Ultra-low Carbon Bainitic Research Articles

Effect of Microstructure on Corrosion Resistance of Ultralow‐Carbon Bainitic Steel

The effect of microstructure on the corrosion behaviors of ultralow‐carbon bainitic (ULCB) steel in 3.5 wt% NaCl solution is discussed. Four ULCB steel specimens with different microstructures are designed via controlling the cooling routes after austenization, namely, furnace cooling (FC), air cooling (AC), oil cooling (OC), and water cooling (WC). Their corrosion behaviors are investigated by alternating immersion test (including weight loss and rust layer observation) and electrochemical tests (including polarization curves and electrochemical impedance spectroscopic measurements). Results show that the corrosion resistance of ULCB steel specimens decreases with increase of cooling rate, that is, in the following sequence: FC > AC > OC > WC. Furthermore, the electron backscatter diffraction characterization and scanning Kelvin probe force microscopy are performed to build the relationship between microstructure and corrosion behaviors of ULCB steels. The reduced corrosion resistance of ULCB steels results from the increased number of microgalvanic couples because of the significantly increased density of grain boundaries with increase of cooling rate.

Microstructure, Mechanical Properties, and Corrosion Behavior of Ultra-Low Carbon Bainite Steel with Different Niobium Content.

Four types of ultra-low carbon bainite (ULCB) steels were obtained using unified production methods to investigate solely the effect of niobium content on the performance of ULCB steels. Tensile testing, low-temperature impact toughness testing, corrosion weight-loss method, polarization curves, electrochemical impedance spectroscopy (EIS), and the corresponding organizational observations were realized. The results indicate that the microstructure of the four steels comprise granular bainite and quite a few martensite/austenite (M/A) elements. The niobium content affects bainite morphology and the size, quantity, and distribution of M/A elements. The elongation, yield strength, and tensile strength of the four types of ULCB steels are above 20%, 500 MPa, and 650 MPa, respectively. The impact toughness of the four types of ULCB steels at −40 °C is lower than 10 J. Steel with Nb content of 0.0692% has better comprehensive property, and maximum charge transfer resistance in 3.5 wt.% NaCl solution at the initial corrosion stage. The corrosion products on the surface of steel with higher niobium content are much smoother and denser than those steel with lower niobium content after 240 h of corrosion. The degree of corrosion decreases gradually with the increase of niobium content at the later stage of corrosion.

Weldability of 1000 MPa Grade Ultra-low Carbon Bainitic Steel

Maximum hardness test in weld heat-affected zone (HAZ), oblique Y-groove cracking test and mechanical property test of welding joint of 1000 MPa grade ultra-low carbon bainitic steel were carried out, so as to research the weldability of the steel. The results show that the steel has lower cold cracking sensitivity, and preheating temperature of 100 °C can help completely eliminate cold cracks, generating good process weldability. The increase of preheating temperature can reduce the hardening degree of heat-affected zone. The strength of welding joint decreases and hardness reduces when heat inputs increase, and excellent mechanical properties can be obtained when low welding heat inputs are used. Fine lath bainites of different orientations combined with a few granular bainites that effectively split the original coarse austenite grains are the foundation of good properties.

Influence of Inclusions on Early Corrosion Development of Ultra-Low Carbon Bainitic Steel in NaCl Solution

The early corrosion development of ultra-low carbon bainitic (ULCB) low alloy steel in NaCl solution was studied by ex situ imaging of corrosion morphology and in situ monitoring of microarea current density and potential, and the corrosion mechanism from initial localized corrosion to uniform corrosion was interpreted. The results indicate that the corrosion development of ULCB steel from initial localized corrosion around inclusions to the uniform corrosion on the whole steel surface is controlled by the galvanic couple effect between different phases resulting from their electrode potential difference in electrolyte solution. The early localized corrosion of steel matrix is initiated and accelerated by the galvanic couple effect between MnS inclusions and steel matrix to form the initial corrosion gaps and the circular corrosion spots around inclusions. The ohmic drop caused by solution resistance influences the acceleration effect of the galvanic couple. With the separation of inclusion from steel matrix, this galvanic couple effect becomes invalid, which results in the expansion from localized corrosion to uniform corrosion. The microgalvanic couple between martensite/residual austenite (M/A) islands and bainite ferrite also accelerates the anodic dissolution of bainite ferrite phase; however, its acceleration corrosion effect is much weaker than that caused by MnS inclusion.

