Ultra-high Strength Dual-phase Steel Research Articles

Research on strain aging behavior of ultra-high strength dual-phase steel

The bake hardening value is one of the vital strength indexes of dual-phase steel, representing the strengthening ability of materials after pre-strain and baking, playing an important role in vehicle safety and lightweight design. Studying and improving the strain aging mechanism of dual-phase steel helps one to understand the material characteristics and enhances its utilization value. However, the ultra-high strength dual-phase steel is often prone to fracture outside the gauge length of a tensile specimen of the bake hardening value test. No suitable theory explains the fundamental law of dislocation pinning during the saturation stage at present. This paper used FEA, DIC, SEM, TEM, internal friction, and metallographic methods to study the strain aging behavior of dual-phase steels under different pre-strain, bake time, and bake temperature conditions. The results show that the fracture outside the gauge length is related to factors such as the uneven distribution of pre-strain and the ultra-high upper yield strength. The rolling pin shape tensile specimen testing has successfully solved this testing problem. The measured results at the saturation stage of dislocation pinning are in good agreement with the fitting results of the dislocation pinning strengthen mechanism based on the probability event quantization assumption.

Towards ultra-high strength dual-phase steel with excellent damage tolerance: The effect of martensite volume fraction

Ultra-high strength (> 1 GPa) dual-phase (DP) steels have been extensively investigated in literature. Yet, the damage tolerance of these DP steels remains mostly unexplored, whereas this parameter is crucial for the sheet formability and anti-crushing performance. In this work, a medium-Mn DP steel was proposed in order to develop strong but damage-resistant DP steel by refining the microstructure. Ultra-fine grained (< 1 µm) DP steels were obtained by a simple cold rolling and intercritical annealing process. The developed DP steel can achieve an outstanding ultimate tensile strength (UTS) of over 1470 MPa with appreciable elongation to failure (EtF) of over 9 % when the martensite volume fraction (Vm) is 55 %. Higher Vm results in higher strength. However, the fracture strain, which is connected with damage tolerance, decreases significantly with increasing Vm, and it is even much lower than that of a full-martensite steel. Furthermore, the necking behaviour transfers from diffuse plus localized necking to diffuse necking with increasing Vm. The sufficient ferrite content in the proposed DP1470 steel enables void growth after nucleation, consequently leading to the occurrence of localized necking and higher fracture strain. Thus, a moderate Vm ranging from 45 % to 65 % is suggested for the development of a 1470 MPa grade DP steel with excellent damage tolerance.

A novel approach to producing architectured ultra-high strength dual phase steels

An ultrafine-grained dual phase (DP) microstructure has been produced in a steel that contains a combination of composition and deformation gradients. A carbon gradient was created by controlled decarburization of the steel. The compositionally graded steel was cold rolled to produce a large number of shear bands prior to recrystallization annealing at 650 °C for 5 h followed by intercritical annealing at 740 °C for 10 min. Microstructural characterization reveals a very fine heterogeneous dual phase microstructure with submicron grains appearing along the traces of the prior shear bands that had developed during rolling. Martensite volume fraction changes from the surface to the center and also within these shear band traces. Tensile tests of the new dual phase structure reveal that the work hardening rate changes spatially with martensite volume fraction. Detailed fractography using ex-situ X-ray computed tomography (XCT) reveals a unique form of multiple cup and cone fracture features that can be related to the evolution of damage along the shear band traces with a gradient of void density from the surface to the center. This strategy for fabricating dual phase steels enables the development of architectured ultra-high strength DP steels with different properties that can be tailored by controlling the composition and deformation gradients and subsequent annealing treatments.

Investigation of the critical factors controlling sheared edge stretching of ultra-high strength dual-phase steels

The present study aimed to explore dual-phase (DP) steels with a good combination of high strength and reasonable global ductility (i.e., total elongation and general stretch formability) and local ductility (i.e., sheared edge ductility or hole expansion ratio). Therefore, a series of ultra-high strength dual-phase steels were designed, melted, rolled, annealed and formed. These steels contained various aluminum additions and vanadium contents and were processed with different coiling temperatures and continuous galvanizing line (CGL) thermal path simulations conducted using a Gleeble 3800 system. The microstructures, tensile properties and hole expansion behaviors of all candidate DP steels were determined and compared. The microstructural and damage evolutions in the process of both hole punching and hole expansion were examined. The results indicated that hole expansion ratios of DP steels could be correlated well with (i) the burnished-to-fracture zone ratios in shear surfaces after hole punching, (ii) the values of reduction in area of tensile specimens after fracturing, and (iii) nanohardness difference between soft ferrite and hard constituents. The micro-voids and micro-cracks introduced by hole punching acted as crack initiation sites, which severely affected the subsequent hole expansion process. Therefore, better sheared edge ductility may benefit from microstructures that retard the crack propagation or void growth and coalescence during hole expansion.

