Showing papers on "TRIP steel published in 2009"

Third Generation of AHSS: Microstructure Design Concepts

Abstract: In recent years there has been an increased emphasis on the development of new advanced high strength sheet steels (AHSS), particularly for automotive applications. Descriptive terminology has evolved to describe the “First Generation” of AHSS, i.e. steels that possess primarily ferrite-based microstructures, and the “Second Generation” of AHSS, i.e. austenitic steels with high manganese contents which include steels that are closely related to austenitic stainless steels. First generation AHSS have been referred to by a variety of names including dual phase (DP), transformation induced plasticity (TRIP), complex-phase (CP), and martensitic (MART). Second generation austenitic AHSS include twinninginduced plasticity (TWIP) steels, Al-added lightweight steels with induced plasticity (L-IP®), and shear band strengthened steels (SIP steels). Recently there has been increased interest in the development of the “Third Generation” of AHSS, i.e. steels with strength-ductility combinations significantly better than exhibited by the first generation AHSS but at a cost significantly less than required for second generation AHSS. Approaches to the development of third generation AHSS will require unique alloy/microstructure combinations to achieve the desired properties. Results from a recent composite modeling analysis have shown that the third generation of AHSS will include materials with complex microstructures consisting of a high strength phase (e.g. ultra-fine grained ferrite, martensite, or bainite) and significant amounts of a constituent with substantial ductility and work hardening (e.g. austenite). In this paper, design methodologies based on considerations of fundamental strengthening mechanisms are presented and evaluated to assess the potential for developing new materials. Several processing routes will be assessed, including the recently identified Quenching & Partitioning (Q&P) process developed in the authors’ own laboratory.

Deformation behavior of TRIP and DP steels in tension at different temperatures over a wide range of strain rates

Abstract: The dependence of the mechanical behavior of DP and TRIP steels on temperature and strain rate is still not completely understood. Therefore, the mechanical properties of a DP 600 and a TRIP 700 steel were characterized by tensile tests over a temperature range of −100 ≤ T ≤ 235 °C at strain rates ranging from 10−3 to 1250 s−1. The results show that the strain hardening behavior of the TRIP steel depends strongly on the initial test temperature, to which the DP steel seems to be relatively insensitive in the studied temperature interval. On the other hand, the tensile strength of the TRIP steel appears to be much less sensitive to strain rate than that of the DP steel. This is explained by the effects of deformation induced heating on the martensite transformation based hardening of the TRIP steel, partially offsetting the direct effects of strain rate.

Hydrogen Delayed Fracture Properties and Internal Hydrogen Behavior of a Fe-18Mn-1.5Al-0.6C TWIP Steel

Abstract: The hydrogen delayed fracture (HDF) properties and internal hydrogen behavior were investigated in a Fe–18Mn–1.5Al–0.6C steel, a representative twinning induced plasticity (TWIP) aided steel. Slow strain rate tests (SSRT) were employed on both smooth and notched specimens to evaluate the effects of diffusible hydrogen on the HDF properties of the steel. Results showed that the fracture stress, fracture strain and time to fracture of the hydrogen pre-charged specimens were relatively insensitive to the amount of diffusible hydrogen. Fracture surface exhibited a ductile dimple fracture mode regardless of the diffusible hydrogen concentration. It was found that most hydrogen became non-diffusible after SSRT. The major trapping sites of hydrogen were dislocations, grain boundaries and twins. The activation energies for detrapping of hydrogen were estimated 35 kJ/mol for dislocations or grain boundaries, and 62 kJ/mol for twins. A comparison of the HDF properties of the present steel with those of other high strength steels revealed that the TWIP steel appeared to be relatively immune to hydrogen delayed fracture. This was due to the combined effects of (a) higher hydrogen solubility of austenite matrix (b) negligible portion of diffusible hydrogen to the total hydrogen, (c) decrease of diffusible hydrogen content during the deformation, and (d) no transformation of austenite to either e or α′ martensite.

