Microstructural development during the quenching and partitioning process in a newly designed low-carbon steel

An improved X-ray diffraction line profile analysis method is developed to determine dislocation density of lath martensitic steels. This method combines the modified Warren–Averbach (MWA) and the modified Williamson–Hall (MWH) methods. The developed method is robust and leads to unique values for the dislocation density, the effective outer cut-off radius of the dislocations ( R e ) and the dislocations distribution parameter ( M ). Dislocation structures of lath martensite in a steel, in the as-quenched condition as well as in tempered conditions, are characterized by using the proposed method. The calculated dislocation density is compared with the values obtained from the MWH method by considering a constant value for M . It was found that both methods provide dislocation densities in the range of the values calculated from the dislocation strengthening component of the yield strength.

An improved X-ray diffraction analysis method to characterize dislocation density in lath martensitic structures

A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T
 M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T
 M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T
 M temperature.

On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

Disclosed is a high-strength steel sheet having excellent processability and a tensile strength of 980 MPa or more. The high-strength steel sheet has the following composition (in mass%): C: 0.1 to 0.3% inclusive, Si: 2.0% or less, Mn: 0.5 to 3.0% inclusive, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less, with the remainder being Fe and unavoidable impurities. The steel structure of the high-strength steel sheet comprises 50% or more of martensite, 50% or less (including 0%) of ferrite, 10% or less (including 0%) of bainite, and 10% or less (including 0%) of residual austenite. In the high-strength steel sheet, the half width in the degree distribution of the nano-hardness obtained by the measurement of the hardness distribution of the martensite is 2.0 GPa or more, and the steel sheet has a tensile strength of 980 MPa or more.

High strength steel sheet and method for manufacturing the same

The effect of nanosized (Ti,Mo)C precipitates on hydrogen embrittlement of tempered lath martensitic steel

X-ray diffraction (XRD)-based modified Warren–Averbach (MWA) analysis, in comparison with the Williamson–Hall (WH) analysis, was applied to 0.3 mass% carbon martensitic steels, as-quenched and subsequently tempered at various temperatures, to give their dislocation densities. For the as-quenched martensite, the WH method gives a value of around 2.0×1016 m−2, which could be overestimated. Meanwhile, the MWA method gives a value of around 6.3×1015 m−2, which is below the possible upper limit of dislocation density, 1016 m−2. The MWA-derived value for the as-quenched steel seems to be 1.6–4.8 times higher than those expected from the precedent results derived by transmission electron microscope (TEM) observations. However, considering that the TEM-derived value gives the microscopically local average while the XRD-derived value gives the macroscopic average, such discrepancy between the TEM-derived value and MWA-derived value is tolerable. For the steels tempered at 723 K and 923 K, the MWA and WH methods give comparable values ranging in 1014 m−2, where the rearrangement of dislocation structure is observed by TEM. However, in these steels where the XRD peaks are narrower and the instrumental width of the present XRD system could be significant, care should be taken over the peak width correction.

https://www.jstage.jst.go.jp/article/isijinternational/50/6/50_6_875/_pdf

Comparison of the Dislocation Density in Martensitic Steels Evaluated by Some X-ray Diffraction Methods

The deformed structure of cold rolled IF steels consists of a large number of α fibre and γ fibre oriented grains and a small number of (110) oriented grains, each with a distinctive grain shape, subgrain structure, hardness and etching appearance. Early nucleation occurs mainly in γ fibre grains and (110) grains. The texture of the early nuclei consists of a γ fibre with a spread towards (221) together with a weak // ND fibre. Numerous nuclei develop within a single deformed grain. α fibre grains are consumed only after 70% of the materials is recrystallised.

Recrystallisation of Ti IF Steel Investigated with Electron Back-scattering Pattern (EBSP)

A high-strength steel sheet includes, by mass %, C: 0.03% to 0.30%, Si: 0.08% to 2.1%, Mn: 0.5% to 4.0%, P: 0.05% or less, S: 0.0001% to 0.1%, N: 0.01% or less, acid-soluble Al: more than 0.004% and less than or equal to 2.0%, acid-soluble Ti: 0.0001% to 0.20%, at least one selected from Ce and La: 0.001% to 0.04% in total, and a balance of iron and inevitable impurities, in which [Ce], [La], [acid-soluble Al], and [S] satisfy 0.02≦([Ce]+[La])/[acid-soluble Al]<0.25, and 0.4≦([Ce]+[La])/[S]≦50 in a case in which the mass percentages of Ce, La, acid-soluble Al, and S are defined to be [Ce], [La], [acid-soluble Al], and [S], respectively, and a microstructure includes 1% to 50% of martensite in terms of an area ratio.

High-strength steel sheet and method for producing same

The present invention provides a steel sheet excellent in workability, which may be used for components of an automobile or the like, and a method for producing the same. More specifically, according to one exemplary embodiment of the present invention, a steel sheet excellent in workability, including in mass, 0.08 to 0.25% C, 0.001 to 1.5% Si, 0.01 to 2.0% Mn, 0.001 to 0.06% P, at most 0.05% S, 0.001 to 0.007% N, 0.008 to 0.2% Al, at least 0.01% Fe. The steel sheet having an average r-value of at least 1.2, an r-value in the rolling direction of at least 1.3, an r-value in the direction of 45 degrees to the rolling direction of at least 0.9, and an r-value in the direction of a right angle to the rolling direction of at least 1.2.

Steel plate exhibiting excellent workability and method for producing the same

A ferritic steel sheet wherein a mean value of X-ray random intensity ratios of a group of {100} to {223} orientations is 3.0 or more and a mean value of X-ray random intensity ratios of three crystal orientations of {554} , {111} , and {111} is 3.5 or less and further at least one of the r values in a rolling direction and a direction at a right angle of that is 0.7 or less.

Naoki Yoshinaga

Papers

Comparison of the Dislocation Density in Martensitic Steels Evaluated by Some X-ray Diffraction Methods

Recrystallisation of Ti IF Steel Investigated with Electron Back-scattering Pattern (EBSP)

High-strength steel sheet and method for producing same

Steel plate exhibiting excellent workability and method for producing the same

Steel plate excellent in shape freezing property and method for production thereof