Ti-stabilized Interstitial Free Steel Research Articles

Textures and precipitates in Ti-stabilized interstitial-free steel

Abstract Samples of titanium-stabilized interstitial-free steel were coiled at 600 and 700 °C. The effect of the coiling temperature on the formation of textures and precipitates during coiling, cold rolling and recrystallization annealing was studied by means of neutron texture measurements, small angle neutron scattering analysis and transmission electron microscopy observations. Whereas coiling at the lower temperature resulted in an insufficient scavenging of carbon in solid solution, coiling at the higher temperature promoted scavenging of carbon in solid solution, and consequently the formation of a large number of precipitates. The large amount of carbon in solid solution caused the retardation of softening in the cold-rolled sample during annealing below 700 °C. Upon annealing above 700 °C the impact of the coiling temperature on grain size and precipitates was not observed. The samples coiled at different temperatures exhibited a nearly identical cold rolling texture. However, the cold-rolled sample having a large amount of carbon in solid solution displayed a weak recrystallization texture.

Insight into TiN inclusion induced pit corrosion of interstitial free steel exposed to aerated NaCl solution

Abstract The mechanism of step-like corrosion pit induced by TiN inclusion in Ti-stabilized interstitial free steel was systematically investigated. There was neither micro-crevice nor local stress in the steel matrix adjacent to the TiN inclusion, but a significant Volta potential difference was detected between the conductive TiN and steel matrix. The corrosion pits induced by the TiN inclusion had a step-like structure, composed of an inner pit and a shallow ring-like corroded region (RCR). Three stages, initiation, transition, and propagation stages, involved in the formation of the step-like corrosion pits were observed. The three stages were manifested with the initiation of inner pit, the deepening of inner pit, and RCR formation, respectively. The inner pit was triggered by the Volta potential difference in the initiation stage. In the transition stage, the extreme gradients of H+ and Cl− occurred through the hydrolysis of Fe2+ and electromigration of Cl− in the deepening inner pit. Finally, the extreme gradients of aggressive ions between the inner pit and NaCl bulk solution resulted in the formation of RCR surrounding the inner pit in the propagation stage.

Segregation mechanism of phosphorus in Ti-stabilized interstitial-free steel

The grain boundary segregation of phosphorus in an interstitial-free (IF) steel aged for different times at 600 and 720°C was investigated using Auger electron spectroscopy. Results revealed that a significant degree of phosphorus segregation occurred during annealing for Ti-stabilized IF steel with 0.045wt% bulk content of phosphorus. In addition, the segregation degree increased with increased holding time. The segregation behavior of phosphorus in IF steel was analyzed based on the theory of grain boundary segregation, along with the present experimental data and previous results. The phosphorus segregation had equilibrium segregation characteristics, and the bulk concentration of phosphorus and holding temperature strongly influenced the final grain boundary segregation concentration of phosphorus in IF steel during annealing. Furthermore, the segregation level of phosphorus substantially decreased with increased annealing temperature or decreased bulk concentration of phosphorus. Results of theoretical calculation were consistent with the experimental ones, indicating that the grain boundary segregation behavior of phosphorus in IF steel during annealing can be qualitatively and quantitatively explained by the theory of equilibrium segregation.

Surface hardening of IF steel by plasma nitriding: Effect of a shot peening pre-treatment

Shot peening and plasma processes are widely used to improve surface properties of several alloys. In this work, triode plasma nitriding (TPN) was applied to Ti-stabilized interstitial free (IF) steels in an attempt to increase their hardness without compromising their excellent conformability. Shot peening was also trialed before triode plasma nitriding in an attempt to enhance nitriding kinetics and achieve deeper case depths. Triode plasma nitriding was performed at 450°C, 475°C and 500°C for 4h on Ti-stabilized IF steel. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the steel structure. Instrumented indentation hardness–depth profiles indicated that a significant hardening effect was achieved after plasma nitriding at 500°C for 4h. These nitriding conditions doubled the near-surface hardness of the parent IF steel and led to a case depth of 500μm. Instrumented indentation hardness-profile data also indicated that the shot peening pre-treatment did not have any beneficial effect on nitriding kinetics as a reduction in case depth occurred after this mechanical pre-treatment. Increased surface roughening promoted by shot peening reduced the nitrogen uptake during nitriding. Dry sliding wear tests also corroborated the benefits of plasma nitriding on Ti-stabilized IF steels, as significantly lower wear volumes resulted after this surface hardening treatment. Although oxidative wear was found to occur in all IF steel samples, wear performance was found to be influenced by load support provided by underlying steel substrates and thickness of compound layers. The best wear performance of solely plasma nitrided samples could be attributed to thicker compound layers and deeper hardened cases compared to shot-peened+plasma nitrided samples, which exhibited shallower case depths and thinner compound layers.