Effects of annealing conditions on microstructure and mechanical properties of low carbon, manganese transformation-induced plasticity steel

Abstract: The effects of annealing conditions on microstructural evolution and mechanical properties have been investigated in low carbon, manganese TRIP (Mn TRIP) steel based on a 0.12C-6Mn-0.5Si-3Al alloy system. The microstructure of cold-rolled sheet subjected to annealing at 760 °C to 800 °C for 30 s to 1800 s consists of a recrystallized ferrite matrix and fine-grained austenite with a phase fraction of 25 % to 35 %. Variation of the annealing conditions remarkably influenced the characteristics of constituent phases and thus affected the tensile strength and elongation. Optimization of microstructural parameters such as grain size and fraction of constituent phases, which control the yield strength, overall work hardening, and the kinetics of strain-induced martensite formation, is thus critical for obtaining an exceptional mechanical balance of the alloy.

Cyclic Deformation of Advanced High-Strength Steels: Mechanical Behavior and Microstructural Analysis

Abstract: The fatigue properties of multiphase steels are an important consideration in the automotive industry. The different microstructural phases present in these steels can influence the strain life and cyclic stabilized strength of the material due to the way in which these phases accommodate the applied cyclic strain. Fully reversed strain-controlled low-cycle fatigue tests have been used to determine the mechanical fatigue performance of a dual-phase (DP) 590 and transformation-induced plasticity (TRIP) 780 steel, with transmission electron microscopy (TEM) used to examine the deformed microstructures. It is shown that the higher strain life and cyclic stabilized strength of the TRIP steel can be attributed to an increased yield strength. Despite the presence of significant levels of retained austenite in the TRIP steel, both steels exhibited similar cyclic softening behavior at a range of strain amplitudes due to comparable ferrite volume fractions and yielding characteristics. Both steels formed low-energy dislocation structures in the ferrite during cyclic straining.

Microstructure and Compression Strength of Novel TRIP‐Steel/Mg‐PSZ Composites

Abstract: Novel composites on basis of austenitic stainless TRIP-steel as matrix with reinforcements of Mg-PSZ are presented. Compact rods were produced by cold isostatic pressing and sintering, square honeycomb samples by the ceramic extrusion technique. The samples are characterized by optical and scanning electron microscopy before and after deformation, showing the microstructure and the deformation- induced martensite formation. The mechanical properties of samples with 5 vol% zirconia are superior compared to zirconia-free samples and composites with higher zirconia contents in terms of bending and compression tests. The honeycomb samples exhibit extraordinary high specific energy absorption in compression.

Energy-Absorbing TRIP-Steel/Mg-PSZ Composite Honeycomb Structures Based on Ceramic Extrusion at Room Temperature

Abstract: Porous materials have received extensive attention for energy absorption in the last few years. In terms of this study, austenitic TRIP-steel/Mg-PSZ-composite honeycomb structures are formed with different mixing proportions due to ceramic extrusion at room temperature. Their specific energy absorption SEA as well as their compression strength have been registered as a function of the compressive strain. X-ray diffractometry (XRD), electron backscatter diffraction (EBSD) as well as electron dispersive X-ray (EDX) analysis support the microstructure characterization. The zirconia addition has mainly contributed as a hard phase in a ductile TRIP steel matrix and has reinforced the composite material up to a compressive strain of about 24%.

Internal Oxidation during Intercritical Annealing of CMnSi TRIP Steel

Abstract: The equilibrium internal oxidation of CMnSi TRIP steel at intercritical annealing temperatures in a +3°C dew point N2+10%H2 atmosphere was investigated by means of high resolution transmission electron microscopy of cross-sectional samples The experimental conditions are considered to lead to the selective internal oxidation of Mn and Si The intercritical annealing however resulted in the formation of three types of isolated particles on the surface: 200–340 nm size single crystal MnO oxide particles, crystalline 30–60 nm size xMnO·SiO2 (1≤x≤4) and amorphous α-xMnO·SiO2 (0

