The recent situation of IF steel is reviewed and discussed with regard to following items: 1) The production of IF steel in Japan, 2) Featuring conditions to decarburize the molten steel effectively in steelmaking process, 3) The effect of Ti and Nb on metallurgical behaviors of IF steel, 4) The effect of processing conditions on metallurgical properties of IF steel, 5) The effect of soluted carbon on the powdering and the cold working embrittlement of IF steel, and 6) Metallurgical properties of IF ferritic stainless steel.

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To predict solute carbon and nitrogen in Nb and Ti added extra-low carbon steels quantitatively, the phase stability of the carbonitride in austenite and/or ferrite has been calculated by using Thermo-Calc software to which some thermodynamic parameters were added for the Fe-Nb-Ti-C-N system. It has been confirmed that the (Nb, Ti) (C, N) phase has a miscibility gap island, which is predicted from the tieline sets between TiN-like and NbC-like phases, and the phase can be revealed in the ferrite region, that is, duplex precipitates coexist in the matrix of ferrite in the equilibrium state, whereas only one TiN-like phase precipitates in the austenite region. As a consequence of the above phase stabilities, it has been predicted that each amount of nitrogen and carbon in solution changes only in the austenite region and the ferrite region, respectively. A quasi-equilibrium precipitation as a two-step heat-treatment has also been demonstrated.

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Bake hardening properties of Ti-bearing ultra low carbon steels are affected by the precipitation behavior of TiN, MnS, TiS and Ti₄C₂S₂. Thus the methods of isolation and determination of these precipitates have been developed and applied to the study on the precipitate change with reheating of the steels.
All precipitates, TiN, MnS, TiS and Ti₄C₂S₂ were extracted by MS electrolysis. TiN was extracted by 10% (v/v) bromine-methanol. MnS was isolated from the residues (extracted by MS electrolysis) by 50°C HCl treatment. TiS in the 50°C HCl treated residues was decomposed by 10% (m/v) iodine-methylacetate and Ti₄C₂S₂ remained. Mn, Ti and S in the residues were determined directly by X-ray fluorescence method. N was determined by neutral titration method. The determined compositions of precipitates except TiS almost corresponded with their stoichiometries. TiS was determined to TiS_1.3.
Whole nitrogen precipitated as TiN and the amount did not change with reheating temperatures between 950 and 1250°C. Total amount of the sulfides such as MnS, TiS and Ti₄C₂S₂ changed scarcely with reheating temperatures, but the amount of each sulfide changed largely. Precipitates in the steels reheated below 1050°C were mainly Ti₄C₂S₂. It decomposed and transfomed into TiS and MnS with raising the reheating temperature, and decreased drastically at 1150°C. The precipitates above 1150°C were mainly MnS and TiS, and the fraction depended on the ratio of Ti to Mn concentration in the steels.

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The influences of S and Ti contents as well as slab reheating temperature on the precipitation behavior of sulfides in hot-rolled bands in Ti-added ultra low-carbon steels have been investigated. The results obtained are as follows: In the specimens with the same 0.02%Ti content, Ti₄C₂S₂ decreases and TiS increases in number as S content increases while in the specimens with the same 0.008% S content, TiS decreases and Ti₄C₂S₂ increases in number as Ti content increases. The number of TiS increases when the slab reheating temperature is high.
The solubility prodects of TiS and Ti₄C₂S₂ in austenite have been also experimentally determined as follows:
log [Ti][S] = -3252/T-2.01
log [Ti][C]^0.5[S]^0.5 = -5208/T-0.78
The experimental results above agree well with the results calculated using these solubility products. The reliability of solubility products determined in this study are checked and it is confirmed that the solubility products in this study is in good agreement with the published experimental results.

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α+γ and γ phases annealing in Ti and Nb-added ultra low-carbon sheet steels with and without Mn, P, and/or Cr has been investigated.
In high-strength steels with Mn, P and/or Cr, BH increases as the annealing temperature increases, whereas YP-EI after aging at 100°C for 1 hr clearly decreases. It is speculated that the coexistence of BH and nonaging properties stems from the high dislocation density introduced by γ→α transformation. In contrast, YP-EI in mild steel without Mn, P, and/or Cr increases as BH increases because dislocation density is not high enough to provide non-aging property.
In mild steel, r-value distinctly decreases and texture is randomized by γ phase annealing, while r-value increases and {111} component develops by intercritical annealing. On the contrary, in high-strength steel, recrystallization texture in α is assumed to be inherited even after α→γ→α transformation takes place. Apparently, variant is distinctly selected when at least γ transforms into α in high-strength sheet steels. The variant selection in γ→α transformation is speculated to be caused by residual stress introduced by α→γ transformation.

