[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Metallographic Methods for Revealing the Multiphase Microstructure of TRIP-Assisted Steels - some applications for steels

The effect of heat input on microstructural zones, hardness and fracture of T-joints in EN S235JR carbon steel was evaluated using the Vickers hardness, SEM, and fractography on impact fractured specimens. The gas metal arc welding (GMAW) process with pulsed metal arc transfer and different welding speeds was performed by an industrial robot using a teaching pendant. Microstructural observations revealed differences in the size and quantity of phases exhibited in the solidified weld bead and recrystallized heat affected zone (HAZ) as well as a refinement of ferrite with different morphologies when a greater heat input was generated. The hardness results indicated greater hardening with greater heat input for the weld bead than the HAZ, which was associated with fine secondary Widmastätten ferrite sideplates growing from columnar grains separated by a larger amount of fine acicular ferrite. At a speed of 35 cm/min, the greater heat input produced a greater increase in volume fraction and width of HAZ with complex microstructures formed by fine-grained ferrite and acicular ferrite. The weld bead reached a greater hardening and volume fraction compared to the HAZ linked to fine acicular ferrite. The hardening of weld bead was confirmed by fractography that showed the presence of a small microvoid morphology I. Guzman-Flores • J.J. Gasca-Dominguez • J. del Prado • L. Rios-Vargas Manufacturera de cigueñales de México (MACIMEX), Centro de Investigación Innovación y Desarrollo Avanzado, Km. 38.5 Carretera la Marquesa, Tenango del Valle, Estado de México 52300, México e-mail: Isidro.Guzman@macimex.com; JuanJose.Gasca@macimex.com; Joaquin.DelPrado@macimex.com; Leopoldo.Rios@macimex.com B. Vargas-Arista (*) División de Estudios de Posgrado e Investigación, Instituto Tecnológico de Tlalnepantla, Av. Instituto Tecnológico s/n, Col. La Comunidad, Tlalnepantla de Baz, Estado de México 54070, México e-mail: benjaminvar@yahoo.com.mx E. Garfias-Garcia División de CBI, Departamento de Materiales, Área de Ingeniería de Materiales, UAM –Azcapotzalco Av. San Pablo No. 180, Col. Reynosa Tamaulipas, Azcapotzalco 02200, México e-mail: D.F.elgg@correo.azc.uam.mx

Materials Characterization I

Abstract Martensitic and austenitic steel alloys were designed to optimize the performance of structures subjected to impulsive loads. The deformation and fracture characteristics of the designed steel alloys were investigated experimentally and computationally. The experiments were based on an instrumented fluid–structure interaction apparatus, in which deflection profiles are recorded using a shadow Moire technique combined with high speed imaging. Fractographic analysis and post-mortem thickness reduction measurements were also conducted in order to identify deformation and fracture modes. The computational study was based on a modified Gurson damage model able to accurately describe ductile failure under various loading paths. The model was calibrated for two high performance martensitic steels (HSLA-100 and BA-160) and an austenitic steel (TRIP-120). The martensitic steel (BA-160) was designed to maximize strength and fracture toughness while the austenitic steel (TRIP-120) was designed to maximize uniform ductility, in other words, to delay necking instability. The combined experimental–computational approach provided insight into the relationships between material properties (strength, uniform ductility, and post-necking ductility) and blast resistance of structures. In particular, the approach allowed identification of material/structure performances by identifying impulse-center deflection behavior and the impulse leading to panel fracture.

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Design and identification of high performance steel alloys for structures subjected to underwater impulsive loading

Silicon existence in conventional low carbon TRIP steel is essential for inhibiting cementite precipitation, despite that it tends to obstruct the hot-dip galvanizing process. Aluminum is considered the mostly preferable substituent of silicon for such purpose. However, it has a counter effect on the strength/ductility balance of TRIP steel as a result of ferrite grain growth. Thereby, this study aimed to completely replace silicon by aluminum in the presence of vanadium in order to enhance the coatability of such grade of steel. Microstructure of the studied steel was examined by using optical microscope and FE-SEM. XRD analysis and tent-etching technique were performed to study the fractions and morphology of the retained austenite. Moreover, the influence of the selected parameters on the mechanical properties were investigated. Showcased results promoted the significant potential of aluminum and vanadium as a substituent to the conventional silicon-based TRIP steel. This substitution has a great influence on promoting the retained austenite stability and hence the mechanical performance of TRIP steel.

Replacement of silicon by aluminum with the aid of vanadium for galvanized TRIP steel

Continuous annealing simulation tests were conducted by using a continuous annealing thermomechanical simulator. Holding times of 5, 60, 180, and 480 seconds for an intercritical annealing temperature of 820 °C were adopted to investigate the evolution of the microstructure and mechanical properties of ferrite-bainite dual-phase steel. The ferrite-bainite dual-phase steel was characterized by high strength and low yield ratio due to the presence of the constituents (polygonal ferrite, bainite, martensite and retained austenite) of the steel microstructure. Specimen 3 exhibits the highest value of A50 (7.67%) and a product of Rm × A50 (10453MPa%) after a 180s holding. This is likely attributed to the presence of a C-enriched retained austenite in the microstructure. And the effect of martensite islands and carbide precipitate is thought to be able to contribute in strengthening the present steel. It is expected that equilibrium of austenite fraction would be reached for reasonable intercritical holding period, regardless of the heating temperature. The results suggest that long increasing holding times may not be needed because the major phase of the microstructure does not change very significantly. It is favorable for industrial production of DP steels to shorten holding times.