Charpy Impact Properties of Grain Boundary Allotriomorphic Ferrite and Granular Bainite Duplex Microstructure

A new type of high strength and low cost bainitic steel with ultra-low carbon content and high Si content has been developed on the basis of Mn-series air-cooling bainitic steels. The tensile properties of YS>690MPa and the impact toughness of AKV>60J at-40°C were obtained by controlling the processing parameters. This was attributed to the formation of the grain boundary allotriomorphic ferrite (FGBA) and the granular bainite (GB) with different shape of M/A islands. The high strength due to the inter-lath lamellar M/A islands or retained austenite companying with high dislocated bainitic ferrite laths of average 300nm width. The effect of microstructure on the impact crack initiation and propagation was studied. The results showed that crack initiation occurred in two different types of sites: at interphase boundaries of bainite ferrite (BF) and M/A islands, at grain boundaries. The FGBA and bainite ferrite (BF) both had blunting effect on microcrack tip to reduce the crack propagation path. Because of the presence of FGBA, the unit crack path was short, at less than 5μm. The blunting effect of BF could be enhanced by the M/A islands, which force the cracks change the propagation path and reduce the unit crack path to less than the size of bainite packets. The mechanism of low temperature microcrack origin of the ultra-low carbon bainitic (ULCB) steel with the microstructure of the FGBA and GB was also discussed.

The Evolution of M/A in Ultra-Low Carbon Bainite Steel

The amount, size and structure of M/A in ultra low carbon bainitic steels，which are mainly affect by the transformation process, the cooling rate and the tempering treatments, have been investigated. The different cooling rate induce the variation of distribution of amount and size of M/A. There are bigger M/A mass with harder phase at low cooling rate while smaller M/A with less hard phase at high cooling rate. A series tempering tests have been carried out, which indicating that the tempering treatment has significant effect on the evolution of M/A.

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bainitic steel during continuous cooling. Continuous cooling transformation (CCT) curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln[1/(1 −f)] and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth patterns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agreement with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.

Improved properties of Mn series ultralow carbon steel with ultrafine bainitic structure

In order to reduce the cost of alloying elements, Mn series ultralow carbon bainitic (ULCB) steels have been developed. The different cooling conditions have been used to improve the mechanical properties of Mn series ULCB steels in the present study. The hot rolled ULCB steel plates with 26 mm thickness were water quenched to different finishing quenching temperatures (550, 450 and 350°C) followed by air cooling. Results show that the as rolled ULCB steel plates exhibit high yield strength of 775 MPa and low energy transition temperature of −55°C when the finishing quenching temperature is chosen at 450°C. Low temperature impact toughness was improved through control of the finishing quenching temperature. Microstructure observations show that a fine granular bainite structure is obtained, and the granular bainite structure exhibited shorter crack propagation path than both granular ferrite and lath martensite structures, which contributed to the improved combination of strength and toughness. Furthermore, the formation process of fine granular bainite structure was investigated.

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 coating material and lubricant on forming force and surface defects in wire drawing process

A pilot wire drawing machine as well as wire end-pointing roller was developed. Using these machines, a wire drawing test for four different coating materials and two different lubricants was performed as the reduction ratio increased from 10% to 30%. Materials used for a substrate in this study are plain carbon steel (AISI1045) and ultra low carbon bainite steel. To compute the friction coefficient between the coating layer of wire and the surface of die for a specific lubricant, a series of finite element analyses were carried out. SEM observations were also conducted to investigate the surface defects of wire deformed. Results show that the behavior of drawing force varies with the lubricant-type at the initial stage of drawing. The powder-typed lubricant with a large particle causes the retardation of full lubrication on the entire contact surface and the local delamination of coating layer on the wire surface. As the flow stress of a substrate increases, the delamination becomes severe.

The corrosion resistance of ultra-low carbon bainitic steel

The corrosion resistance of ultra-low carbon bainitic (ULCB) steel was compared with a weathering steel 09CuPCrNi through accelerated corrosion tests. The results indicated that the corrosion resistance was almost the same for ULCB and 09CuPCrNi based on the weight loss. It can be seen that the grain refinement did not deteriorate corrosion resistance property. The homogeneous microstructure, lower carbon content and random distributing ∑3 boundary could effectively increase the corrosion resistance of the ULCB steel. The characteristics of the rust layers indicated that the inner rust layer contained nanocrystalline Fe 3− x O 4 particles, while copper and chromium alloying additions were enriched at the rust layer and substrate interface in ULCB steel. These factors played important roles in forming a compact protective rust layer.

