Low Carbon Bainitic Steel Research Articles

Effect of Tempering Temperature on Microstructure and Properties of Cold Deformed Low Carbon Bainitic Steel

In order to research the cold deformation work hardening characteristic of new type low carbon bainitic steel, this article studies the effect of different degrees of cold deformation (elongation and compression) and different tempering temperatures on microstructure and mechanical properties of 15SiMn2Mo low carbon beinaitic steel. The results showed that with the tempering temperature increasing after 10% pre-tension deformation, the tensile strength and yield strength of the test material increased first and then decreased, and reached its peak value at 300°C, roughly the same as the strength of hot-rolling and 300°C tempering. With the compression deformation degree rising, the hardness of test material increased and showed the test material has good work hardening performance. Streamline and "z" shape ferrite banding appeared in microstructure. With the tempering temperature increasing, the microstructure of compressed deformation steel recoveried and recrystallized, the tendency of ferrite along the streamline was weakened, the new refining granular phase was enhanced and uniformity of microstructure was improved. The microstructure refinement was significantly increased with the compressive deformation degree rising.

A Study on Recrystallization Behavior of Low Carbon Bainitic Steel during Multi-Pass Deformation

The multi-pass compression deformation of low carbon bainitie steel was carried out on a thermal simulator between a temperature range of 1050~800°Cwith accumulated 60.7% deformation and a deformation rate of 2s-1. The recrystallization behavior of the tested steel during high temperature deformation was discussed after comparatively analyzing the stress-stain curves and austenite deformation structures under different deformation conditions. The results show that with the same strain rate and total deformation, the grain size of recrystallized austenite is mainly determined by pass temperature and pass deformation, especially the latter, while has little relation with quantity of pass. The ultimate deformation stress is determined by ultimate deformation temperature and improved by the increasing quantity of pass.

The Corrosion Behavior of a Low Carbon Bainite Steel in Simulated Atmosphere Environment Containing Chloride and/or Sulfide

The corrosion behaviors of a low carbon bainite steel in three atmosphere environments simulated by corresponding solutions were investigated in this accelerated test. It is found that the corrosion rate of the low carbon bainite steel is lower than that of the commercial weathering steel 09CuPCrNi in all the three solutions. The corrosion rates of the bainite steel in the three solutions are also different, which is most serious in the solution containing both chloride and sulfide. There is no serious selective corrosion, for the corrosion has little effect on the mechanical property of the bainite steel. Corrosion rates are relation to the amount of Cu and Cr enrichment in rust layers, which are well corresponding to the specific surface area values of corrosion products. Inhibit effect can be found in the solution containing both chloride and sulfide.

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.

Variations of microstructure and properties of 690 MPa grade low carbon bainitic steel after tempering

The variations of microstructure, mechanical properties and electrical resistivity of 690MPa grade low carbon bainitic steel tempered at different temperatures were investigated with Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and double-arm Bridge. The results show that the appearance of granular bainite, decomposition of retained austenite, variations of dislocation density and solid solution of microalloying elements are the main reasons for variations of mechanical properties and electrical resistivity. Electrical resistivity reflects the solution content of microalloying elements and variations of dislocation density, which can be used as a fast and effective way to analyze the microstructure of materials.

The Tempering on Microstructure and the Yield-to-Tensile Ratio of High Performance Steels

The effects of tempering temperature on the microstructure and mechanical properties of 600MPa grade low carbon bainitic steel were investigated. The cause for the microstructure evolution has been investigated and the best tempering process was chosen to decrease the yield ratio of the steel. The influence of tempering process on the yield-to-tensile ratio of steels has been investigated by the aid of optical microscopy, SEM and XRD. The results show that after the TMCP processing the microstructure of steels mainly consist of lath martensite and bainite. The bainite and martensite have been refined markedly after the relaxation processing, therefore the properties of steels has been improved evidently. In order to decrease the yield-to-tensile ratio the steels underwent high temperature tempering. It has been found that during the tempering with the tempering temperature increased the yield-to-tensile ratio of steels decreased. The XRD and EBSD results show tempering temperature has considerable influence on the yield strength, but the influence on the tensile strength is not considerable. With the increase in tempering temperature, the low temperature toughness of steel can be improved considerably. The yield ratio of the steel was reduced after tempering at 650 °C and higher temperatures due to reversed austenitic phase transformation.

