled with each other, leading to a three-dimensional interwoven structure with highly misoriented grains. Because of the formation of three-dimensional structures, we propose that the terms “acicular ferrite” and “bainitic ferrite,” which are currently used in steel, be replaced by the terms “interwoven acicular bainite” and “lath bainite,” respectively. Moreover, we also confirmed that the cooling rate is an important factor in determining whether bainitic microstructures occur in the form of granular bainite, interwoven bainite, or lath bainite.

Effects of Cooling Conditions on Microstructure, Tensile Properties, and Charpy Impact Toughness of Low-Carbon High-Strength Bainitic Steels

In this study, four low-carbon high-strength bainitic steel specimens were fabricated by varying finish cooling temperatures and cooling rates, and their tensile and Charpy impact properties were investigated. All the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituents. The specimens fabricated with higher finish cooling temperature had a lower volume fraction of martensite-austenite constituent than the specimens fabricated with lower finish cooling temperature. The fast-cooled specimens had twice the volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fast-cooled specimen fabricated with high finish cooling temperature and fast cooling rate showed the lowest energy transition temperature among the four specimens because of the lowest content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, despite the presence of some hard microstructural constituents such as bainitic ferrite and martensite-austenite.

Surface and Electrochemical Behavior of HSLA Steel in Supercritical CO2-H2O Environment

General corrosion was observed on high strength low alloy carbon steel after electrochemical impedance spectroscopy experiments (EIS) performed in H2O saturated with CO2 at 50 °C and 15.2 MPa. However, general and localized were observed on the same material surfaces after the EIS experiments performed in supercritical CO2 containing approximately 6100 ppmv H2O at 50 °C and 15.2 MPa. The general corrosion areas were uniformly covered by the FeCO3-like phase identified by X-ray diffraction (XRD). In the area of localized corrosion, XRD also revealed FeCO3-rich islands embedded in α-iron. The energy dispersive X-ray (EDX) analysis revealed high concentrations of iron, carbon, and oxygen in the area affected by general corrosion and in the islands formed in the area of localized corrosion. The real and imaginary impedances were lower in H2O saturated with CO2 than those in the supercritical CO2 containing the aqueous phase indicating faster corrosion kinetics in the former.

Similar and dissimilar resistance spot welding of martensitic advanced high strength steel and low carbon steel: metallurgical characteristics and failure mode transition

Safety, emissions and fuel economy of vehicles have always been of great importance to automakers. Advanced high strength steels (AHSSs) have proved to be effective in fulfilling these needs of the industry, and this has led to great opportunities for increasing usage of AHSSs. The present work aimed to investigate the resistance spot welding characteristics of martensitic steel (M130) having the thickness of 2 mm, utilising metallographic observation, microhardness and tensile–shear testing. The results were compared for different combinations of martensitic steel and low carbon steel (LCS). In the study of interfacial to pull-out failure mode transition, dissimilar M130/LCS welds exhibited the lowest tendency to fail in the interfacial failure mode. Weld nugget diameter is proposed as a main variable controlling the peak load and energy absorption of the welds along with other factors such as electrode indentation and hardness/strength of the failure location.

The formation mechanisms of interlocked microstructures in low-carbon high-strength steel weld metals

Microstructural features and the formation mechanisms of interlocked microstructures of acicular ferrite in a low-carbon high-strength steel weld metal were investigated by means of computer-aided three-dimensional reconstruction technique and electron backscattered diffraction analysis. Multiple nucleation on inclusions, sympathetic nucleation or repeated nucleation, hard impingement, mutual intersection, and fixed orientation relationships of acicular ferrite grains were observed. They were all responsible for the formation of interlocked microstructures in the weld metal. During the process of isothermal transformation, the pre-formed acicular ferrite laths or plates partitioned austenite grains into many small and separate regions, and the growth of later formed acicular ferrite grains was confined in these small regions. Thus, the crystallographic grain size became smaller with the increasing holding time. ► Acicular ferrite is formed by multiple nucleation and sympathetic nucleation. ► Hard impingement and intersection of ferrite grains occur at later stages. ► The pre-formed ferrite laths partition austenite grains into smaller regions. ► The growth of later formed ferrite grains is confined in the smaller regions.

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

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.