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Isothermal decomposition processes of austenite in ultra-low carbon 3%Mn-1%Cr steels have been investigated mainly by means of surface relief and microstructural observations and the following results have been obtained:
(1) Bainitic reaction occurs even in the very low carbon steels where carbide particles do not precipitate.
(2) Widmanstatten ferrite sub-units grow often from grain boundary ferrite allotriomorphs, while bainite nucleates specifically at an austenite grain boundary. These structures grow into austenite in the shape of laths, the longitudinal growth of them, however, being thought to be displacive.
(3) The coalescence of them and the further diffusional growth lead to the formation of Widmanstatten ferrite and bainite laths, respectively.
(4) The differences in the formation mechanism between Widmanstatten ferrite and bainitic ferrite exist in the diffusional nucleation events, i.e., the former occurs probably in the orthoequilibrium condition while the latter is in the paraequilibrium condition.

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To realize a remarkable improvement of the deep drawability of steel sheets, the formation of texture of Ti-bearing IF steel sheets has been investigated throughout the rolling and annealing processes.
Initially, the formation of hot band textures has been investigated in terms of the influence of rolling temperature and reduction. A low carbon steel sheet has been also studied to clarify the features of IF steel sheets.
Subsequently, the textures formed by rolling and annealing has been studied at the surface and the midplane of IF steel sheets. The formation of the recrystallization texture has been discussed taking into consideration of the stored energy and the crystal rotation relationship around the axes between the orientation of the recrystallized grain and the surrounding deformed matrix. It was also shown that the consideration of the local crystal rotations in the vicinity of grain boundaries due to rolling facilitates the understanding of the formation of the recrystallization texture.
The obtained results led to the conclusion that the α-region hot rolling under cartain conditions is a very effective means of improving the deep drawability of IF steel sheets.

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A quantitative study was undertaken to characterise the size, dispersion and microchemistry of precipitates and the texture of two hot-dip galvanised Ti stabilised interstitial free steels of comparable chemistry and grain size but with a distinct difference in Lankford values. Small angle neutron scattering technique was used complementary to electron optical techniques to resolve the ultra-fine precipitates (<5 mm). The specimen with a dense dispersion of fine precipitates exhibited a weak intensity of {111} texture and a low Lankford value. In order to clarify whether the development of {111} texture was suppressed by increased solute carbon in the matrix or by the Zener pinning force exerted by the ultra-fine precipitates during recrystallisation growth, internal friction measurements were carried out. The effect of ultrafine precipitates in retarding the grain boundary mobility is confirmed. The possible roles of solute carbon effect, and particle pinning effect on the evolution of {111} texture are discussed.

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The changes in recrystallization behavior and r-value by reducing impurity elements in Ti-added ultra low carbon sheet steel were investigated. The recrystallization temperature of high-purity steel is lower than that of conventional impure steel when compared at a given finish-rolling temperature or solute titanium content. This is attributed to the retarding effect of phosphorus, which can be interpreted on the basis of a commonly accepted solute-drag effect since sulfur and nitrogen have almost no effect on recrystallization behavior. The half-recrystallization temperature of Ti-added ultra low carbon steel is more closely related to the content of solute titanium, rather than the amount of TiC precipitates. The superior difference of r-value in high-purity steel in comparison to conventional impure steel is assumed to have arisen from the balance between an unfavorable effect, which may be a suppression in grain growth, caused by the increase in sulfur and nitrogen contents, and a favorable effect caused by the addition of phosphorus.

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High strain-rate rolling in the ferrite region has been adopted to develop the {111} recrystallization texture in extra-low C steel without cold-rolling. The intensity of the {222} component in the recrystallized sheet steel can be increased by increasing the rolling strain rate. Conversely, the texture of hot-rolled sheet steel, does not depend on the strain rate. The {222} redidual strain in specimens rolled at a high strain-rate is significantly higher than that in the low strain-rate specimens, while the {211}, {200} and {110} residual strains are independent of the strain rate. High strain-rate rolling can lead to more rapid recovery of {222} residual strain during annealing. As a result, it is considered that high strain-rate rolling in the ferrite region provides high stored energy preferentially in the {111} component, with the result that the region of high stored energy in the {111} component recovers and nucleates swiftly and a strong {111} recrystallization texture is developed. Moreover, high strain-rate rolling of high-purity iron makes it possible to recrystallize the material quickly and obtain the high r-value associated with a {111} recrystallization texture without annealing as cold-rolling.

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The influences of Mn of the recrystallization behavior and the formation of recrystallization texture in Ti-added ultralow-C steels were investigated and compared with that of low-C Al-killed steels. In Ti-added ultralow-C steels, Mn does not influence the recovery, the nucleation and growth rate of recrystallization, or the recrystallization. On the other hand, in low-C Al-killed steel, Mn inhibits the recovery and retards the growth rate of recrystallized grains. Consequently, the development of ND//<111> is impaired, and the intensity of ND//<110> increases after recrystallization. The differences in recrystallization behavior and annealing texture formation between the two kinds of steel were discussed from the viewpoint of the roles of precipitates and solute Mn. It was determined that Mn-C complex is the main reason for these differences.