Design of Composition in (Al/Si)-alloyed TRIP Steels

Abstract: There is an increasing interest in the progressive substitution of Si by Al in TRIP steels in order to obtain alloys with excellent mechanical properties and improved coatability. In this paper, thermodynamic calculations have been carried out with the help of JMatProTM software in order to assess and compare the effects that Si and Al additions exert on the phase transformation, carbon enrichment and alloying element content of phases during continuous galvanizing of multiphase steels. These simulations have provided important implications regarding the optimal combination of Si and Al. It has been found that Al causes a more pronounced increase of A3 temperature and a wider extension of the intercritical range than Si. For a constant volume fraction of phases, the carbon content in austenite is similar for Al and Si-alloyed steels. However, ferrite in Al-alloyed is richer in carbon and consequently an increase in its strength could be expected. The hardenability of intercritically annealed austenite has been estimated for alloys with different combinations of Mn, Al and Si. Finally, simulated CCT diagrams predict for Al-alloyed steels a higher amount of new ferrite formed during cooling from intercritical annealing and the need of shorter isothermal holding times at 460°C. However, Si-TRIP steels would need faster cooling rates to prevent pearlite formation and longer isothermal holding times to complete the bainitic transformation and to obtain a microstructure with high retained austenite.

Effect of testing temperature and strain rate on the transformation behaviour of retained austenite in low-alloyed multiphase steel

Abstract: The influence of temperature and strain rate on the evolution of the retained austenite content during plastic straining has been studied by tensile tests performed at 4 × 10−4, 10−2 and 0.4 s−1 between −100 and 450 °C. Up to 150 °C, the austenitic volume fraction mechanically transformed decreases with increasing testing temperature being almost independent of strain rate. In contrast, above 150 °C the martensite transformation depends on testing temperature and strain rate since it is related to the ability of carbon atoms for pinning dislocations.

Influence of Manufacturing Processes and Microstructures on the Performance and Manufacturability of Advanced High Strength Steels

Abstract: Advanced high strength steels (AHSS) are performance-based steel grades and their global material properties can be achieved with various steel chemistries and manufacturing processes, leading to various microstructures. In this paper, we investigate the influence of supplier variation and resulting microstructure difference on the overall mechanical properties as well as local formability behaviors of advanced high strength steels (AHSS). For this purpose, we first examined the basic material properties and the transformation kinetics of TRansformation Induced Plasticity (TRIP) 800 steels from three different suppliers under different testing temperatures. The experimental results show that there is a significant supplier (i.e., manufacturing process) dependency of the TRIP 800 steel mechanical and microstructure properties. Next, we examined the local formability of two commercial Dual Phase (DP) 980 steels during stamping process. The two commercial DP 980 steels also exhibit noticeably different formability during stamping process in the sense that one of them shows severe tendency for shear fracture. Microstructure-based finite element analyses are carried out next to simulate the localized deformation process with the two DP 980 microstructures, and the results suggest that the possible reason for the difference in formability lies in the morphology of the hard martensite phase in the DPmore » microstructure.« less

Method for displaying and quantitatively determining retained austenite or island martensite- austenite in TRIP steel

Abstract: The invention relates to a display and quantitative detection method of residual austenite or island martensite-austenite (MA island for short) in TRIP steel. The method comprises the steps: (1) a TRIP tapping sample is prepared into a metallographic specimen; (2) after grinding and polishing, the metallographic specimen is light etched for 5 to 8 seconds in Nital of 1 to 3 percent by weight, and water is sprinkled on a flannel polishing disk for 1 to 2 seconds; sodium pyrosulfite water solution of 1 to 3 percent by weight and picral of 2 to 6 percent by weight are mixed by volume ratio of 1:1 to prepare corrosive liquid, then the metallographic specimen is put into the corrosive liquid for etching of 55 to 65 seconds; (3) the structure of the TRIP steel is observed under a metallurgical microscopy: the delta ferrite is minus blue, the residual austenite or MA island is white, and bainite is brownish-black; and (4) each phase in the structure is carried out quantitative analysis. The method presents different colors, each phase of structure has distinct layers which are easy to distinguish; and a quantitative image analysis method is utilized to fix amount of the feature object structure, which is objective and accurate and has good repeatability.