Study of the Microstructure and Strain Induced Precipitation during Thermomechanical Processing of Low Carbon Microalloyed Steels

A series of anisothermal multipass hot torsion tests were carried out to simulate hot rolling on three high-strength low-carbon steels with different amounts of Mn, Mo, Nb and Ti and designed for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling. The effect of austenite strengthening obtained at the end of thermomechanical processing on the final microstructure obtained after cooling was studied. Higher levels of austenite strengthening before cooling promote a refinement of final microstructure but can also restrict the fraction of low-temperature transformation products such as acicular ferrite. This combined effect gives rise to a wide range of final microstructures and mechanical properties depending on the composition, processing schedule and cooling rates applied. On the other hand, the precipitation state obtained at diverse temperatures during and at the end of hot rolling schedule was evaluated by means of transmission electron microscopy (TEM) in two microalloyed steels. It was found that two families of precipitates with different morphology, composition and mean size can coexist in microalloyed steels.

Influence of Ti and Nb on the Strength–Ductility–Hole Expansion Ratio Balance of Hot-rolled Low-carbon High-strength Steel Sheets

To satisfy the demand for high-strength steel sheets with excellent strength–ductility–hole expansion ratio balance, high-strength steels consisting of a ferrite or bainitic ferrite matrix strengthened by a large amount of finely dispersed precipitates have been developed.In this study, the influence of the precipitation-forming elements Ti and Nb on the strength–ductility–hole expansion ratio balance of these steels has been investigated. The strength–ductility–hole expansion ratio balance of steel with Ti added was superior to that with Nb added. The reason for the superiority of steel with added Ti was discussed from various viewpoints such as microstructure, texture, the condition of the pierced surface, sulphide formation, the nucleation and propagation behaviour of cracks during hole expansion, etc. We concluded that the inferior strength–ductility–hole expansion ratio balance of the steel with added Nb was mainly caused by the formation of large textural colonies and a detrimental influence of MnS. From these inferences, we proposed a combined addition of Ti and Nb, and succeeded in improving the strength–ductility–hole expansion ratio balance of the advanced high-strength steel.

Voltage-Pulsed and Laser-Pulsed Atom Probe Tomography of a Multiphase High-Strength Low-Carbon Steel

The differences in artifacts associated with voltage-pulsed and laser-pulsed (wavelength = 532 or 355 nm) atom-probe tomographic (APT) analyses of nanoscale precipitation in a high-strength low-carbon steel are assessed using a local-electrode atom-probe tomograph. It is found that the interfacial width of nanoscale Cu precipitates increases with increasing specimen apex temperatures induced by higher laser pulse energies (0.6-2 nJ pulse(-1) at a wavelength of 532 nm). This effect is probably due to surface diffusion of Cu atoms. Increasing the specimen apex temperature by using pulse energies up to 2 nJ pulse(-1) at a wavelength of 532 nm is also found to increase the severity of the local magnification effect for nanoscale M2C metal carbide precipitates, which is indicated by a decrease of the local atomic density inside the carbides from 68 ± 6 nm(-3) (voltage pulsing) to as small as 3.5 ± 0.8 nm(-3). Methods are proposed to solve these problems based on comparisons with the results obtained from voltage-pulsed APT experiments. Essentially, application of the Cu precipitate compositions and local atomic density of M2C metal carbide precipitates measured by voltage-pulsed APT to 532 or 355 nm wavelength laser-pulsed data permits correct quantification of precipitation.

Special features of the evaluation of the weldability of low-carbon high-strength pipe steels

The special features of the weldability of high-strength low-carbon microalloyed steels are investigated.

Effect of Heating Rate on the Austenite Formation in Low-Carbon High-Strength Steels Annealed in the Intercritical Region

Austenite formation during intercritical annealing was studied in a cold-rolled dual-phase (DP) steel based on a low-carbon DP780 composition processed in the mill. Two heating rates, 10 and 50 K/s, and a range of annealing temperatures from 1053 K to 1133 K (780 °C to 860 °C) were applied to study their effects on the progress of austenitization. The effect of these process parameters on the final microstructures and mechanical properties was also investigated using a fixed cooling rate of 10 K/s after corresponding annealing treatments. It was found that the heating rate affects the austenite formation not only during continuous heating, but also during isothermal holding, and the effect is more pronounced at lower annealing temperatures. Faster heating delays the recrystallization kinetics of the investigated steel. The rate of austenite formation and its distribution are strongly influenced by the extent of overlapping of the processes of recrystallization and austenitization. It appeared that the heating rate and temperature of intercritical annealing have a stronger effect on the final tensile strength (TS) of the DP steel than holding time. Both higher annealing temperatures and long holding times minimize the strength difference caused by a difference in heating rate.

