TRIP-aided Steel Sheets Research Articles

Effect of Thermomechanical Rolling on Mechanical Properties of TRIP-Aided Steel Sheet

To attain the aim of weight reduction and safety improvement of vehicles, some high strength steel sheets have been developed and investigated. TRIP-aided steel sheets with transformation-induced plasticity (TRIP) of the retained austenite have high strength and ductility, and excellent hydrogen embrittlement resistance. In previous study, as high strength TRIP-aided steel for forging parts, the volume fraction of retained austenite in the TRIP-aided steel could be increased by hot forging with austempering. Similarly, our research group reported that the thermomechanical process of hot rolling following by austempering could also increase the amount of retained austenite in the TRIP-aided steel sheet. The tensile properties and formabilities of TRIP-aided steel sheet subjected to the thermomechanical rolling just before austempering possess obvious advantages compared with those of TRIP-aided steel sheet without thermomechanical rolling process (with only austempering). These excellent mechanical properties may be caused by the finely dispersed retained austenite and refined bainitic ferrite and/or martensite brock by thermomechanical rolling process.

V-Bendability of Ultrahigh-Strength Low Alloy TRIP-Aided Steel Sheets with Bainitic Ferrite Matrix

V-Bendability of Ultrahigh-Strength Low Alloy TRIP-Aided Steel Sheets with Bainitic Ferrite Matrix

Thermal Stability of Retained Austenite in Advanced TRIP Steel with Bainitic Ferrite Matrix for Automotive Industries

Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Microstructure and Tensile Properties of TRIP-Aided Steel Sheet Subjected to Hot-Rolling and Austempering

Various high strength steel sheets for weight reduction and safety improvement of vehicles have been developed. TRIP-aided steel with transformation induced plasticity of the retained austenite has high strength and ductility. Conventional TRIP-aided steels are subjected to austempering process after austenitizing. Generally, elongation and formability of TRIP-aided steel are improved by finely dispersed retained austenite in BCC phase matrix. The finely dispersed retained austenite and grain refinement of TRIP-aided steel can be achieved by hot rolling with heat treatment. Therefore, the improvement of mechanical properties of TRIP-aided steel is expected from the manufacturing process with hot rolling and then isothermal transformation process. In this study, thermomechanical heat treatment is performed by combining hot rolling and isothermal holding as the manufacturing process of TRIP-aided steel sheets. The complex phase matrix is obtained by hot rolling and then isothermal holding. Although the hardness of the hot rolled and isothermal held TRIP-aided steel is decreased, the volume fraction of retained austenite is increased.

Effect of heating rate on microstructure and mechanical properties of TRIP-aided multiphase steel

Optimization of high strength and high formability of multiphase cold rolled TRIP-aided steel sheet is based on the continuous annealing conditions. The influence of heating rate on the microstructure and mechanical properties of TRIP steel was investigated with Gleeble-3500 and a thermal system using resistance heating method. Experiments with different heating rates (5 and 500K/s) and annealing temperatures from 883K to 1223K were applied to study effects on the progress of austenitization and recrystallization kinetics. Different initial heating rates were applied to examine the influence of heating rate on the mechanical properties of TRIP steel. It appeared that the final tensile strength of the TRIP steel increased with increasing heating rate held at 1073K. The phase-transformation kinetics during the intercritical annealing was investigated by means of dilatometry. The results indicated that samples with higher heating rates have a much higher austenite volume fraction for a given annealing time of 80s held at 1073K. In addition, the higher heating rate results in a larger volume fraction of retained austenite (RA) and lower retained austenite carbon content in comparison to the lower heating rate after held at the same intercritical temperature. The morphology of RA under different annealing conditions was also tentatively discussed.

Microstructure and Mechanical Properties of Cold-Rolled TRIP Steel Sheets

Transformation induced plasticity (TRIP) steels have complex multiphase microstructure composed of ferrite, bainite and retained austenite [1]. These metastable retained austenite can transforms into martensite during plastic deformation, which generates a TRIP effect resulting in excellent combination of high strength and ductility even at high strength level [2-5]. For this reason, the TRIP-aided steel sheets are suitable to fabricate automobile parts, as they can offer excellent formability without sacrifice the strength and safety requirement of the steel sheets. As a result, the development of TRIP-aided steels has been a very important issue in the automobile field.

Stretch-flangeability of Ultra High-strength Low Alloy TRIP-aided Sheet Steels with Mixed Structure Matrix of Bainitic Ferrite and Martensite

The microstructure, retained austenite characteristics, tensile properties and stretch-flangeability of ultra high-strength 0.2%C–1.5%Si–1.5%Mn (mass%) TRIP-aided bainitic ferrite cold-rolled sheet steel, “TBF steel”, were investigated for automotive applications. When isothermally held at temperatures less than martensite-start temperature for 300–3000 s after annealing or austenitizing, the TBF steel possessed mixed matrix structure of bainitic ferrite and martensite, with retained austenite of about 4 vol%. The TBF steel achieved the tensile strength higher than 1400 MPa and hole-expanding ratio of 40%. The good combination of tensile strength and hole-expanding ratio was mainly caused by highly carbon–enriched retained austenite and softened matrix structure composing of bainitic ferrite and martensite.

