Low Carbon High Strength Research Articles

Steel Microstructure Effect on Mechanical Properties and Corrosion Behavior of High Strength Low Carbon Steel

Different thermomechanical treatments were applied to a high strength low carbon steel with a novel chemical composition. As a result, three different microstructures were produced with dissimilar mechanical and corrosion properties. Subsequently, a tempering heat treatment was applied to redistribute the phases in the steel. Microstructure A with 56 pct martensite and 32 pct bainite presented high strength but medium ductility; microstructure C with 95 pct ferrite and 3 pct martensite/austenite resulted in low strength and high ductility, and finally microstructure B with 98 pct bainite and 2 pct martensite/austenite resulted in high strength and ductility. Alternatively the corrosion behavior obtained by polarization curves was characterized in 0.1 M H2SO4, 3 M H2SO4, 3.5 wt pct NaCl, and NS4 solutions resulting in similar magnitudes, while the corrosion behavior acquired by electrochemical impedance spectroscopy had slightly differences in 3 M H2SO4.

TRIP Effect in a Hot-Rolled Low-Carbon High Strength Complex Phase Steel

In the present work, a study has been made of the hot-rolling process for a transformation induced plasticity (TRIP) steel Fe-0.12C-0.5Si-1.4Mn-0.5Cr (wt%). The volume fractions of retained austenite before and after a deformation were determined by X-ray diffraction (XRD). The microstructure was characterized by optical microscope, scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). A uniaxial tension text indicated that the steel possesses ultimate tensile strength of 748 MPa with yield ratio of 0.7 and elongation of 20%. The steel with the volume fraction of retained austenite of 12.5 % exhibits significant TRIP effect.

Structure–mechanical property relationship in a high strength low carbon alloy steel processed by two-step intercritical annealing and intercritical tempering

The influence of annealing and tempering temperature on the microstructure and mechanical properties was investigated in a low carbon alloy steel that was processed by a two-step intercritical annealing and intercritical tempering heat treatment. In general, the microstructure of the processed steel comprises intercritical lath-like ferrite, bainitic/martensitic lath and acicular-type retained austenite. The lower intercritical annealing temperature resulted in lower fraction of intercritical ferrite with finer grain size and consequently higher strength. On the other hand, the intercritical tempering temperature significantly influenced retained austenite content and precipitation. High fraction of retained austenite was obtained at a temperature slightly above Ac1 temperature and retained austenite content decreased with increase in tempering temperature. This behavior is attributed to the competition between the enrichment of Mn and Ni and the fraction of reversed austenite. Fine niobium carbide precipitates of size ~2–6nm and copper precipitates of size range ~10–30nm were obtained. The optimal intercritical annealing and tempering temperatures to obtain the product of tensile strength and elongation % of ~30GPa% were 780°C and 660°C, respectively and the volume fraction of retained austenite was ~29%.

中間段階変態組織を有する調質型低炭素高張力鋼の延性脆性遷移挙動

中間段階変態組織を有する調質型低炭素高張力鋼の延性脆性遷移挙動

Concurrent engineering solution for the design of ship and offshore bracket parts and fabrication process

Brackets in ships and offshore structures are added structures that can endure stress concentrations. In this study, a concurrent engineering solution was proposed, and a high strength low carbon cast steel alloy applicable to offshore structures was designed and developed. The yield strength and ultimate tensile strength of the designed steel were 480 and 600 MPa, respectively. The carbon equivalent of the steel was 0.446 with a weld crack susceptibility index of 0.219. The optimal structural design of the brackets for offshore structures was evaluated using ANSYS commercial software. The possibility of replacing an assembly of conventional built-up brackets with a single casting bulb bracket was verified. The casting process was simulated using MAGMAsoft commercial software, and a casting fabrication process was designed. For the proposed bulb bracket, it was possible to reduce the size and weight by approximately 30% and 50%, respectively, compared to the conventional type of bracket.

Concurrent engineering solution for the design of ship and offshore bracket parts and fabrication process

Brackets in ships and offshore structures are added structures that can endure stress concentrations. In this study, a concurrent engineering solution was proposed, and a high strength low carbon cast steel alloy applicable to offshore structures was designed and developed. The yield strength and ultimate tensile strength of the designed steel were 480 and 600 MPa, respectively. The carbon equivalent of the steel was 0.446 with a weld crack susceptibility index of 0.219. The optimal structural design of the brackets for offshore structures was evaluated using ANSYS commercial software. The possibility of replacing an assembly of conventional built-up brackets with a single casting bulb bracket was verified. The casting process was simulated using MAGMAsoft commercial software, and a casting fabrication process was designed. For the proposed bulb bracket, it was possible to reduce the size and weight by approximately 30% and 50%, respectively, compared to the conventional type of bracket.

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% 5μm 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 570MPa, 615MPa, and 95%, respectively, which meet the requirements of the wheel-disc application. The steel consists of 20% 3μm 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 600MPa, 655MPa, 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.

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.

High strength–ductility and engineering realisation of a novel quenching–partitioning–tempering steel

By applying appropriate quenching–partitioning–tempering (Q–P–T) processes, proposed by T. Y. Hsu (Xu Zuyao), new types of low carbon high strength steels with adequate plasticity and toughness can be obtained (Fig. 1) In this work, the effects of tempering parameters on the microstructures and mechanical properties of Q–P–T steels were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and characterised using a Gleeble thermo-mechanical simulator and Zwick/Sans universal testing machine. The work hardening behaviour during plastic deformation was also analysed. Additionally, the characteristics of microstructures and the retained austenite stability of Q–P–T steels under different tensile temperatures were determined. Lastly, the possibility of the application of the Q–P–T processes in engineering practice was explored, and two optimised Q–P–T processes, for 12 and 20 mm thick hot rolled plates respectively, were d...

