High-strength Low-carbon Steel Research Articles

Microstructural Characteristics and Impact Fracture Behaviors of a Novel High-Strength Low-Carbon Bainitic Steel with Different Reheated Coarse-Grained Heat-Affected Zones

To obtain the correlation of microstructural characteristics and toughness in a novel high-strength low-carbon bainitic structural steel with medium and heavy plate after multipass welding, a welding thermal simulation experiment was conducted to simulate different subregions in the reheated coarse-grained heat-affected zones (CGHAZ). The microstructure evolution was then analyzed and factors that influence the fracture behavior were studied. The results show that the brittle zone appeared in subcritical reheated CGHAZ, and the fractured morphology was cleavage fracture. Supercritical reheated CGHAZ had the highest impact toughness, and the fractured morphology was primarily the ductile fracture with dimples formed via the micropore polycondensation mechanism. With an increase in the secondary pass welding thermal cycle peak temperature (tp2), the average length size of martensite and austenite (M-A) decreased from 9 to 2 μm. The coarsening of M-A constituents was the main reason for decrease in the crack initiation absorbed energy. A large number of retained austenite and cementite precipitates in subcritical reheated CGHAZ clearly worsened the impact toughness, and the massive austenite and cementite precipitates more than offset the beneficial effects of high-angle boundaries. This phenomenon led to disappearance of the effect of high-angle grain boundary of prior austenite and lath bainite on arresting crack propagation. In supercritical reheated CGHAZ, crack propagation absorbed energy was increased because of grain refinement, fine precipitates, lamellar residual austenite at corners, and high-angle grain boundary.

Complex isothermal precipitation behavior of V(C, N) in V-, N-added low carbon steel

Precipitation behavior of carbide/carbonitride in high-strength low-carbon steel is significantly affected by the ferrite transformation modes, which may change from the negligible partitional local equilibrium mode to the partitional local equilibrium mode as holding temperature increases or holding time increases at a certain temperature for ferrite phase transformation. Herein, the isothermal precipitation behavior of V(C, N) under negligible partitional local equilibrium mode of ferrite transformation in V-, N-added low carbon steel has been investigated. Not only the complex precipitation coexisting phenomenon of random precipitation and fibrous precipitation but also that of random precipitation and interphase precipitation was observed and certificated by the selected area electron diffraction patterns. In addition to Baker–Nutting orientation relationship, the Kurdjumov–Sachs OR [111]α//[110]P, $$\left( {1\overline{1}0} \right){\upalpha }//\left( {1\overline{1}\overline{1}} \right)$$ P between ferrite and interphase precipitation were found. Due to big variation in the movement velocity of austenite/ferrite interface, non-uniform distribution of precipitates occurred, resulting in hardness of ferrite grains changing from 185 to 215 HV.

Anisotropic mechanical properties and deformation behavior of low-carbon high-strength steel component fabricated by wire and arc additive manufacturing

Wire and arc additive manufacturing (WAAM) is an efficient technique for fabricating large and complex components that are applied in the manufacturing industry. In this study, anisotropic mechanical properties of a low-carbon high-strength steel component fabricated by WAAM were investigated via mechanical testing, and the transversal and longitudinal deformation behavior of the component were studied using the digital image correlation (DIC) method. Additionally, the features of microstructure, texture, and fracture mode of the inter-layer area and deposited area were also investigated to reveal the mechanism of anisotropy. The results showed the mechanical properties of longitudinal specimens were inferior to that of the transversal specimens. Several strain concentration zones in the longitudinal specimen were relevant to the inter-layer characteristics observed from the fracture surface and macrostructure, which was confirmed by the strain evolution recorded by DIC. The inter-layer areas were proved to be the weak link in the deposited component by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) analysis results, including various phase composition, phase morphology, misorientation angle, grain size, Schmid factor, and texture. Finally, based on the fractography analysis, anisotropy resulted from inter-layer zones is also confirmed via the comparison of transversal and longitudinal fracture morphology.