Formability of Al–Nb Bearing Ultra High-strength TRIP-aided Sheet Steels with Bainitic Ferrite and/or Martensite Matrix

Microstructure, tensile properties and stretch-flangeability of 980–1470 MPa grade Al or Al–Nb bearing TRIP-aided cold-rolled sheet steels with bainitic ferrite and/or martensite matrix microstructure (TBF steels) were investigated for automotive applications such as impact member reinforcements, sheet flames and so on. In addition, these properties were related with the microstructure and the retained austenite characteristics. Complex additions of 0.5% Al and 0.05% Nb into a base steel with chemical composition of 0.2% C, 1.5% Si and 1.5% Mn (in mass%) significantly enhanced the total elongation and stretch-flangeability, especially when austempered at temperatures below martensite-start temperature. The excellent stretch-flangeability was primarily associated with (i) refined prior austenitic grain by NbC precipitates and (ii) uniform fine mixed matrix microstructure of bainitic ferrite and martensite, as well as (iii) TRIP effect of metastable retained austenite.

Effect of YAG Laser Cutting on Stretch-flangeability of Ultra High Strength TRIP Sheet Steel with Bainitic Ferrite Matrix

The effect of YAG laser cutting on stretch-flangeability of ultra high strength TRIP-aided steel sheets with bainitic ferrite matrix (TBF steel sheet) austempered at 375 or 450°C, was investigated for automotive applications. Holes of 5 mm diameter for hole-expanding test were produced by YAG laser cutting and mechanical punching and the stretch-flangeability was evaluated by measurement of the hole-expanding ratio (λ).In TBF steel sheet, laser cut specimens show higher stretch-flangeability than mechanically punched specimens. The hole-expanding ratio (λ) of TBF steel sheet austempered at 375°C in the case of laser cutting at powers between 50 and 100 W was higher than those austempered at 450°C. Furthermore, the strength–stretch-flangeability balance (TS×λ) of TBF steel sheet austempered at 375°C showed the highest value after laser cutting at 100 W. Compared to mechanical punching, YAG laser cutting contributed to the improvement of the TS×λ to 1100 MPa with TBF steel sheet possessing fine bainitic ferrite matrix.

Microstructure and mechanical behavior of cold-rolled CMnAlSi TRIP-aided steel sheets

The microstructure, mechanical properties and retained austenite stability of CMnAlSi-TRIP steels were investigated. Optical microscopy and X-ray diffractometry were used to analyze the microstructure, volume fraction and carbon concentration of retained austenite, while the tensile tests were performed to study the retained austenite stability against strain-induced martensitic transformation. The results show that the mechanical properties of the CMnAlSi cold-rolled TRIP-aided steels fabricated by proper heat treatment display good combination of strength and ductility after isothermal bainitic transformation (IBT) treatment at 420–450 °C for 60–120 s. The austenite transformation kinetics is influenced significantly by the IBT temperature, and the high mechanical stability of retained austenite, governed by its carbon concentration, and determines high strain hardening during deformation.

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.

Effect of cold rolling reduction and intercritical annealing temperature on the bulk texture of two TRIP-aided steel sheets

High strength TRIP-aided steel sheets with high formability and better ductility are of industrial interest. Texture control and retained austenite characterization are considered as the main factors with respect to the formability and ductility. Processing parameter such as initial conditions of hot rolling, cold rolling to final size of sheet, intercritical annealing after cold rolling and subsequent isothermal heat treatment to achieve TRIP specification in product, affect bulk texture of sheets. Moreover, chemical composition of steel should be noted especially because it can change the intensity of orientation and final macro texture of sheet. In this work, the effect of cold rolling and intercritical annealing on texture development has been investigated for two TRIP-aided steel sheets, which are different in C, Si and Al content. Results show that the cold rolling extends the γ-fiber on these grades of steel, whereas any intensive α-fiber components were not observed by change of cold rolling reduction. Also the effect of cold rolling on final texture of experimented materials is different and depends on chemical composition of steel. In contrary, intercritically annealing of cold rolled sheets decreases the intensity of γ-fiber so that the chemical composition of TRIP steel would influence effectively.

Formability of C-Si-Mn-Al-Nb-Mo Ultra High-strength TRIP-aided Sheet Steels

Formable 980–1470 MPa grade ultra high-strength TRIP-aided sheet steels with bainitic ferrite matrix (TBF steels) have been developed for cold forming of automotive applications. Complex additions of 1.0%Al–0.05%Nb–0.2%Mo to 0.2%C–1.5%Si–1.5%Mn base steel increased both the volume fraction and carbon concentration of retained austenite films of the TBF steel, with refining of microstructure. Extremely much retained austenite was realized in the TBF steel austempered at 450–500°C, corresponding to hot-dip galvanization just after annealing. Good combinations of strength and elongation and strength and stretch-flangeability were achieved when the TBF steel was austempered at 450–500°C and 425–475°C, respectively. These good combinations were caused by fine lath structure and a large amount of metastable retained austenite film. It is noteworthy that the TBF steel exhibited an excellent combination of total elongation and stretch-flangeability when austempered at 325–350°C lower than MS.