Impulse Excitation Internal Friction Study of Dislocation and Point Defect Interactions in Ultra-Low Carbon Bake-Hardenable Steel

The simultaneous presence of interstitial solutes and dislocations in an ultra-low carbon bake-hardenable steel gives rise to two characteristic peaks in the internal friction (IF) spectrum: the dislocation-enhanced Snoek peak and the Snoek–Kê–Köster peak. These IF peaks were used to study the dislocation structure developed by the pre-straining and the static strain aging effect of C during the bake-hardening process. A Ti-stabilized interstitial-free steel was used to ascertain the absence of a γ-peak in the IF spectrum of the deformed ultra-low carbon steel. The analysis of the IF data shows clearly that the bake-hardening effect in ultra-low carbon steel is entirely due to atmosphere formation, with the dislocation segment length being the main parameter affecting the IF peak amplitude. Recovery annealing experiments showed that the rearrangement of the dislocation structure lead to the elimination of the C atmosphere.

Relation of ductile-to-brittle transition temperature to phosphorus grain boundary segregation for a Ti-stabilized interstitial free steel

Abstract Equilibrium grain boundary segregation of phosphorus in a Ti-stabilized interstitial free (IF) steel is measured using Auger electron spectroscopy (AES) after the specimens are aged for adequate time at different temperatures between 600 and 850 °C. Based on the experimental data of equilibrium grain boundary segregation along with the McLean equilibrium segregation theory, the free energy of segregation of phosphorus is evaluated to be ∼44.8 kJ/mol, being independent of temperature. With the AES results being combined with the ductile-to-brittle transition temperatures (DBTTs) determined by impact tests, a relationship between DBTT and phosphorus boundary concentration is established. Predictions with the relationship indicate that cold work embrittlement may be severe if the steel is annealed at relatively low temperatures after cold rolling.

Effect of Cold Rolling on as–ECAP Interstitial Free Steel

Ti-stabilized interstitial free steel subjected to eight passes, route BC room temperature equal channel angular pressing (ECAP) additionally was cold rolled (CR) up to 95 pct thickness reduction. Electron back-scattering diffraction and transmission electron microscopy characterized microstructural refinement and microtexture evolution, whereas the mechanical properties were assessed by uniaxial tensile tests. After 95 pct CR, the average high-angle grain boundary spacing reduces to 0.14 μm, whereas the high-angle boundary fraction increases to ~81 pct. The ECAP negative simple shear texture components rotate by ~15 deg around the transverse direction toward the rolling direction for up to 50 pct CR, with typical rolling textures observed at 95 pct CR. The decrease in boundary spacing produces a ~500 MPa gain in 0.2 pct proof stress, a ~600 MPa increase in ultimate tensile strength (UTS), and a ~4 pct loss in total elongation after 95 pct CR. Similar rates of decrease in work hardening correspond to comparable rates of cross and/or multiple slip events irrespective of the processing regime and substructural refinement. The fracture mode of the tensile samples changes from ductile to brittle type between ECAP and 95 pct CR and is attributed to the reduced work hardening capacity of the latter. The modified Hall–Petch equation shows that the convergence of high-angle boundary spacing values with their low-angle counterparts results in an increased contribution via boundary strengthening to the 0.2 pct proof stress and UTS.

Mechanical Properties and Microstructures of Ferritic-Rolled High-Strength Interstitial-Free (IF) Steel Sheets

High-strength IF steel sheet has increasingly attracted more attention of steelmakers in recent years as it has the potential to lighten the weight of automobiles and save energy and lower the production cost. In this paper, the effect of processing parameters on microstructures and mechanical properties especially deep drawability of ferritic-rolled P-added high strength Ti-stabilized IF steels were investigated and the precipitates in the steels were also analyzed. The results show that lubricant condition has great influence on the r-value and deep drawability of high-strength IF steels. And with the decrease of rolling temperature in ferrite region, the deep drawability of steels is improved.

Precipitation Behavior of Warm-Rolled Ti-Stabilized Interstitial-Free (IF) Steel Sheets

Precipitates have great influence on the recrystallization, texture evolution and thus the final mechanical properties of the Interstitial-Free (IF) steel sheets, however, very few studies have dealt with the precipitation behavior of IF steels warm rolled in ferrite region. In the present work, the precipitate characteristics (type, morphology, size and amount) of warm-rolled ordinary Ti-stabilized Interstitial-Free (Ti-IF) steel and p-added high-strength Ti-IF steel were investigated by Transmission Electron Microscope (TEM) and Energy Dispersion Spectrometer (EDS). The results show that most precipitates in warm-rolled ordinary Ti-IF steels are TiN, TiS, Ti4C2S2 and TiC. Besides these precipitates, a great amount of FeTiP precipitates exist in warm-rolled P-added high-strength Ti-IF steel. The precipitation of FeTiP retards the migration of grain boundary in the recrystallization annealing, so the {111} texture and thus deep drawability of warm-rolled high-strength Ti-IF steel is deteriorated.