Effect of isothermal bainitic transformation temperature on retained austenite fraction in C-Mn-Si-Al-Nb-Ti TRIP-type steel

Abstract: Purpose: The aim of the paper is to determine the influence of isothermal bainitic transformation temperature on a fraction of retained austenite for a new-developed C-Mn-Si-Al-Nb-Ti TRIP-type steel. Design/methodology/approach: The thermo-mechanical processing was realized in a multi-stage compression test by the use of the Gleeble 3800 thermomechanical simulator. The steel was subjected to six variants of processing with an isothermal bainitic transformation temperature in a range from 250 to 500°C. Identification of phase composition was achieved using microstructure observations and X-ray diffraction. To determine the fraction of retained austenite the Rietveld method was applied. Findings: The maximum fraction of retained austenite equal up to 16% can be obtained for the temperatures of isothermal bainitic transformation from 400 to 450°C, while the maximum carbon content in the γ phase equal 1.5 wt.% is present at the temperature of 350°C. Below 350°C due to high Ms temperature, the largest grains of retained austenite located in the ferritic matrix transform to marteniste. In a temperature range from 350 to 450°C, the Msγ temperature has a negative value, stabilizing the retained austenite. Research limitations/implications: To determine in detail the influence of isothermal bainitic transformation conditions on a fraction of retained austenite, the knowledge of the effect of isothermal holding time is also important. Practical implications: The obtained microstructures and especially retained austenite fraction dependent on an isothermal bainitic transformation temperature can be useful in optimization of thermo-mechanical processing conditions of C-Mn-Si-Al TRIP-type steels. Originality/value: Combined colour etching and X-ray diffraction methods were applied for microstructure identification of modern group of TRIP steels predicted to use in the automotive industry.

High-strength steel machined product and method for manufacturing the same, and method for manufacturing diesel engine fuel injection pipe and common rail

Abstract: A high-strength steel machined product giving excellent hardenability has a metal microstructure with excellent balance of strength and toughness and high stability of retained austenite. The product is composed of an ultra-high low-alloy TRIP steel having a metal microstructure which contains an appropriate quantity of two or more of Cr, Mo, and Ni, and an appropriate quantity of one or more of Nb, Ti, and V, and having an appropriate value of carbon equivalent; the metal microstructure has a mother-phase structure composed mainly of lathy bainitic ferrite with a small amount of granular bainitic ferrite and polygonal ferrite, and has a secondary-phase structure composed of fine retained austenite and martensite.

TRIP steel plate for high strength cold rolling hot dip galvanizing and preparation thereof

Abstract: The invention pertains to the metal material processing field and particularly provides a TRIP steel sheet used for high-strength cold rolling and hot galvanizing and a manufacturing method thereof. The TRIP steel sheet comprises the following components: 0.1wt percent to 0.4wt percent of C, not more than 0.06wt percent of Si, 0.5wt percent to 2.5wt percent of Mn, 0.5wt percent to 2.0wt percent of Al, not more than 0.03wt percent of P, not more than 0.02wt percent of S, 0.01wt percent to 0.10wt percent of Nb, 0.01wt percent to 0.10wt percent of Ti, 0.1wt percent to 1.0wt percent of Cu, 0.1wt percent to 0.6wt percent of Ni and the rest is iron. Through the measures of the control of the content of the elements such as C, Mn, Nb and Ti in the steel, and the temperature and time of hot rolling, reeling, cold rolling and annealing, etc., the comprehensive performance of the TRIP steel sheet is improved. Compared with the traditional transformation-induced plasticity (TRIP) steel, the TRIP steel sheet has the following advantages of: 1) good platability: due to the low silicon adopted, the requirements of the plating layer of hot plated pure zinc and alloy plated zinc can be met to obtain the plating layer with good performance; 2) good grain refining effect: due to Nb micro-alloying, grain can be effectively refined.