Formability of Nb bearing ultra high-strength TRIP-aided sheet steels

Microstructure, tensile property and formability of formable ultra high-strength 0.2% C–1.5% Si–1.5% Mn–0.02–0.11% Nb TRIP-aided sheet steels with bainitic ferrite matrix (TBF steels) were investigated for automotive applications such as impact member reinforcements, seat frames etc. Nb addition of 0.08% achieved the best combination of total elongation and stretch-flangeability, accompanied with an increase in peak austempering temperature for total elongation. The excellent total elongation and stretch-flangeability were primarily associated with microstructure of the matrix, retained austenite characteristics, morphology of matrix and retained austenite, etc. It was expected that hot-dip galvanizing just after continuous annealing can be realized in the 0.08% Nb TBF steels.

Effects of Nitrogen on the Mechanical Properties of Cold Rolled TRIP-aided Steel Sheets

Tensile tests were conducted in order to study the effects of nitrogen on the mechanical properties of the 0.2C–1.5Mn–1.5Si–0.04Al–(0.003–0.015)N steels often annealing at 800–830°C, followed by the austempering at 400–450°C. The results show that both tensile strength and elongation increase and the balance of tensile strength×elongation are improved upon nitrogen addition to the TRIP steel. It was found that the density of AlN precipitates increases with addition of nitrogen. The volume fraction of retained austenite increases due to AlN precipitation, because the precipitates retard the transformation of austenite during cooling and austempering. As a larger volume fraction of austenite remained after annealing has been transformed to martensite during deformation, elongation as well as strength has increased in the nitrogen added steel. It is also observed that the average grain size of ferrite and bainite decreases because of AlN precipitation that hinders grain growth. The refinement of ferrite and bainite by AlN precipitates also contributes to the increase in the strength of nitrogen added steels.

Development of Cold Rolling and Intercritical Annealing Texture in Two TRIP-Aided Steels

High strength TRIP-aided steel sheets with high formability and better ductility are of industrial interest. Texture control and retained austenite characterization are considered as the main factors with respect to the formability and ductility. In this work, the effect of cold rolling and intercritical annealing on texture development has been investigated for two TRIP-aided steel sheets, which are different in Si and Al content. Experiments show that the cold rolling extends the a and g–fibers on these grades of steel. Intercretically annealing decreases the intensity of a–fiber and leads to sharpness of g–fiber, especially for Al steel.

超高強度低合金TRIP型ベイニティックフェライト鋼板の温間成形性

超高強度低合金TRIP型ベイニティックフェライト鋼板の温間成形性

Bake-hardening behavior of cold-rolled CMnSi and CMnSiCu TRIP-aided steel sheets

The aim of this paper is to investigate the bake-hardening (BH) behavior of cold-rolled 0.2C–1.5Mn–1.5Si and 0.2C–1.5Mn–1.5Si–0.5Cu TRIP-aided steels. By analyzing the variation of the mechanical properties and microstructure of the two steels during paint baking, it is found that the BH values of both steels are about 25MPa, and that some retained austenite changed into bainite during paint baking, copper addition has less influence on the BH value although, it leads to the increase of tensile strength.

Structure–properties relationship in TRIP steels containing carbide-free bainite

The purpose of the present contribution is to review the current knowledge about the relationship between the micro-structure of cold rolled intercritically annealed low alloy TRIP-aided sheet steels and their mechanical properties from a materials engineering point of view. The focus is on their production in existing industrial lines and on their application in the manufacture of passenger cars with a body-in-white which offers an improved passive safety. The review aims to make clear that although low alloy TRIP-aided sheet steel is by now starting to be an established structural material in BIW manufacturing, there is still room for the further optimization of the composition and the processing. In addition, there are still a number of problems related to their physical metallurgy that require a better fundamental understanding.

Ductility of 0.1-0.6C-1.5Si-1.5Mn Ultra High-strength TRIP-aided Sheet Steels with Bainitic Ferrite Matrix

The effects of heat treatment and forming conditions on retained austenite characteristics and ductility of 0.1-0.6C-1.5Si-1.5Mn, mass%, ultra high-strength TRIP-aided sheet steels with bainitic ferrite matrix were investigated. These steels possessed large total elongations of about 20-25 % in a tensile strength ranging from 700 to 1 300 MPa when austempered at temperatures above martensite-start temperature (M S ). The total elongations were enhanced by warm forming at two temperatures, T P 1 and T P 2 . The first peak forming temperatures T P 1 s were between 0°C and 75°C and were nearly constant regardless of carbon content of the steels. This was associated with the strain-induced martensite transformation of a large amount of metastable retained austenite which suppressed a rapid fall of strain-hardening rate in an early strain range to resultantly increase the uniform and total elongations. On the other hand, the second peak forming temperatures T P 2 s were between 200 and 300°C and further large total elongations beyond 30% were achieved in high carbon steels (0.4% C and 0.6% C steels) with tensile strength of 1 300-1 500 MPa. The large improvement was controlled by both the strain-induced bainite transformation and dynamic strain aging.