A Study on the Microstructure of Ultra Low Carbon Bainitic Steels by RPC Technique

The effect of tempering process on the microstructure of ultra low carbon bainitic (ULCB) steel produced by relaxation precipitation controlled phase transformation (RPC) has been investigated by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The results showed that the final microstructure mainly contained lath-like bainitic ferrite, granular bainite and martensite-austenite (MA) constituent in ULCB steels. On tempering at 650°C a slight increase was detected in the effective grain size as the strain-induced precipitates pinned up the dislocation walls and subgrains. After tempering at 700°C, bainitic ferrite laths started to coarsen and polygonal ferrite occurred. The effective grain size of ULCB steels in as-rolled condition was 1.5 μm at the tolerance of 10o~15o measured by EBSD technique.

Processing Low and Ultra-Low Carbon Bainitic Steels with Excellent Property Combinations

Six experimental low and ultra-low carbon C-Mn-Mo-Nb-B and one conventional TMCP steel heats have been prepared to study the effects of chemical composition and hot deformation on the microstructure and the strength-toughness properties. In physical simulation tests, it was found that the deformation of austenite below the non-recrystallization temperature enhances the formation of higher-temperature bainitic morphologies and polygonal ferrite. On the other hand, hardness exhibits relatively low sensitivity to the degree of deformation below Tnr, whereas the deformation results in a distinct refinement in the microstructures, as determined by SEM-EBSD measurements, suggesting an improvement in the impact toughness. Simultaneous alloying with Mo-Nb-B seemed to be most efficient to provide high hardness and strength. Hot rolling trials indicated that the yield strength in the range 500-700 MPa with the excellent toughness down to –80 °C can be achieved in low carbon (≈ 0.03%) bainitic grades.

Development of alternative as-rolled alloys to replace quenched and tempered steels with tensile strength in the range of 600–800 MPa

The fierce competition between steelworks and different alloy design approaches are the motivation behind the development of new microalloyed structural steels which must satisfy increasingly severe mechanical strength and toughness requirements. Other equally desirable aspects for these new materials are the suppression of heat treatments after hot rolling and better weldability, which makes this alloy evolution even more complex. As weldability improvement requires minimization of the carbon content of the steel, these new steels must present hardening mechanisms which does not require the presence of this element. Up to this moment, the most feasible answers to this challenge are steels hardened by copper precipitation, like HSLA-80 or ASTM A710, and the so-called ultra low carbon bainite (ULCB) steels. The aim of this work was to study the effects of some controlled rolling parameters over the mechanical properties of these relatively new steels. The knowledge of these effects certainly will help in the definition of optimised process conditions for these new steel alloy designs. It was verified here that the total strain applied during hot rolling and the finishing temperature were essential to improve the toughness of both alloys; the effect of the former parameter revealed to be more important. The aged HSLA-80 steel showed greater values of yield strength, but lower toughness than the as-rolled ULCB alloy. This fact is certainly due to the aging treatment that is normally applied to the first alloy.

Comparison of Cold Formability of Cold Drawn Non-heat-treated Steels Having Similar Strength

The cold formability of the drawn non-heat-treated steels, i.e. dual phase (DP) steel, low Si steel and ultra low carbon bainitic (ULCB) steel, was examined in terms of the deformation resistance and the forming limit. The present investigation was aimed at elucidating the effect of drawing on the cold formability of non-heat-treated steels which is directly affected by drawing since no heat treatment are involved during forming processes for them. A special care was taken for the present steels to exhibit the similar strength after drawing, so eliminating the strength effect. The present steels after drawing revealed the elastic-near perfect plastic behavior in compression. After drawing, the low Si steel exhibited the lowest deformation resistance estimated by the absorbed energy during deformation. In case of the forming limit in terms of the critical strain under which no cracking occurs during the upsetting test of the drawn steels, the low Si steel and the ULCB steel were better than the conventional heat-treated steel. Accordingly, among several non-heat-treated steels which can replace the conventional heat-treated steel as forging steels, the low Si steel seems to exhibit the best performance if they have the similar strength. The compressive deformation behavior of the present drawn non-heat-treated steels was discussed in association with the strain hardened state and the Bauschinger effect developed by drawing. In addition, their cold formability was explained by the plastic incompatibility between the constituent phases of each steel.