Study on Bending Beam Delayed Cracking of Ultra High-strength DP Steel for Automotive Use

Automobile lightweight project can effectively reduce energy consumption and exhaust emission, but it will reduce automobile safety. Therefore, using a large number of high-strength or ultra-high-strength steels in automobiles is an effective way to give consideration to both lightweight and safety. Delayed cracking is a symptom-free cracking phenomenon, which is extremely destructive. It is necessary to evaluate the delayed cracking performance of ultra-high strength steel before it is used. In this paper, the delayed cracking properties of three ultra-high strength dual-phase steels DP780, DP980 and DP1180 for automobile are studied by bending beam delayed cracking test. The experimental results show that the delayed cracking resistance of DP780, DP980 and DP1180 steel becomes worse in turn, and DP1180 steel has the worst delayed cracking resistance. In addition, the content and morphology of martensite will affect the delayed cracking performance. The morphology of martensite in the tested steel is lathy, and the higher the martensite content, the worse the delayed cracking resistance of the steel.

Role of Interface between Ferrite and Martensite in Hydrogen Embrittlement Behavior of Ultra-high Strength Dual-phase Steel Sheets

Role of Interface between Ferrite and Martensite in Hydrogen Embrittlement Behavior of Ultra-high Strength Dual-phase Steel Sheets

Study of Sheared Edge Formability of Ultra-High Strength DP980 Sheet Metal Blanks

Advanced high strength steels (AHSS) and ultra-high strength steels (UHSS) have been increasingly implemented by the automotive industry for better crashworthiness and fuel economy. However, these steels are often sensitive to the trimmed edge cracking. The objective of the present paper is to study the sheared edge of ultra-high strength dual-phase steel, DP980, in mechanical trimming and hole punching by sheared edge quality assessment, stretchability, and hole expansion tests as well as finite element analysis. Furthermore, the mechanism of fracture propagation in trimming and hole punching processes of DP980 was discussed. Rather a unique fracture mechanism was observed for trimming of DP980 steel leading to the burr removal at the final stage of the trimming process. Finite element analysis revealed that, under very large clearances, a secondary crack initiates from the edge of the lower tool, and the primary propagated crack turns toward it simultaneously. Intersecting of these two cracks leads to the total separation and leaves the edge of the trimmed part with a broken burr. Fracture observation of trimmed specimens revealed that crack initiation sites under tension moved from the middle of the trimmed surface toward the burr tip with increasing the clearance. This study demonstrates the importance of stretchability tests for designing the stamping dies as well as a reliable finite element simulation for characterizing the material behavior during the shearing process.

Optimization of the Continuous Galvanizing Heat Treatment Process in Ultra-High Strength Dual Phase Steels Using a Multivariate Model

The main process variables to produce galvanized dual phase (DP) steel sheets in continuous galvanizing lines are time and temperature of intercritical austenitizing (tIA and TIA), cooling rate (CR1) after intercritical austenitizing, holding time at the galvanizing temperature (tG) and finally the cooling rate (CR2) to room temperature. In this research work, the effects of CR1, tG and CR2 on the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of cold rolled low carbon steel were investigated by applying an experimental central composite design and a multivariate regression model. A multi-objective optimization and the Pareto Front were used for the optimization of the continuous galvanizing heat treatments. Typical thermal cycles applied for the production of continuous galvanized AHSS-DP strips were simulated in a quenching dilatometer using miniature tensile specimens. The experimental results of UTS, YS and EL were used to fit the multivariate regression model for the prediction of these mechanical properties from the processing parameters (CR1, tG and CR2). In general, the results show that the proposed multivariate model correctly predicts the mechanical properties of UTS, YS and %EL for DP steels processed under continuous galvanizing conditions. Furthermore, it is demonstrated that the phase transformations that take place during the optimized tG (galvanizing time) play a dominant role in determining the values of the mechanical properties of the DP steel. The production of hot-dip galvanized DP steels with a minimum tensile strength of 1100 MPa is possible by applying the proposed methodology. The results provide important scientific and technological knowledge about the annealing/galvanizing thermal cycle effects on the microstructure and mechanical properties of DP steels.

Hydrogen Embrittlement Behavior of Ultra-high Strength Dual Phase Steel Sheet under Sustained Tensile-loading Test

Hydrogen Embrittlement Behavior of Ultra-high Strength Dual Phase Steel Sheet under Sustained Tensile-loading Test

Microstructures, Mechanical Properties, and Strain Hardening Behavior of an Ultrahigh Strength Dual Phase Steel Developed by Intercritical Annealing of Cold-Rolled Ferrite/Martensite

A dual phase (DP) steel was produced by a new process utilizing an uncommon cold-rolling and subsequent intercritical annealing of a martensite–ferrite duplex starting structure. Ultrafine grained DP steels with an average grain size of about 2 μm and chain-networked martensite islands were achieved by short intercritical annealing of the 80 pct cold-rolled duplex microstructure. The strength of the low carbon steel with the new DP microstructure was reached about 1300 MPa (140 pct higher than that of the as-received state, e.g., 540 MPa), without loss of ductility. Tensile testing revealed good strength–elongation balance for the new DP steels (UTS × UE ≈ 11,000 to 15,000 MPa pct) in comparison with the previous works and commercially used high strength DP steels. Two strain hardening stages with comparable exponents were observed in the Holloman analysis of all DP steels. The variations of hardness, strength, elongation, and strain hardening behavior of the specimens with thermomechanical parameters were correlated to microstructural features.

