Polygonal Ferrite Matrix Research Articles

Formability Evaluation of TRIP-aided Steel Sheets with Different Microstructures: Polygonal Ferrite and Bainitic Ferrite Matrix

Recently, application of TRIP-aided steel sheets has been developed due to their good balance of strength and formability. In this work, a comparative study was performed on the formability of two different microstructures of TRIP-aided steel with different retained austenite characteristics. Microstructures along with their specific retained austenite characteristics such as volume fraction, stability and morphology were achieved by annealing of cold rolled sheet at fully austenite and intercritical region during subsequent isothermal treatment in the range of 350–410°C. SEM and XRD measurements were used to evaluate retained austenite characteristics. Formability and mechanical properties of sheets were determined by Erichsen test and tensile test, respectively. Results indicate that steel sheets with polygonal ferrite (PF) matrix present better formability. However, at identical volume fraction of retained austenite, sheets with bainitic ferrite (BF) matrix show better stretch formability due to high retained austenite stability.

The Structure Dependence of Deformation Behavior of Transformation-Induced Plasticity–Assisted Steel Monitoring by In-Situ Neutron Diffraction

Tensile deformation behavior of two transformation-induced plasticity (TRIP)–assisted multiphase steels with slightly different microstructures due to different thermomechanical treatment conditions applied was investigated by in-situ neutron diffraction. The steel with lower austenite volume fraction (f γ = 0.04) and higher volume fraction of needlelike bainite in the α-matrix exhibits higher yield stress (sample B, 600 MPa) but considerably lower elongation in comparison to the steel with higher austenite volume fraction (f γ = 0.08), granular bainite, and polygonal ferrite matrix (sample A, 500 MPa). The neutron diffraction results have shown that the applied tensile load is redistributed at the yielding point in such a way that the retained austenite bears a significantly larger load than the α matrix during the TRIP-assisted steel deformation.

Ultrafine Grained Ferrite/Martensite Dual Phase Steel Fabricated by Large Strain Warm Deformation and Subsequent Intercritical Annealing

An ultrafine grained (UFG) ferrite/cementite microstructure was produced by use of large strain warm deformation in two plain C-Mn steels. In order to overcome the characteristic restricted tensile ductility of this steel, an intercritical annealing was applied to obtain an UFG ferrite/martensite dual phase (DP) steel. Suitable intercritical annealing parameters have been worked out using dilatometry. Microstructure evolution during intercritical annealing has been investigated by means of scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The study revealed that increasing the Mn content from 0.87 to 1.63 mass% was highly beneficial for the formation of martensite. This effect is explained by the enrichment of Mn in cementite which is partly inherited by austenite. The final microstructure consists of martensite islands embedded in an ultrafine grained polygonal ferrite matrix. The average grain size is 1-2 μm. Small amounts of retained austenite (<1 μm) are finely dispersed. The grain size is hardly affected by the intercritical annealing, whereas the fraction of high-angle grain boundaries in the ferrite matrix is reduced. Tensile tests revealed that strain hardenability is drastically improved by the introduction of martensite as a second phase. The UFG-DP steel exhibits a good combination of high strength and uniform elongation and considerable strain hardenability.

Microstructural characterization of TRIP-aided steels

Cold rolled steel sheets containing 0.4C, 0.5Si, 1.5Mn, 1.0Al, 0.02Nb and 0.1Mo (mass%) were subjected to three heat treatment schedules to produce TRIP-aided steels with annealed martensite (AM), bainitic ferrite (BF) and polygonal ferrite (PF) matrix microstructures. The distribution of different phases in these three varieties of steels was estimated using some existing quantitative relationships. In most of the cases, distribution of different constituents of the microstructure and the partitioning of carbon among the different phases could be estimated.

Strain-induced Transformation Behaviour of Retained Austenite and Tensile Properties of TRIP-aided Steels with Different Matrix Microstructure

The effects of strain rate and temperature on the strain induced transformation behaviour of retained austenite and hence mechanical properties of TRIP-aided steels with annealed martensite (AM), bainitic ferrite (BF) and polygonal ferrite (PF) matrix microstructures were examined. For this, tensile tests were carried out at four different strain rates varying from 3.33×10−5 s−1 to 3.33×10−2 s−1 at room temperature and at 150°C. Additional intermittent tests were carried out at intermediate strain rates to estimate the extent of strain induced transformation of austenite to martensite using X-ray diffraction analysis. Tensile strength (TS) decreased with strain rate at both temperatures, though the TS at 150°C was lower for all steels except for BF steel tested at 3.33×10−5 s−1. The total elongation (TEL) for PF steel significantly improved at 150°C at all strain rates, with a maximum of 78% obtained at 3.33×10−3 s−1, the corresponding TS being 930 MPa. A similar effect was seen in AM steels tested at 3.33×10−3 s−1 and 3.33×10−4 s−1. However, in BF steel, higher temperature had a detrimental effect on TEL, especially at strain rates of 3.33×10−2 s−1 and 3.33×10−3 s−1.

Acoustic Emission Technique to Study the Effect of Strain Rate on the Deformation Behaviour of TRIP Aided Steels with Different Matrix Microstructures

Acoustic Emission (AE) signals generated during the tensile deformation of three TRIP-aided steels with different matrix microstructures were monitored. The tensile tests were carried out at four different strain rates varying from 3.33×10−5 to 3.33×10−2 s−1 and additional intermittent tests were carried out at the lowest and highest strain rates to estimate the extent of deformation induced austenite to martensite transformation using X-ray diffraction analysis. Good correlation was obtained between AE and X-ray diffraction analyses. Both techniques showed that increasing strain rate restricted progressive transformation of retained austenite to martensite. The energy of continuous AE signals generated during tensile test conducted at high strain rate (3.33×10−2 s−1) was found to be directly proportional to the fraction of martensite formed. The proportionality constant (k) was influenced by the matrix microstructure; steels with polygonal ferrite matrix manifesting a higher k value than steels with annealed martensite matrix or bainitic ferrite matrix.