Effect of holding time on the microstructure and mechanical properties of dual-phase steel during intercritical annealing

Thermal simulations of Ti, Nb and V microalloyed steel were conducted using a thermomechanical simulator, and the microstructural evolution and the precipitation behavior during isothermal processing were analyzed. The results show that with increasing holding time, the microstructural constituents change from the martensite and bainitic ferrites to granular bainite and polygonal ferrite. The maximum hardness is obtained for the specimen after isothermal holding for 5 s due to the martensite strengthening effect. The hardness of the specimen decreases after isothermal holding for 10 s, because the strengthening contribution of fine dispersed precipitates becomes weaker. The hardness values of the specimens increase and then remain high after isothermal holding for 60 and 600 s. This is attributed to the contribution of the interphase precipitation hardening to the hardness of the studied steel. The precipitates in the specimen are coarsened after isothermal holding for 3600 s, even though the coarsening is not remarkable. These precipitates are fcc (Ti, Nb)(N, C) particles and belong to the MX-type precipitates. The beneficial effects of precipitation strengthening are lost. The hardness decreases to a minimum hardness value due to the presence of large amounts of polygonal ferrite after isothermal holding for 3600 s. Relatively coarse precipitates are the primary origin of the hardness decrease.

Microstructural features and precipitation behavior of Ti, Nb and V microalloyed steel during isothermal processing

The effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region was investigated using a Gleeble 1500 thermomechanical simulator and a continuous annealing thermomechanical simulator. The results showed that the microstructure consisted of ferrite and bainite as the main phases with a small amount of retained austenite and martensite islands at cooling rate of 5 and 50 °/s, respectively. Fast cooling after continuous annealing affected all constituents of the microstructure. The mechanical properties were improved considerably. Ultimate tensile strength (UTS) increased and total elongation (TEL) decreased with increasing cooling rate in all specimens. The specimen 1 at a cooling rate of 5 °/s exhibited the maximum TEL and UTS×TEL (20% and 27 200 MPa%, respectively) because of the competition between weakening by presence of the retained austenite plus the carbon indigence by carbide precipitation, and strengthening by martensitic islands and precipitation. The maximum UTS and YS (1 450 and 951 MPa, respectively) were obtained for specimen 2 at a cooling rate of 50 °/s. This is attributed to the effect of dispersion strengthening of finer martensite islands and the effect of precipitation strengthening of carbide precipitates.

Effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region

Effects of thermo-mechanical control processing (TMCP) on microstructure and mechanical properties of hot-rolled ultra-high strength ferrite-bainite dual phase steel were investigated on a laboratory hot rolling mill. The results have shown that the microstructure containing ferrite and bainite can be obtained by TMCP. Ultimate tensile strength of all the specimens exceeded 1000MPa. Finish rolling temperatures affect the mechanical properties of ultra-high strength ferrite-bainite dual phase steel. Ultimate tensile strength reached 1078MPa at relatively low finish rolling temperature because of the ferrite grains refined. Fast cooling after low temperature rolling results in the ferrite grains refined and the formation of martensite islands. As a result, the product of ultimate tensile strength and total elongation (Rm×A50) of specimen 4 with fast cooling after low temperature rolling reaches the maximum value (18096MPa%).

Study of Microstructure and Mechanical Properties of Hot-Rolled Ultra-High Strength Ferrite-Bainite Dual Phase Steel

Effect of controlled rolling and cooling process on the mechanical properties of low carbon cold forging steel was investigated for different processing parameters of a laboratory hot rolling mill. The results show that the specimens with fast cooling after hot rolling exhibit very good mechanical properties, and the improvement of the mechanical properties can be attributed mainly to the ferrite-grain refinement. The mechanical properties increase with decreasing final cooling temperature within the range from 670 °C to 570 °C due to the finer interlamellar spacing of pearlite colony. The specimen with fast cooling after low temperature rolling shows the highest values of the mechanical properties. The effect of the ferrite grain size on the mechanical properties was greater than that of pearlite morphology in the present study. The mechanical properties of specimens by controlled rolling and cooling process without thermal treatment were greatly superior to that of the same specimens by the conventional rolling, and their tensile strength reached 490 MPa grade even in the case of low temperature rolling without controlled rolling. It might be expected to realize the substitution medium-carbon by low-carbon for 490 MPa grade cold forging steel with controlled rolling and cooling process.

Zhuang Li

Papers

Effect of holding time on the microstructure and mechanical properties of dual-phase steel during intercritical annealing

Microstructural features and precipitation behavior of Ti, Nb and V microalloyed steel during isothermal processing

Effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region

Study of Microstructure and Mechanical Properties of Hot-Rolled Ultra-High Strength Ferrite-Bainite Dual Phase Steel

Controlled rolling and cooling process for low carbon cold forging steel