Mechanical Properties and Microstructures of Ferritic-Rolled Ultra-Low Carbon (ULC) and Ti-Stabilized Interstitial-Free (Ti-IF) Steels

In this paper, effects of ferritic rolling process on microstructures and mechanical properties especially drawability of ultra-low carbon (ULC) and Ti-stabilized interstitial-free (Ti-IF) steels were investigated and the precipitates of ferritic-rolled Ti-IF steels were also analyzed. The results show that good deep drawability are achieved as IF steels are ferritic rolled in good lubricant condition; on the contrary, lubricant condition has less influence on the deep drawability of ULC steels. And with the decrease of rolling temperature in ferrite region, deep drawability of Ti-IF steels are improved, but for ULC steels, when ferritic rolled at low temperature, the r-value is still less than one, which means insignificant deep drawability.

Influence of H 2–N 2 atmosphere composition and annealing duration on the selective surface oxidation of low-carbon steels

The selective surface oxidation of the Ti-stabilized interstitial free (Ti-IF) and dual phase (DP) steel during recrystallization annealing at 820 °C under the variety of exposure time and composition of protective H 2–N 2 atmosphere at low dew point (−40 °C) was examined. It was found that Mn, Al, Si and Cr oxides are formed in all cases, but higher Mn and Si concentration in DP steel leads to an increase in a density of coarse nonwettable MnSiO 3 particles. An increase in annealing time enhances surface coverage with oxides, while increased H 2 content in gas atmosphere results in less coverage with external oxides improving the wettability. The influence of the stronger reducing atmosphere is more exhibit on Ti-IF steel where after 120 s of annealing in 15vol.%H 2–85vol.%N 2 atmosphere the wetting angle θ stabilized to a value of the order of 6°. The presence of surface BN and the TiN particles also affects the full surface coverage by external oxides.

Grain Refinement and Texture Change in Interstitial Free Steels after Severe Rolling and Ultra-Short Annealing

A Ti-stabilized interstitial free steel was highly cold deformed to a reduction of 95% and subsequently submitted to extremely short annealing cycles with heating rates varying between 300°C/s and 4500°C/s followed by water quench at various temperatures. The microstructural and textural development was studied through various consecutive stages: partially recrystallized, fully recrystallized and after α→γ→α transformation. It was found that irrespective of the heating rate the recrystallization has completely terminated before the onset of the ferrite to austenite phase transformation. In the fully recrystallized condition, ultra-rapid heating gave rise to substantially refined structures with an average ferrite grain size of 6µm. It was also observed that this grain refinement saturates with heating rates beyond 1000°C/s. With regard to the texture formation, the characteristic {111} deep drawing fibre of cold rolled IF steels was observed, irrespective of the heating rate, in an annealing treatment as short as 0.3s. After the forward and reverse α→γ transformation, the ensuing ferrite texture displayed a strong memory effect, as the {111} fibre was even more intense after the double transformation than before.

Adhesion of Enamel Layer after Repeated Firings in Ti-Stabilized IF Steel Sheet for Porcelain Enamel Use and Its Mechanism

Adhesion of enamel layer in Ti-stabilized Interstitial-Free steel (Ti-IF) for porcelain enamel was studied through pickling loss weight, smuts on a pretreated Ti-IF steel, chemical compositions of steel and the repeated number of firing to form enamel, comparing with high oxygen steel for porcelain enamel. When pickling loss weight in Ti-IF steel is below 30g/m 2 , the adhesion of enamel layer can be over 80% even after firing is repeated four times High oxygen steel, in contrast, has over 80 % adhesion of enamel layer, regardless of the repetition of firing, and its adhesion is not related to pickling loss weight. TEM observation reveals that the smut on pretreated Ti-IF steel is mainly round-shaped TiS (100-300 nm) and the one on pretreated high oxygen steel is mainly Cr 2 MnO 4 (100-300 nm). During the reaction between enamel and pretreated steel for firing process, TiS is presumed to worsen adhesion, while Cr 2 MnO 4 is not related to adhesion. Therefore, reducing TiS (lowering pickling loss weight) is an effective measure to keep satisfactory adhesion in Ti-IF steel at repeated firings. Under conventional pretreatment conditions (ex. pickling for about 5 min with 12-15 % H 2 SO 4 at 70-80°C and Ni-deposition for about 5 min with 13-18 g/L NiSO 4 .7H 2 O at 70-80°C), Cu should be higher than about 0.04 % and P shouid be lower than about 0.013 % to be below 30g/m 2 as pickling loss weight in order to keep over 80 % adhesion in Ti-IF steel.