The effect of Mn partitioning in Fe–Mn–Si alloy investigated with STEM-EDS techniques

The partitioning of Mn in the multiphase structure of a low—alloy 04G4 steel, showing a transformation—induced plasticity effect, was investigated using transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometry (EDS). The application of a soft annealing at 898K for 1–60h between hot rolling and cold rolling activates Mn diffusion in the ferrite matrix and Mn enrichment processes in the cementite. During continuous annealing at 973K in the (α+γ) temperature range new austenite (γ) is formed. A non-uniform distribution of Mn between bainitic and polygonal ferrite (α) matrix and retained austenite was observed after accelerated cooling. An increase of Mn content up to 10wt.% was detected in proeutectoid cementite and in martensite–austenite islands after water quenching from 973K.

Effects of Prestraining on High-Cycle Fatigue Strength of High-Strength Low Alloy TRIP-Aided Steels

The effects of prestraining on high-cycle fatigue strength of newly developed low alloy TRIP-aided steels with different matrix structures and different retained austenite characteristics were investigated for automotive applications. Prestraining to 10% in tension increased the fatigue limit of the TRIP-aided steels, especially in steel with a polygonal ferrite matrix. It was considered that the polygonal ferrite matrix brought on high a fatigue limit mainly due to TRIP of the retained austenite and high compressive internal stress in the matrix resulting from a hard second phase on prestraining. On the other hand, the steel with bainitic ferrite lath matrix exhibited only a small increase in the fatigue limit after prestraining. This was expected to be mainly due to “strain-induced martensite hardening” on prestraining, with small contributions of TRIP of compressive internal stress in the matrix. In addition, a very interesting finding was obtained that the internal stress is the most effective parameter among some parameters to increase the fatigue strength in low alloy TRIP-aided steels.

Development of high strength low alloy TRIP‐aided steels with annealed martensite matrix

Formable high‐strength low‐alloy TRIP‐aided sheet steels with annealed martensite matrix or TRIP‐aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate‐like retained austenite along annealed martensite lath boundary, the stability of which against the strain‐induced transformation was higher than that of the conventional TRIP‐aided dual‐phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the TRIP‐aided annealed martensitic steels exhibited superior large elongation and reduction of area. In addition, the steels possessed the same excellent stretch‐flangeability and bendability as TRIP‐aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress in matrix and so on.

Ductility and Formability of Newly Developed High Strength Low Alloy TRIP-aided Sheet Steels with Annealed Martensite Matrix.

Formable high-strength low-alloy sheet steels with annealed martensite matrix or TRIP-aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate-like retained austenite along annealed martensite lath boundary, whose stability against the strain-induced transformation was higher than that of the conventional TRIP-aided dual-phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the annealed martensite steels exhibited a superior large elongation and reduction of area. In addition, they possessed the same excellent stretch-flangeability and bendability as TRIP-aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress and so on.

疲労 高強度低合金ＴＲＩＰ鋼板の高サイクル疲労強度に及ぼす予ひずみの影響

The effects of prestraining on high cycle fatigue strength of newly developed low alloy TRIP steels with different matrix structure and different retained austenite characteristics were investigated for the automotive applications. The prestraining to 10% in tension increased fatigue limit of the TRIP steels, especially in steel with polygonal ferrite matrix. It was considered that the polygonal ferrite matrix in the steel brought high fatigue limit mainly due to TRIP of the retained austenite and high compressive internal stress in the matrix, resulting from hard second phase on prestraining. On the other hand, the TRIP steel with bainitic ferrite lath matrix exhibited a small increment in fatigue limit after prestraining. This was supposed to be mainly owing to strain-induced martensite content increased by prestraining, with small effects of TRIP of retained austenite and compressive internal stress in the matrix. In addition, a very interesting finding was obtained that the internal stress is the most effective parameter among some parameters to increase the fatigue strength in the low alloy TRIP steels.

Development of grade API X80 pipeline steel plates produced by controlled rolling

The future demand for high-grade pipeline materials (API X80 and higher) calls for the development of steels with microstructures showing acicular phases (acicular ferrite, bainite, and martensite-austenite (M A) constituent). A systematic investigation has heen made (of the mechanical properties and microstructures exhibited hy a wide range of controlled-rolled plates (of final thickness 18 mm) of various chemical compositions, produced on a laboratory scale. Low-carhon (∼ 0·06% C) boron-free and boron-containing steels have been examined, as has a boron steel with a Imrer carbon content (0–02% C). The boron-free steels (carbon equivalent ≤0.46%) always exhihited a polygonal ferrite matrix with a dispersed phase consisting mainly of M A constituent, while the horoncontaining steels (carhon equivalent ≤0.40%) invariahly showed an acicular matrix with a lower volume fraction of finely dispersed M A island. Quantitative relationships are ohtained hetween the volume fractions of the microstructural components and the mechanical properties. These relationships clearly show that the type of microstructure found in the horon steels gives better strength toughness combinations (in terms of the Charpy-V shelf energy). With both the boron-free and the boron-containing steels, the best results have been obtained after adding ∼0·10% Ti (together with ∼0·03% Nb), rather than vanadium, to increase precipitation strengthening. This increase has been confirmed directly for polygonal ferrite hy quantitatively comparing the number and size distrihution of precipitates in a Nb-V and a Nb-Ti steel having otherwise identical compositions.