SPLIT FRACTURE PHENOMENON AND MECHANISM IN TENSILE TESTS OF HIGH STRENGTH LOW CARBON BAINITIC STEEL

Split fracture phenomenon in tensile tests of a high strength low carbon bainitic steel plate was investigated. Regular tensile tests in the longitudinal direction and short bar tensile tests in three principal axis directions were performed. The results showed that split fracture always occurred when the tensile direction was longitudinal or transverse. The splitting surface was almost parallel to the direction of thickness. According to SEM observation of splitting crack, it was found that the split had features of cleavage fracture. Meanwhile, the original steel plate was experimentally proved to have similar strength and ductility in the longitudinal direction, the transverse direction and the direction of thickness. Based on a finite element analysis simulation, the lateral tensile stress within the necked zone was calculated to be much lower than the principle tensile stress. Thus, a mechanism transition from ductile to brittle during the tension process was proposed. Moreover, it was revealed that the split fracture is not definitely the result of lower performance along the direction of thickness, but it is caused by the comprehensive influence of texture evolution, the redistribution of grain boundaries and the state of three dimensional stresses with large tensile plastic deformation, which can be considered as the characteristic of mechanical property of bainitic steel.

Effect of single pass welding heat input on microstructure and hardness of submerged arc welded high strength low carbon bainitic steel

The bead on plate test method was employed to investigate the effect of single pass welding heat input on microstructure and hardness of submerged arc welded high strength bainitic steel. The weld metal microstructure contains very high volume fraction of acicular ferrite regardless of heat input. By contrast, with the increase in heat input, the main microstructure in coarse grained heat affected zone (HAZ) changes from a mixture of lath martensite and ferrite sideplate to full ferrite sideplate. Microhardness measurement showed that maximum hardness values always present at the coarse grained HAZ under lower heat input condition, which indicates that the minimum heat input of experimental steel should be higher than 0·92 kJ mm−1 to avoid the formation of welding cold crack. The size distributions of complex inclusions in the weld metal are very similar for all heat inputs mainly because the short residence time does not make the inclusion grow rapidly.

Analysis of microstructural variation and mechanical behaviors in submerged arc welded joint of high strength low carbon bainitic steel

Microstructural variation in high strength low carbon bainitic steel weldment was investigated in detail by means of optical microscope, transmission electron microscope and scanning electron microscope equipped with electron backscattered diffraction. The results showed that the welded joint has various microstructures such as acicular ferrite, coarse granular ferrite and fine polygonal ferrite. The martensite–austenite (MA) constituent has a variable structure in each sub-zone, which includes fully martensite and fully retained austenite. Meanwhile, the fine grained heat affected zone has higher content of retained austenite than the welded metal (WM) and coarse grained heat affected zone (CGHAZ). The orientation relationship between retained austenite and product phases in the WM and CGHAZ is close to Kurdjumov–Sachs relationship. However, the polygonal ferrite in the fine grained HAZ has no specific orientation relationship with the neighboring retained austenite. The toughness of the coarse grained region is much lower than that of the WM because the coarse bainite contains many large MA constituents to assist the nucleation of microcracks and coarse cleavage facet lowers the ability to inhibit the crack propagation.

Microhardness characterisation in developing high strength, high toughness and superior ballistic resistance low carbon Ni steel

To develop new steels that exceed the ballistic resistance, strength and toughness of current naval ship steel plates, we designed optimally quenching–lamellarising–tempering treated low carbon 10%Ni steel plates. These plates displayed exceptional properties in all aspects; when compared to the widely used HSLA-100 steel plates, they showed superior toughness and an improvement of over 15% in strength and 20 mm fragment simulation projectile ballistic limit V50. A preliminary vibrating sample magnetometer study on the effect of quenching–lamellarising–tempering process and dynamic deformation on the microstructure evolution of the 10%Ni steel during ballistic impact was valuable to our understanding of the steel. The data led us to conclude that dynamic deformation during ballistic perforation induced an austenite to martensite phase transformation, which improves the global dynamic plasticity and ballistic resistance of the steel target. This mechanism, similar to the transformation induced plasticity effect, can be appropriately named ballistic induced plasticity. Characteristics of the microhardness maps of the sectioned craters created by ballistics are consistent with ballistic induced plasticity.

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.

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.

High strength ductility steels treated by novel quenching–partitioning–tempering process

This paper reviews the novel quenching–partitioning–tempering (Q–P–T) process based on the design of microstructure proposed by Hsu and its direct application to a medium carbon ultrahigh strength Q–P–T steel. Further developments of low carbon high strength Q–P–T steels and a superior strength–ductility Q–P–T steel are presented. Finally, the mechanism of retained austenite on the ductility enhancement of advanced high strength steels is introduced.

High strength low carbon hot rolled δ-ferritic steel: microstructure and mechanical properties

The present study concerns the development of high strength low carbon hot rolled bainitic and martensitic δ transformation induced plasticity steels. Equilibrium and para-equilibrium phase evolution have been examined by carrying out thermodynamic calculation using MT-DATA software. Microanalysis demonstrates that both manganese and aluminium partition between liquid and solid phases. Isothermal treatment and tempering at 350°C for bainitic and martensitic microstructures respectively have yielded the best combination of strength and ductility. All the steels have exhibited the continuous yielding behaviour and favourable ratio of yield and tensile strength, which are desirable for formability. The annealed steel has yielded a high level of tensile strength with the static toughness value in between the conventional transformation induced plasticity assisted and dual phase steels.