Textures and precipitates in Ti-stabilized interstitial-free steel

Samples of titanium-stabilized interstitial-free steel were coiled at 600 and 700 °C. The effect of the coiling temperature on the formation of textures and precipitates during coiling, cold rolling and recrystallization annealing was studied by means of neutron texture measurements, small angle neutron scattering analysis and transmission electron microscopy observations. Whereas coiling at the lower temperature resulted in an insufficient scavenging of carbon in solid solution, coiling at the higher temperature promoted scavenging of carbon in solid solution, and consequently the formation of a large number of precipitates. The large amount of carbon in solid solution caused the retardation of softening in the cold-rolled sample during annealing below 700 °C. Upon annealing above 700 °C the impact of the coiling temperature on grain size and precipitates was not observed. The samples coiled at different temperatures exhibited a nearly identical cold rolling texture. However, the cold-rolled sample having a large amount of carbon in solid solution displayed a weak recrystallization texture.

Grain Refinement and Texture Change in Interstitial Free Steels after Severe Rolling and Ultra-short Annealing

A Ti-stabilized interstitial free steel was highly cold deformed to a reduction of 95% and subsequently submitted to extremely short annealing cycles with heating rates varying between 300°C/s and 4500°C/s followed by water quench at various temperatures. The microstructural and textural development was studied through various consecutive stages: partially recrystallized, fully recrystallized and after α→γ→α transformation. It was found that irrespective of the heating rate the recrystallization has completely terminated before the onset of the ferrite to austenite phase transformation. In the fully recrystallized condition, ultra-rapid heating gave rise to substantially refined structures with an average ferrite grain size of 6 μm. It was also observed that this grain refinement saturates with heating rates beyond 1000°C/s.The transformation structures obtained by ultra fast heating to temperatures in the full austenite region exhibited remarkably coarser grains than the ones observed after ferrite annealing. This coarsening effect was attributed to the effects of heterogeneous nucleation during phase transformation as C was initially present as TiC precipitates in the ferrite matrix.With regard to the texture formation, the characteristic {111} deep drawing fibre of cold rolled IF steels was observed, irrespective of the heating rate, in an annealing treatment as short as 0.3 s. After the forward and reverse α→γ transformation, the ensuing ferrite texture displayed a strong memory effect, as the {111} fibre was even more intense after the double transformation than before.

Recrystallization kinetics and associated textures of Cu-bearing Ti-IF steels

The influence of copper on recrystallization of Ti-stabilized interstitial free steels has been studied. It is shown that the recovery and the recrystallization kinetics are slowed down. Thus the final recrystallization texture is modified: whereas the γ-fiber is still present, its intensity is lower. The deformed α-fiber contribution is larger than without copper and its magnitude depends on the annealing conditions: it is all the more intense as the heating rate is small. These effects are related to the presence of copper-based precipitates. The recrystallization textures are discussed in function of the precipitates evolution.

The Segregation Behavior of Phosphorus in Ti and Ti+Nb Stabilized Interstitial-Free Steels.

The phenomenon of cold work embrittlement (CWE) in the P-added high strength interstitial-free steels has been associated with the segregation of P to the ferrite grain boundaries. This segregation by P is believed to decrease the cohesive strength of the grain boundaries by weakening their bonding. Hence, the resistance of the steel to brittle fracture, i.e., intergranular fracture, is greatly decreased. The goal of the present study was to investigate the segregation behavior of P during the different stages of processing (prior to and after coiling, and after cold rolling and annealing) in Ti and Ti+Nb stabilized interstitial-free steels. It was found that a considerable amount of segregation of P to the ferrite grain boundaries occurred during the coiling process in Ti-stabilized interstitial-free steels. However, with the addition of Nb in the Ti+Nb-stabilized interstitial-free steels, the segregation of P was decreased in the as-coiled condition. The P content on the ferrite grain boundaries in the final cold rolled and annealed condition was found to depend on two factors; (1) the segregation of P in the as-coiled condition, and (2) availability of Ti to form phosphides during the annealing process. It was confirmed in this study that the addition of P decreases the CWE resistance of the steel. Furthermore, it appears that the CWE resistance of the Ti+Nb-stabilized interstitial-free steels is improved by the presence of solute Nb on the ferrite grain boundaries.