On the role of Cu addition in toughness improvement of coarse grained heat affected zone in a low carbon high strength steel

In this article, the coarse grained heat affected zone (CGHAZ) of a low carbon high strength steel was simulated at two heat inputs on a Gleeble-3800 thermo-mechanical simulator. A comparative study was conducted to reveal the role of Cu addition in toughness improvement by microstructure characterization and Charpy impact test. Microstructure observation suggested that there is no observable difference in the microstructure and prior austenite grain size between Cu-free steel and Cu-bearing steel, but a higher density of high-angle grain boundary (HAGB) was obtained in Cu-bearing steel with 50 kJ/cm. This was intrinsically associated with the configuration of three Bain groups from the aspect of crystallography. The optimum configuration can be achieved by adjusting transformation driving force, cooling rate and nucleation characteristics during martensite/bainite transformation. In this study, Cu addition played a determining role in increasing transformation driving force at high cooling rate, and altering the diffusion kinetics of alloying elements at low cooling rate. Accordingly, the density of HAGB can be optimized to be beneficial for high toughness in CGHAZ. Therefore, the optimum toughness was obtained in Cu-bearing steel with heat input of 50 kJ/cm, which was featured by the impact energy of ~ 198 J at − 40 °C.

Bainitic Transformation and Mechanical Properties of Low-Carbon High-Strength Bainitic Steels with Mo Addition

Two low-carbon high-strength bainitic steels with/without molybdenum (Mo) addition were designed to investigate the effect of Mo on bainitic transformation, microstructure and properties. The results show that during austempering, Mo addition increases the amount of isothermal bainite transformation and slightly accelerates the bainite transformation kinetics. It is mainly attributed to the fact that there is the ferrite transformation in Mo-free steel but not in Mo-added steel even cooled at a very high cooling rate before austempering. In Mo-free steel, the formation of ferrite leads to the higher carbon content in untransformed austenite with higher stability and thus retards the isothermal bainite transformation kinetics. The ferrite transformation controlled by carbon diffusion is strongly inhibited in the Mo-added steel because Mo addition decreases obviously the diffusion coefficient of carbon and grain boundaries energies. In addition, the promoted effect of Mo addition on isothermal bainite transformation is stronger at lower austempering temperature. Mo addition leads to the more and finer bainite plates and increases the strength of low-carbon bainitic steel. Moreover, the volume fraction of retained austenite first decreases and then increases in Mo-free steel with the increase in austempering temperature, whereas it presents the inverse trend in Mo-added steel. Similarly, during continuous cooling process, Mo addition effectively retards the formation of high-temperature products and thus improves the tensile properties of low-carbon steels.

Microstructure and Mechanical Properties of Low-Carbon High-Strength Steel Fabricated by Wire and Arc Additive Manufacturing

Wire and arc additive manufacturing (WAAM) is a novel technique for fabricating large and complex components applied in the manufacturing industry. In this study, a low-carbon high-strength steel component deposited by WAAM for use in ship building was obtained. Its microstructure and mechanical properties as well as fracture mechanisms were investigated. The results showed that the microstructure consisted of an equiaxed zone, columnar zone, and inter-layer zone, while the phases formed in different parts of the deposited component were different due to various thermal cycles and cooling rates. The microhardness of the bottom and top varied from 290 HV to 260 HV, caused by temperature gradients and an inhomogeneous microstructure. Additionally, the tensile properties in transversal and longitudinal orientations show anisotropy characteristics, which was further investigated using a digital image correlation (DIC) method. This experimental fact indicated that the longitudinal tensile property has an inferior performance and tends to cause stress concentrations in the inter-layer areas due to the inclusion of more inter-layer zones. Furthermore, electron backscattered diffraction (EBSD) was applied to analyze the difference in Taylor factor between the inter-layer area and deposited area. The standard deviation of the Taylor factor in the inter-layer area was determined to be 0.907, which was larger than that in the deposited area (0.865), indicating nonuniform deformation and local stress concentration occurred in inter-layer area. Finally, as observed from the fracture morphology on the fractured surface of the sample, anisotropy was also approved by the comparison of the transversal and longitudinal tensile specimens.