Analytical Investigation of Prior Austenite Grain Size Dependence of Low Temperature Toughness in Steel Weld Metal

Prior austenite grain size dependence of the low temperature impact toughness has been addressed in the bainitic weld metals by in situ observations. Usually, decreasing the grain size is the only approach by which both the strength and the toughness of a steel are increased. However, low carbon bainitic steel with small grain size shows a weakening of the low temperature impact toughness in this study. By direct tracking of the morphological evolution during phase transformation, it is found that large austenite grain size dominates the nucleation of intragranular acicular ferrite, whereas small austenite grain size leads to grain boundary nucleation of bainite. This kinetics information will contribute to meet the increasing low temperature toughness requirement of weld metals for the storage tanks and offshore structures.

Analysis of martensite–austenite constituent and its effect on toughness in submerged arc welded joint of low carbon bainitic steel

Martensite–austenite (M–A) constituent formed during welding is generally recognized as an important factor to decrease the toughness of welded joint. In this article, the morphology and chemical composition of M–A constituent in the low carbon bainitic steel welded joint was analysed in detail by means of optical microscope, transmission electron microscope and scanning electron microscope with electron probe microanalysis. The experimental results show that the M–A constituent formed in the different sub-zones presents different morphologies and different amounts. The maximum amount of M–A constituent occurs in the coarse grained heat affected zone (HAZ). It is evident that the carbon atoms segregate on the M–A constituent and carbon concentration on the slender M–A constituent is higher than that on the massive M–A constituent. Meanwhile, the distribution profile of silicon on the M–A constituent shows an obvious inhomogeneity. Most of M–A constituents have a twinned structure and/or a high dislocation density. According to impact testing results, the crack initiation energy in the HAZ specimens deteriorates significantly because the large M–A constituent can assist the formation of cleavage crack. On the other hand, the coarse prior austenite grain in the HAZ lowers the crack propagation energy.

Corrosion Behavior of New Weathering Steel in the Environment Simulating Coastal Industrial Atmosphere

The corrosion behavior of low carbon bainitic steel with Cu-P alloying in the environment simulating coastal industrial atmosphere was investigated by using dry-wet cycling corrosion test. 09CuPCrNi steel and low carbon bainitic steel without Cu-P alloying were used as comparative steels. The corrosion kinetics and electrochemical impedance spectra of the steels were measured, respectively. The morphologies of rust layers were observed by SEM and the phase constitutes of the rust layers were analyzed by XRD. Low carbon bainitic steel with Cu-P alloying behaves the lowest corrosion rate and the highest resistance of rust layer. Bainite microstructure is responsible for the uniform corrosion and the formation of dense rust layer. Cu-P alloying accelerates the transformation of gamma-FeOOH and Fe3O4 to thermodynamic stable phase alpha-FeOOH, which improves the protective effect of the rust layer.

Effect of Aging Temperature on the Microstructure and Mechanical Properties of 1000MPa Grade Low Carbon Bainitic Steel

A kind of 1000MPa low carbon bainitic steel belonged to the Fe-Cu-Nb series was hot rolled and aged, the influence of aging temperatures on the microstructure and mechanical properties of the steel were investigated by using Scanning electron microscopy (SEM) and transmission electron microscopy(TEM). The results show that the microstructure of the low carbon bainitic steel consisted of lath-shaped bainite(LB), granular bainite(GB) and quasi-polygonal ferrite(QF), and the proportion of each kind of microstructure changed with the aging temperatures. The strength of steel with the increase of aging temperature first increased, then decreased, Aging temperatures had distinct effect on yield strength of the tested steel, and less effect on the ultimate tensile strength, we can get the best comprehensive properties yield strength 1011.87 MPa and elongation rate 16.38% of good tough match aged at 450°C. Through analysis it is concluded that the strength of the tested steels aged at 450°C reaches the maximum value, which is attributed to the precipitation of a large amount of fine ε-Cu particles(5~10nm) and a small number of(Nb,Ti)(C,N) precipitates.

Influence of bainite morphology on impact toughness of continuously cooled cementite free bainitic steels

The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size.

Effect of boron addition on the microstructures and mechanical properties of thermomechanically processed and tempered low carbon bainitic steels

Effect of boron addition on the microstructures and mechanical properties of thermomechanically processed and tempered low carbon bainitic steels

Effect of Heat Treatment on Microstructure and Mechanical Properties of 800MPa Grade Ultra Low Carbon Bainite Steel

A new kind of bainite steel with ultra-low carbon content and Nb, Ti alloys has been developed. By applying thermomechanical control process, water quenching and tempering at different temperature, excellent properties have been obtained when tempered at 450°C, with the yield strength of 813MPa and elongation of 16.2%. The morphology observed by SEM shows that the microstructure consists of fine lath-shaped bainite, polygonal ferrite, quasi-polygonal ferrite and a small fraction of residual austenite or martensite-austenite constituents. In a TEM study plenty of precipitates with the size about 5-10nm were observed interacting with the dislocations, which is very significant for the optimization of strength and ductility.