Aços alternativos ao HY-80 sem a necessidade de aplicação de têmpera e revenido

A competitividade cada vez mais acirrada entre as diversas usinas siderúrgicas e também entre diferentes tipos de materiais continuam promovendo o desenvolvimento de novos aços estruturais microligados que atendam a requisitos de resistência mecânica e tenacidade cada vez mais severos. Outros aspectos igualmente desejáveis, como a supressão de tratamentos térmicos e melhor soldabilidade do produto, tornam essa evolução ainda mais complexa. Como a melhoria da soldabilidade do produto exige a minimização do teor de carbono do material, esses novos aços precisam dispor de mecanismos de endurecimento que dispensem a presença desse elemento. As alternativas mais viáveis são os aços endurecíveis por precipitação de cobre, do tipo HSLA-80 ou ASTM A710, ou os aços bainíticos com teor extra-baixo de carbono, do tipo ULCB.

Microstructure Refinement of Structural Steel in China

In Oct. 1998 Chinese government opened national program: Fundamental Research on New Generation of Iron and Steel Materials in China (N.G. Steels). The progress of on-going project has been briefly reported in this paper. In order to get microstructure refinement in structural steels, the following 5 ways have been studied: For Ferrite+Pearlite microstructure in plain carbon steels and low-alloy steels, (1) Purified steel-making→Fully equiaxed continuous casting→Rough-mill with higher Zener-Hollman parameter (y-DRX refinement)→Finishing-mill rolling by Deformation Induce Ferrite Transformation (DIFT) technology. (2) For thin-slab continuous casting and rolling (CSP) process, ultra-fine sulfides (20-60 nm) and oxides (5∼20nm) controlling technology. (3) For Low Carbon Bainite (and Ultra Low Carbon Bainite) microstructure (LCB/ULCB) in microalloyed steels, combination of Deformation Induced Precipitation (DIP) and subsequent middle temperature phase transformation controlling. For quench-temperated martensite microstructure in alloy structural steels: (4) Ultra-fine y grains by innovated heat treatment and alloy design; (5) Carbide-free bainite/martensite microstructure with retained y films along lath/sub-lath boundaries.

Advanced ultra-low carbon bainitic steels with high toughness

The purpose of this paper is to report on investigations with dilatometry, transmission electron microscopy, X-ray microanalysis, mechanical testing and fracture mechanics methods into the strengthening mechanisms operative in ultra-low carbon bainitic (ULCB) steels and high strength low alloy copper precipitation strengthened (HSLA100) steels and their resistance to brittle fracture.

The mechanism of the formation of boron ineffective zone and its effect on the properties of Ultralow carbon bainitic steels

In the manufacturing of ultralow carbon bainitic (ULCB) steels, boron is an alloying element which is essential to promote the desired bainitic transformation. In order to obtain this hardenability effect, boron must be in solution and it must segregate to the austenite grain boundary and where it decreases the contribution of the boundary's interfacial energy to ferrite nucleation. During the development of ULCB steels in China Steel Corporation, a small boron ineffective zone was found at the centre of steel plates. From electron-probe X-ray microanalysis (EPMA) and boron autoradiograph analysis, it was found that the formation of the boron ineffective zone was due to the centre-line segregation of inclusions which strongly combined with boron and formed a boron-free zone in its vicinity. The microstructure of the boron ineffective zone was a conventional ferrite with a strength which was much lower than that of the surrounding bainite. This resulted in crack separation in the tensile and impact specimens. It was found from a hydrogen-induced-cracking (HIC) test, that the HICs had a propensity to propagate along the boron ineffective zone. From a welding y-grooved test, a higher cold-cracking sensitivity at this boron ineffective zone was also found.

The Influence of Plastic Deformation and Cooling Rates on the Microstructural Constituents of an Ultra-low Carbon Bainitic Steel.

An experimental ultra-low carbon bainitic steel was prepared to investigate the effect of a prior compressive deformation on the morphology of the transformation procuct during continuous cooling. It is found that at the higher cooling rate the deformed austenite tends to form the non-parallel plates of acicular ferrite, and that at the lower cooling rate the deformed austenite tends to form the parallel plates of bainitic ferrite. The orientation relationships between adjacent grains of acicular ferrite have been studied using the analysis of axis-angle pair. The result shows that the adjacent variants have nearly the same orientation in space, which is analogous to the case in alloy-steel weld metals. Furthermore, based on a thermodynamic analysis, it is indicated that acicular ferrite transformation (from deformed austenite) also exhibits the phenomenon of incomplete reaction, where the reaction ceases well before the residual austenite achieves its equilibrium carbon concentration.