Behavior of hydrogen in a low carbon high strength steel

Behavior of hydrogen in a low carbon high strength steel

Влияние содержания ниобия на кинетику фазовых превращений аустенита и механические свойства в зоне перегрева сварных соединений малоуглеродистых высокопрочных сталей

Влияние содержания ниобия на кинетику фазовых превращений аустенита и механические свойства в зоне перегрева сварных соединений малоуглеродистых высокопрочных сталей

Investigation on Precipitation Behavior and Morphology of TiN Particles in High-Ti Low-Carbon High-Strength Steel

The precipitation behavior of TiN particles under homogeneous and heterogeneous nucleation conditions in a low-carbon high-strength steel with 0.11 wt% Ti was investigated by thermodynamics and dynamics equations combining with the microstructural observations on the surface and at the core of the casting slabs. The effect of cooling rate on the morphology and dimension of TiN particles was also analyzed in the present study. In addition, the three-dimensional and the spatial tridimensional morphology of compound TiN–Al2O3 was innovatively characterized by a focused ion beam scanning electron microscope. It was found that TiN particles possessed inerratic facet morphology (such as squares and clusters), and the square morphology was predominant. The preferential plane for TiN precipitates in the tested steel was {100}. Moreover, TiN particles precipitated both in solid and liquid phases from the solid–liquid phase region during the solidification process, and preferential precipitation was observed in the solid phase. Furthermore, the size of TiN particles decreased with the increasing cooling rate during the casting process. Al2O3 particles provided nucleation sites for the heterogeneous nucleation of TiN particles.

Influence of the Local Stresses and Strains at the Crack Tip on the Mechanism of Fracture of Hardox-400 Steel

We present the results of strength and crack-growth resistance tests of Hardox-400 low-carbon highstrength steel within the temperature range from – 100 to 20°С. The data of microfractographic analysis reveal the presence of various mechanisms of fracture of specimens with cracks (from brittle to ductile). The distributions of local stresses and strains in the specimen in the vicinity of a crack tip are obtained as a result of modeling and numerical calculations. By analyzing the numerical results and specific features of the fracture surfaces, we establish the critical level of normal stresses such that, for stresses higher than this level, the fracture mechanism becomes brittle. The analyses of the levels of stresses and strains enable us to substantiate the mixed brittle–ductile fracture mechanism.

Grain-boundary segregation of boron in high-strength steel studied by nano-SIMS and atom probe tomography

High resolution imaging by secondary ion mass spectrometry and atom probe tomography have been employed to investigate boron segregation at austenite grain boundaries (γGBs) after soaking in a high-strength low-carbon steel. The combined use of these two analytical techniques is shown to be powerful for quantifying solute and segregated boron levels. Quenching was performed after soaking aiming to clarify the temperature effect on boron distribution under thermal equilibrium. Boron depletion in the γGBs vicinity was observed in the as-quenched states from high temperatures, suggesting that the cooling rate was not fast enough to limit boron diffusion during cooling. We found that boron segregation at γGBs increases with temperature. This is due to the increase of solute boron concentration in the grains, resulting from boride precipitate dissolution. It appears that the segregation magnitude still follows the equilibrium laws as a function of temperature. From our investigations, it was possible to determine the boron equilibrium segregation enthalpy. These results have important practical consequences for controlling the levels of segregated boron in steels.

Effect of simulated cooling time on microstructure and toughness of CGHAZ in novel high-strength low-carbon construction steel

Influences of cooling time (welding heat input) on microstructure, impact toughness and the fracture mechanism of the weakest CGHAZ (coarse-grained heat-affected zone) in a novel high-strength low-carbon microalloyed construction steel were studied for the purpose of laying a theoretical foundation for developing welding support technologies. When the cooling time (t8/5) was increased, the microstructure changed from dot shape M-A constituents and lath martensite/bainite to slender and blocky M-A constituents and coarse granular bainite. Accordingly, the impact toughness deteriorated. Large blocky M-A constituents seriously reduced the impact absorbed energy during crack initiation. For coarse bainite, the high-misorientation boundary almost disappeared. Therefore, crack initiation energy determines the cleavage fracture micromechanism of high heat input construction steel.