Effect of Final Cooling Temperature on Mechanical Properties of a Water Cooled Mn-Series Low Carbon Bainitic Steel as 8Mn2SiNb

The effect of final cooling temperature on the mechanical properties of a water cooled Mn-series low carbon bainitic steel as 8Mn2SiNb has been investigated in this paper. The results indicate that the optimum final cooling temperature is 450 °C, followed by air cooling to room temperature. Compared with air cooling, the condition of water cooling to 450 °C increases the tensile strength and yield strength about 13.3% (From 805MPa to 929MPa) and 59.0%(From 464MPa to 741MPa) respectively, remaining 21.5% elongation and 151J toughness. SEM observation reveals that the microstructure of the steel after water cooling to 450 °C is mainly granular bainite +lath martensite +refined grain boundary allotriomorphic ferrite (FGBA). Compared with air cooling, the condition of water cooling to 450 °C increases the volume fraction of strengthening phase (M-A island) from 28.2% to 38.1%.

Microstructure Evolution in Low-Carbon Bainitic Steel during Tempering

Microstructure evolution in low-carbon bainitic steel during tempering is investigated by hardness measurements and metallographical examinations. It is found that the microstructure evolution and the hardness variation can be divided into four stages when samples were tempered at 600°C and 700°C, and the evolution of bainte is similar to recovery and recrystallization of deformed metals. It is also found that the newly formed ferrite during recrystallization grows more rapidly along the long axis of bainite laths, and there is evidence of composition changing during recrystallization.

Effect of cooling parameters on the microstructure and properties of Mo-bearing and Cr-bearing steels

To develop low-cost low carbon bainitic steel, Mo-bearing and Cr-bearing steels were melted in a vacuum induction furnace and were researched by thermal simulation and hot rolling at the laboratory. As the cooling rate increases from 0.2 to 50°C/s, the transformation temperatures of two steels lie between 650 and 400°C, and the final microstructures of them change from quasi-polygonal ferrite and granular bainite to lath bainite. Compared with cooling in air or by interrupted cooling, Mo-bearing and Cr-bearing steel plates cooled by sprayed water boast higher strength and superior toughness, for large-size islands are responsible for the poor mechanical properties. Compared to Mo, Cr is effective to isolate the bainitic reaction in low carbon steel, and the bainitic microstructure can also be obtained in Cr-bearing steel cooled at a wide range of cooling rate.

Microstructural characteristics and toughness of the simulated coarse grained heat affected zone of high strength low carbon bainitic steel

Abstract The correlation of microstructural characteristics and toughness of the simulated coarse grained heat affected zone (CGHAZ) of low carbon bainitic steel was investigated in this study. The toughness of simulated specimens was examined by using an instrumented Charpy impact tester after the simulation welding test was conducted with different cooling times. Microstructure observation and crystallographic feature analysis were conducted by means of optical microscope and scanning electron microscope equipped with electron back scattered diffraction (EBSD) system, respectively. The main microstructure of simulated specimen changes from lath martensite to coarse bainite with the increase in cooling time. The deterioration of its toughness occurs when the cooling time ranges from 10 to 50 s compared with base metal toughness, and the toughness becomes even worse when the cooling time increases to 90 s or more. The MA (martensite–austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. For crack propagation energy, the small effective grain size of lath martensite plays an important role in improving the crack propagation energy. By contrast, high misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation.

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

Abstract 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.

The Effect of Bainitic Transformation Microstructure under Applied Stress on Corrosion Resistance of HQ785 Steel

In this thesis, based on the low-carbon bainitic steel HQ785, isothermal transformation tests are carried out on the Gleeble-3500 thermo-mechanical simulator under uniaxial external tensile stress state. The fully immersed accelerated corrosion tests for the specimens, with different microstructure after phase transformation are carried out. The corrosion behavior of experimental steel is evaluated, combing with OM, SEM, weight loss method and electrochemical method. The results show that the shape of phase transformation microstructrue varies from granular to lath with the increase of external tensile stress. The corrosion resistance of which is better than that of granular bainite without stress. The sample with rusty layer after full immersion seawater corrosion shows good protectiveness.