Rough Set Approach to Predict the Strength and Ductility of TRIP Steel

Abstract: Transformation Induced Plasticity (TRIP) gives birth to new generation steels with high strength and good ductility. Both these properties of steel depend on a number of compositional and processing parameters, but till date there exist certain gaps in the understanding of the complex role of each parameters on the microstructure and thus the properties of the steel. Rough Set Theory is employed to derive decision rules that attempt to explain this complex behavior. Applying efficient heuristics, the number of attributes are reduced to form a minimal reduct, and their values are at the same time discretized into linguistic intervals. The derived rules could clearly indicate on the relative importance of the compositional and processing variables.

Effect of alloying elements and coiling temperature on the recrystallization behavior and the bainitic transformation in TRIP steels

Abstract: The effects of alloying elements and coiling temperature on recrystallization behavior and bainitic transformation were investigated based on 0.07C-Mn-Cr-Nb steel with a low carbon equivalent. Based on the ferrite recrystallization behavior, the proper intercritical annealing temperature of all studied steels was suggested to produce TRIP steel with good strength and elongation balance. All steels coiled at 550 °C showed much faster ferrite recrystallization behavior than steels coiled at 700 °C. In addition to the coiling temperature, the effect of increasing carbon content on the ferrite recrystallization was minor at a coiling temperature of 550 °C, but much more prominent at a coiling temperature of 700 °C. The highest Mo added steel showed the best strength and elongation balance, and the highest carbon and Mo added steel showed the highest tensile strength at a coiling temperature of 550 °C. The steel containing a higher amount of elemental Al (0.7 wt.% Al) exhibited much better elongation than the lower Al added steel (0.04 wt.% Al) in TS 780 MPa grade, about 24 % and 19 %, respectively.

Research and development of 780 MPa cold rolling TRIP-aided steel

Abstract: As an industry-university cooperative project, an experimental research was conducted to develop a new cold-rolled transformation-induced plasticity (TRIP) steel with a tensile strength of 780 MPa and an elongation of 25% under the conditions that some processing parameters were preestablished according to the actual conditions of factory production lines. The optimal heat treatment conditions for obtaining excellent strength-ductility balance, specifically in intercritical annealing temperature and isothermal treatment temperature, were investigated by means of thermal simulation tests. Ultimately the desirable mechanical properties were attained successfully, and the microstructure and retained austenite stability were studied by optical microscopy (OM) and electron back-scattered diffraction (EBSD). It has been discovered that the sizes of retained austenite grains are generally proportional to the sizes of carbon-clear space (ferrite and bainite) around them, and there is marked selectivity in their transition sequences under stress-strain.

Review and prospect of high strength low alloy trip steel

Abstract: Research status of high strength low alloy TRIP (transformation induced plasticity)steels for automobile structural parts is briefly described. Composition and microstructure factors especially the morphology, size and volume fraction of retained austenite,which largely influence the strength and ductility of the steel, are reviewed and discussed one after another. Modelling of the inter-critical annealing and martempering processes as well as the designing of the TRIP steel aided by commercial software are introduced. Some special aspects of the dynamic mechanical properties of TRIP steel are firstly reported.

Effect of Bake-Hardening on the Structure-Property Relationship of Multiphase Steels for the Automotive Industry

Abstract: The effect of a bake-hardening (BH) treatment on the microstructure and mechanical properties has been studied in C-Mn-Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X-ray diffraction, transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). All steels showed high BH response. however, the DP and trip steels after IA/BH showed the appearance of upper and lower yield points, while the stress-strain behavior of the trip steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the "as-quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.

Metallurgical Phenomena during Processing of Cold Rolled TRIP Steel

Abstract: Metallurgical phenomena taking place during processing of TRIP Steel are investigated and described with the aim of achieving better understanding of the microstructure development throughout the entire integrated processing routes. Different TRIP steel structure sizes were created by controlling the hot rolling process prior to cold rolling. After that the specimens were intercritically annealed under different conditions to obtain prescribed austenite fractions, and subsequently quenched in salt bath at the bainite transformation temperature. The microstructures had been investigated using light optical microscopy (LOM) and the amount of retained austenite was determined by magnetometry.