Effect of Magnesium on the Austenite Grain Growth of the Heat-Affected Zone in Low-Carbon High-Strength Steels

To research the effect of Mg on the austenite grain growth of the heat-affected zone (HAZ) in low-carbon high-strength steels, two steels with different Mg contents were prepared using a laboratory vacuum. It was observed through optical microscope (OM) that the HAZ austenite grain size decreased from 385 to 86 μm after Mg treatment. In-situ observation by a confocal scanning laser microscope showed that the HAZ austenite grain size of the steel with Mg treatment could hold fine-grained structure after 1673 K (1400 °C) heating for 300 seconds, which was mainly attributed to the formation of pinning particles after the addition of Mg. Analysis showed that the number of pinning particles was much increased and the mean size was much decreased by the Mg treatment. It also can be found that more particles occurred with the MgO as the nucleating center. The HAZ toughness was improved from 33 to 185 J by refinement of austenite grains in the simulated HAZ due to the Mg treatment. The results presented in this article point to a potential method for improving HAZ toughness of low-carbon high-strength steels.

Investigation on the Elongation of Tailor Welded Blanks with Different Thickness

Tailor welded blanks (TWBs) get popular used in the automobile for the light-weighting, and the assessments for TWBs formability are more and more important owing to the TWBs with different thicknesses. In this paper, based on the tensile test and low carbon high strength steel (ZStE220P), the influences of seam location and the length of sample on the elongation are studied, and the results shows that the location of seam has greater effect on the formability of TWBs and elongation is getting decreased with the seam shifting to the thinner part.

Nanoscale co-precipitation and mechanical properties of a high-strength low-carbon steel

Nanoscale co-precipitation in a novel high-strength low-carbon steel is studied in detail after isothermal aging. Atom-probe tomography is utilized to quantify the co-precipitation of co-located Cu precipitates and M 2C (M is any combination of Cr, Mo, Fe, or Ti) carbide strengthening precipitates. Coarsening of Cu precipitates is offset by the nucleation and growth of M 2C carbide precipitate, resulting in the maintenance of a yield strength of 1047 ± 7 MPa (152 ± 1 ksi) for as long as 320 h of aging time at 450 °C. Impact energies of 153 J (113 ± 6 ft-lb) and 144 J (106 ± 2 ft-lb) are measured at −30 °C and −60 °C, respectively. The co-location of Cu and M 2C carbide precipitates results in non-stationary-state coarsening of the Cu precipitates. Synchrotron-source X-ray diffraction studies reveal that the measured 33% increase in impact toughness after aging for 80 h at 450 °C is due to dissolution of cementite, Fe 3C, which is the source of carbon for the nucleation and growth of M 2C carbide precipitates. Less than 1 vol.% austenite is observed for aging treatments at temperatures less than 600 °C, suggesting that transformation-induced plasticity does not play a significant role in the toughness of specimens aged at temperatures less than 600 °C. Aging treatments at temperatures greater than 600 °C produce more austenite, in the range 2–7%, but at the expense of yield strength.

The effect of isothermal holding on the microstructures and mechanical properties of a low carbon alloy steel

A low carbon high-strength steel was isothermally treated at 900°C for varying times and was immediately quenched into ice water. The microstructures and grain refinement were characterized by the techniques of serial sectioning and computer-aided three-dimensional reconstruction. The microstructures in the specimens that were isothermally maintained for short periods of time consisted of predominantly martensite and much lower bainite. The lower bainite was lath-like, plate-like, rod-like and lenticular in shape. Most were lath-like or plate-like. The lower bainite laths or plates divided the prior austenite grains into many smaller regions, and the martensite that transformed at lower temperatures was thus confined in the smaller zones and thus had a smaller packet size. The specimens that were isothermally held for short periods thus had higher strength, compared with the directly quenched specimens.

Residual stresses in welded joints of high-strength steels

By using a “GLEEBLE-3800” complex, we perform the experimental investigations of the dynamics of changes in the yield strength of the metal of the heat-affected zone (HAZ) in low-carbon high-strength steels for various rates of cooling of the welded joints. The influence of the rate of energy input and thermal welding cycles on the formation of residual stresses in welded joints of steels with different chemical compositions and properties is determined both experimentally and numerically. The relationship between the residual stresses, modes of heat input, and the period of holding of the HAZ metal at temperatures above the point AC3 is established.