Evaluation of phase transformations and mechanical properties in a copper bearing high-strength low-carbon steel microalloyed with Nb

In this study, the phase transformations and mechanical properties of a Cu-bearing high-strength low-carbon (HSLC) steel microalloyed with Nb were investigated. The phase transformations were studied by dilatometry and then the microstructures were evaluated. Mechanical properties were measured after various cycles of heat treatment. The results showed that the microstructure of this steel, at the cooling rates studied here, includes a mixture of bainite and martensite. The relatively high amounts of the substitutional elements such as Ni, as well as the low carbon content, resulted in the proximity of the bainite and martensite start temperatures. The Bs and Ms temperatures were determined to be 325 and 315 °C, respectively. This led to the formation of coalesced bainite in the microstructure. The maximum hardness of the alloy was obtained at the aging temperature of 450 °C. Additionally, the optimal combination of yield strength and impact energy was obtained by a single-stage aging treatment at 575 °C for 1 h. The reason for the toughness improvement can be attributed to the formation of the precipitated austenite between the martensitic lathes during the aging process.

Characterization and Modeling of Hot Deformation Behavior of a Copper-Bearing High-Strength Low-Carbon Steel Microalloyed with Nb

This study investigates hot deformation behavior of a newly developed Cu-bearing high-strength low-carbon steel microalloyed with Nb (Nb-HSLC). A computational method based on experimental data was employed to design the chemical composition of the alloy. Compression tests were carried out in the temperature range of 850-1100 °C as well as strain rates of 0.001-10 s−1 using BAHR Dil 805 A/D thermo-analyzer equipment. The Arrhenius-type constitutive equations were used to model the hot working behavior of the designed steel. Effects of friction and temperature rise during deformation were corrected to obtain the actual stresses. The results showed that the peak flow stress was increased with increasing Zener–Hollomon parameter. The obtained flow curves at strain rates lower than 0.1 s−1 and temperatures above 950 °C represented the typical dynamic recrystallization (DRX) behavior, while the flow curves at temperatures lower than 950 °C at all strain rates were associated with continuous strain hardening. This feature is in good agreement with the precipitation temperature range of Nb(C, N) particles, i.e., 800-1000 °C. Moreover, the flow curves showed the serrations during hot deformation at strain rates of 0.001 and 0.01 s−1, indicating that the dynamic strain aging (DSA) phenomenon occurred at low strain rates. The best fit between “peak stress” and “deformation conditions” was obtained by a hyperbolic sine-type equation (R2 = 0.993). Therefore, the average activation energy was determined as 348 kJ mol−1. The agreement between the achieved model and experimental flow data was verified using the results of additional tests at a strain rate of 5 s−1. The maximum difference between the measured and predicted “peak stresses” was calculated as 5 Mpa.

Strength and Fracture Toughness of Hardox-400 Steel

This paper presents results of strength and fracture toughness properties of low-carbon high-strength Hardox-400 steel. Experimental tests were carried out for specimens of different thickness at wide temperature range from −100 to 20 °C. The dependences of the characteristic of material strength and fracture toughness on temperature based on experimental data are shown. Numerical calculation of the stress and strain distributions in area before crack tip using the finite element method (FEM) was done. Based on results of numerical calculation and observation of the fracture surfaces by scanning electron microscope (SEM), the critical local stress level at which brittle fracture takes place was assessed. Consideration of the levels of stress and strain in the analysis of the metal state at the tip of the crack allowed to justify the occurrence of the brittle-to-ductile fracture mechanism. On the basis of the results of stretch zone width measurements and stress components, the values of fracture toughness at the moment of crack initiation were calculated.

Development of a High-Strength Low-Carbon Steel with Reasonable Ductility through Thermal Cycling

In this research work, cost-effective structural material synthesis routes have been adopted in the form of thermal cycling designed and applied on a low-cost material (AISI 1010 steel). The main synthesis route consists of two-step thermal cycling (designed with respect to iron–carbon phase diagram) combining short-duration holding in upper critical and intercritical temperature regimes and different cooling conditions (forced air cooling and ice-brine quenching). Accordingly, the adopted hierarchical design approach (from nanoscale onwards) with a combination of hard and soft phases (to attain an adequate balance between mutually exclusive strength and ductility properties) results in origin of a novel microstructure. The microstructure consists of dislocation-enriched martensite regions (possessing plate and lath morphologies in 1:4 ratio) containing nanosized cementite particles and α-ferrite regions containing submicroscopic cementite particles. As a consequence, an excellent combination of strength (UTS = 905 MPa) and ductility (%Elongation = 17) is achieved in the synthesized material.