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The mechanism of improving r-value of continuously annealed steel sheets by high temperature hot-coiling in Cu-bearing ultra low C interstitial-free (IF) steel has been investigated, by using the hot-bands heat-treated at different temperatures from 500 to 800°C in order to change the morphology of pre-precipitates in hot-bands. By the ODF analysis of recrystallization texture, it was clarified that the γ-fiber texture (<111>//ND) extremely developed by the heat-treatment at the temperature ranging from 720 to 760°C. TEM observations also revealed that the complex-precipitates composed of Ti-carbosulfide and Cu were observed only in the same range of heat-treating temperature. Improvement of r-value by high temperature hot-coiling is caused by the precipitation of Cu combined with coarse Ti-carbosulfides which avoids the detrimental influence of fine pre-precipitation of Cu in matrix.

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The mechanism for improving the resistance to the secondary working embrittlement by adding B to extra-low-C, Ti and Nb co-added interstitial-atom free steels with various P contents was studied by investigating the brittle-ductile transition temperature of drawn cups under various drawing ratios, and by evaluating the grain boundary segregation behavior of B and P. The transition temperature was lowered by adding B under all the conditions studied, while it was raised by employing a higher drawing ratio and with increasing P content. The effect of P content on the transition temperature was not affected by B content. By increasing the holding time at 1123 K, the transition temperature was significantly raised irrespective of the B content. It was proved by Auger electron spectroscopy that P atoms segregate to the grain boundary with or without B addition. The results obtained in this study support the mechanism that grain boundary strengthening by segregated B itself was predominant for improving the resistance to the secondary working embrittlement by B addition. However, it was also observed that P segregation was decreased with increasing the B addition. Therefore, it can be concluded that the improvement by B addition to the secondary working embrittlement was predominantly due to grain boundary strengthening by segregated B itself, and additionally by the site competition between B and P at the grain boundary.

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The effect of adding Nb on the Lankford value (r-value) and of the continuous annealing conditions on hardness has been investigated for extra-low-carbon steel sheet used as tin mill black plate (TMBP).
When hot rolling was finished in the ferrite region, adding a small amount of Nb markedly improved both the average r-value and its planar anisotropy. The reason for this improvement is considered to be that, by finishing hot rolling in the ferrite region, a texture like that from cold rolling appeared in the hot band when Nb had been added. By using such hot bands, the cold-rolled texture was more strongly developed in as-cold-rolled steel sheet than was the case when using steel without added Nb. This stronger cold-rolled texture created a strong {554}<225> orientation after continuous annealing and improved the r-value.
The hardness of TMBP is influenced by whether complete recrystallization is achieved or not, the degree of recrystallization depending on the heating rate, annealing temperature and soaking time during the continuous annealing process. However, by soaking for about 10 sec, the effect of heating rate become small. After complete recrystallization, the effect of cooling rate and the other annealing conditions on the hardness is very small. The insensitivity of this steel toward the annealing conditions is favorable for increasing the speed and efficiency of the continuous annealing process.

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The effect of continuous annealing atmosphere on material properties of ultra low carbon Ti-added cold-rolled steels has been investigated. By replacing the conventional annealing atmosphere with the carburizing atmosphere, the improvement of formability and bake hardenability of ultra low carbon Ti-added cold-rolled steel are obtained as compared with those of steels annealed in conventional atmosphere. By carburizing for a short time during continuous annealing, the extremely small amount of carbon is added without changes in the microstructure. The resistance to cold-work embrittlement and fatigue property are improved due to the increase in grain boundary strength. Furthermore, bake hardenability exceeding 30 MPa is achieved through the control of carburizing condition and chemical composition of steel. The excellent combination of bake hardenability with r-value is attainable by carburizing after recrystallization. A part of added carbon atoms exist in solution, and the others are combined with the excess Ti of steel. Furthermore, the solute carbon is distributed nonuniformly in the direction of thickness due to the diffusion of carbon and the combination of added carbon with excess Ti. Consequently, the amount of solute carbon is controlled by the carburizing condition and the amount of excess Ti.

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For the weight reduction of automotive body, an ultra-low C Ti-added cold-rolled sheet steel with a tensile strength of 450 N/mm² has been developed by means of adding P and Mn together. In order to develop the sheet steels of higher tensile strength, the effect of Si on the mechanical property of the ultra-low C Ti-added sheet steel has been studied by using steels containing 0.05-1.5 mass%Mn or 0.07 mass%P and coiled at 500°C in the hot rolling process. As a result it has been found, that ultra-low C Ti-added IF sheet steels with Mn ranging from 0.05 to 1.5 mass% exhibits maximal mean r-value at a medium Si content, which increases with the Mn content. This change in the r-value with the Si and Mn is illustrated in terms of the Mn-C dipole formed at 600°C during heating stage of continuous-annealing. With decrease in the Mn-C dipole the r-value increases. Then it is concluded that the A_r3 transformation temperature should be controlled to be about 900°C and the Mn content should be as low as possible for the improvement of deep drawability.

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