A novel route for development of ultrahigh strength dual phase steels

Abstract Dual phase (DP) steels were produced by a new approach utilizing simple cold-rolling and subsequent intercritical annealing of a ferrite–martensite duplex starting structure. The effects of intercritical annealing temperature on the microstructural evolutions and mechanical properties were studied. It was found that the volume fraction of martensite increased by increasing the intercritical annealing temperature. Tensile testing showed a good strength–elongation balance for DP steels (UTS×UE>100 J cm −3 ) in comparison with the commercially used high strength steels. The strength of the low carbon steel with the new ultrafine grained DP microstructure (average grain size of about 1–2 μm) was reached over 1500 MPa (about 200% higher than that of the as-received state, e.g. 540 MPa), without loss of ductility. The variations of hardness, strength, elongation, strain hardening behavior and fracture mechanism of the specimens with intercritical annealing temperature were correlated to microstructural features.

Microstructure, mechanical properties and fracture behavior of ultra-high strength dual-phase steel

Abstract An ultra-high strength dual-phase steel with main components 0.16C–1.38Si–3.20Mn was produced by cold rolling followed by intercritical heat treatment. Influences of different annealing temperatures on the mechanical properties were investigated, focusing on the development of localized deformation as well as crack initiation and propagation. The work hardening behavior of tested steels after annealing was analyzed in terms of the modified C–J analysis. The experimental results indicate that the microstructures of all the tested steels contain ferrite and martensite. Annealing at 800 °C, the best comprehensive performance with a yield strength of 873 MPa, tensile strength of 1483 MPa, total elongation of 11% and yield ratio of 0.58 was achieved. Besides, the mechanisms of damage taking place in the ferrite and crack initiating at the interface between the ferrite and martensite have been observed. The crack initiating perpendicular to the lath of the martensite and then following at or near the narrowest part of a martensite ligament have also been observed. The original crack propagation exhibits a transgranular fracture feature.

The Effects of Thermomechanical Processing on the Microstructure and Mechanical Properties of Ultra-High Strength Dual Phase Steel

In this paper, ultra-high strength dual phase steel was investigated. Thermomechanical processing was conducted by using a laboratory hot rolling mill. The results have shown that the main transformation products at three different kinds of thermomechanical processing were ferrite, bainite, and small amounts of martensite. Laminar cooling led to ferrite grain refinement. The mechanical properties of specimen 1 which was controlled cooling after a relative lower temperature rolling are much higher than that of specimen 2. The presence of martensite islands and precipitates contributed to the enhancement of strength of the present steel. And the presence of retained austenite resulted in higher toughness. As a result, these specimens exhibited satisfactory mechanical properties.

Research on Austenite Static Recrystallization Kinetics of Ultra-High Strength Dual Phase Steel

In this paper, double deformation test was conducted in a MMS-300 thermomechanical simulator. Austenite static recrystallization kinetics of ultra-high strength dual phase steel was investigated. The results have shown that the softening fractions increase with holding time as a whole. Under deformation temperatures of 950°C, 1000°C and 1050°C, the softening fractions increase with the increase of the deformation temperature. And the softening curve appears relatively flat under deformation temperatures of 950°C. The austenite no-recrystallization temperature (Tnr) of ultra-high strength dual phase steel was between 900°C and 950°C. Double deformation test conditions which promoted Nb(CN) precipitation resulted in a decrease of a austenite recrystallization. Strain induced precipitation of Nb (CN) was the main reason why the softening curve appeared a plateau.

Microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile test. The results show that Si can promote the transformation of austenite (γ) to ferrite (α), enlarge the (α+γ) region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite (M/A) constituents, as well as the decomposition of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.

Design and Characterization of Ultrahigh Strength Dual-Phase Steel with Low Ratio of Yield Strength/Ultimate Tensile Strength

An ultrahigh strength dual-phase (DP) steel with low ratio of yield strength/ultimate tensile strength (YS/UTS) was designed based on the simulation using JmatPro software so as to improve formability as well as to extend its application in automobile industry. Results show the DP steel suffered from water quenching (WQ) technology exhibits high ratio, 0.872, of YS/UTS, which loses the advantage of formability of DP steels and restricts its application in automobile industry. Therefore, the controlled slow-cooling rate (CSCR) technology is employed to this DP steel, and the low ratio, 0.458, of YS/UTS is obtained. Although the tensile strengths of the DP steel suffered from two kinds of technologies are over 1000 MPa, The YS of the DP steel with CSCR technology is 480 MPa and is much lower than 983MPa of the DP steel with WQ technology, which are attributed to relative large grains and small volume fraction of martensite in the former based on the characterization of microstructure by optical microscope, scanning electron microscope, transmission electron microscope and electron backscattering diffraction.