Sulfide Stress Corrosion Cracking Behavior of G105 and S135 High-Strength Drill Pipe Steels in H2S Environment

Slow strain rate testing was employed to investigate the sulfide stress corrosion cracking (SSCC) behavior of low-carbon high-strength drill pipe steels (G105 and S135) in the simulated drilling environment containing H2S. Results reveal that both G105 and S135 steels have a high SSCC susceptibility in the simulated drilling environment containing H2S. In the range of experimental slow strain rate, the SSCC sensitivity of drill pipe steel increases with the decrease in slow strain rate, and the ductility and fracture properties of drill pipe steels also decrease obviously. With the increase in the steel strength, the SSCC sensitivity of the drill pipe steel increases, and the S135 drill pipe steel has higher SSCC sensitivity than G105. There is no obvious plastic deformation on the fracture surface of G105 and S135 steels in H2S solution. The cracks of fracture sprout on the surface of the specimen, the instantaneous fracture zone is in the middle, and the fracture can be divided into the cracking and the final fracture zone. The SSCC mechanisms of two drill pipe steels are hydrogen embrittlement. The hydrogen enters the steel and reduces the binding force of the iron atom, resulting in cleavage fracture.

Experimental determining of fracture behaviour of P460NL1 steel welded joint specimens

This paper is focused on the experimental measuring of impact energy as a part of the doctoral dissertation involving the fatigue behaviour of welded joints, made of fine grain normalised micro-alloyed low carbon high strength steel P460NL1 [1,2], typically used for pressure vessels working at subzero temperatures. VAC 65 [3] was used as filler material, and two plates were welded using the MAG welding procedure (with 82% Ar + 18% CO2 shielding gas).The specimens were taken from two opposite ends of the welded plate, taking into account the measured values of groove edge temperature in these locations, wherein the temperatures in Location 2 (near the end of the weld) were higher than those in Location 1 (near the weld’s beginning). Based on their specific position, in terms of temperature and notch position (weld root or face), the specimens were divided into four groups of three. This testing was performed using a SCHENCK-TREBEL instrumented Charpy pendulum. The total impact energy was determined, along with its components, crack initiation and crack propagation energy.The obtained results have confirmed that the test specimens were of high toughness, with total impact energy ranging from 165-200 J, at room temperature. The crack propagation energy was the dominant component, being several times greater than the crack initiation energy, as was expected.

М/А-Constituent in Bainitic Low Carbon High Strength Steel Structure. Part 1

Results are provided for experimental studies of conditions for the formation of various types of M/A-constituent (structural component consisting of martensite and austenite) in the microstructure of low-carbon pipe steel specimens produced by rolling with thermomechanical treatment (controlled rolling and accelerated cooling). As a result of experiments different types of M/A constituent are obtained and the temperature-time conditions for their formation are determined. A research procedure is developed making it possible with maximum confidence to determine the type, size and volume fraction of M/A-constituent in the microstructure. Depending on cooling conditions and exposure temperature, two critical states of the M/A-constituent are identified: at high exposure temperature, predominantly twinned martensite is formed, and austenite is formed at a low temperature. At intermediate temperatures, various combinations of these phases are observed, while the fraction of martensite in the composition of M/A “islands” decreases with decreasing exposure temperature, but the fraction of austenite increases.

The Influence of Ultra Fast Cooling Processing on Tensile and Charpy Impact Properties of Low-Carbon High-Strength Steel

Low-carbon high-strength steel was investigated by ultra fast cooling (UFC) processing. The results have shown that the microstructures of both steels were all composed of quasi polygonal (QF), acicular ferrite (AF) and granular bainite (GB). The mechanical properties of low-carbon steel all meet the standard requirements of linepipe steels, this is attributed to the effect of the ferrite grain refinement which is resulted in UFC processing. MA islands are more finely dispersed in steel B. Nanoprecipitates which were likely to form at lower temperatures during or after UFC in steel B are finer and more dispersed than those in steel A. Therefore, steel B has higher ultimate tensile strength (UTS) and yield strength (YS). The Charpy absorbed energy (AK) of steel B exhibited a high value. This is because the amount of AF in steel B is much more than that in steel A. A great deal of acicular ferrite grain boundary inhibit growing cleavage crack in steel B.