Strength Steel Sheets Research Articles

Effect of Stress Relaxation on Springback of Steel Sheet in Warm Forming

Springback of a high strength steel (HSS) sheet of 980 MPa grade was investigated at elevated temperatures ranging from room temperature to 973 K. From U-and V-bending experiments it was found that springback was decreased with increasing temperature at temperatures of above 573 K. Furthermore, springback was decreased with punch-holding time because of stress relaxation. In this work, the stress relaxation behavior of the steel was experimentally measured. By using an elasto-vicoplasticity model, the stress relaxation was described, and its effect on the springback of sheet metals in warm forming was discussed theoretically.

Fracture Prediction for High-strength Steel Sheet Subjected to Draw-bending Using Forming Limit Stress Criterion

A fracture criterion for sheet metals subjected to draw-bending is investigated using the concept of the forming limit stress criterion. The test material used is a 1.0-mm-thick high- strength steel sheet with a tensile strength of 590MPa. The specimen undergoes bendingunbending under tension when passing over the die profile. The drawing speed was set to 5-100 mm • s-1. The magnitude of true stress σDB when a specimen fractured has been precisely determined. Moreover, multiaxial tube expansion tests of the test material are performed to measure the forming limit stress σPT of the test material under plane-strain tension. It is found that σDB is larger than σPT by 2.8-6.3%. Therefore, it is concluded that the forming limit stress criterion is effective as a fracture criterion in draw-bending.

SPRINGBACK EFFECT OF AUTOMOTIVE LOWER ARM COMPONENT PREPARED VIA BURRING PROCESSING

Complex components of the sheet metal forming process need to be designed with high precision and accuracy in order to prevent defects and misalignment of the end products. One of the sheet metal cool stamping process for these complex automotive components is burring which is the forming of a flange around a hole made in a piece of sheet metal. Springback is a common defect during the burring process. The aims of this paper are to investigate the springback effect and improve shape accuracy of hole burring by inner burring process of lower arm part for automotive lower arm part. The springback defects at hole burring usually happened on the inner burring process. Experimental stretch flanging for cold stamping process of inner burring process was used to investigate the reasons of springback effect around the burred hole for a lower arm part of high strength steel (HSS) sheets SPFH590. From the two designs of burring punch dies, the result shows the values of springback effect for clearance -0.15 which have a big gap at hole burring A arm and B arm diameters, are larger than clearance -0.34 which have small gap for inner burring process of lower arm part. The experimental analysis shows that springback is proportionally related to the punch-die clearance parameter of the tool profile where the springback increase as the clearance increases.

Microstructural Developments Leading to New Advanced High Strength Sheet Steels: A Historical Assessment of Critical Metallographic Observations

Journal Article Microstructural Developments Leading to New Advanced High Strength Sheet Steels: A Historical Assessment of Critical Metallographic Observations Get access David K Matlock, David K Matlock Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Larrin S Thomas, Larrin S Thomas Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Mark D Taylor, Mark D Taylor Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Emmanuel De Moor, Emmanuel De Moor Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar John G Speer John G Speer Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 5–6, https://doi.org/10.1017/S1431927615000823 Published: 23 September 2015

Hydrogen embrittlement of DP-1000 flat steel sheet: Influence of mechanical properties, specimen geometry, pre-damaging and electrolytically zinc galvanizing

Studies towards the impact of solute hydrogen on the mechanical properties of ultra-high strength sheet steel grades (UHSS) for the automotive industry have been increasingly conducted in the past years. This paper focuses on the pre-damaging of a dualphase DP-1000 automotive sheet steel, as found in the manufacturing of automotive body parts, in connection with hydrogen embrittlement. It is shown that the kind of pre-damaging of the specimens has a severe influence on the occurrence of hydrogen embrittlement. Hydrogen embrittlement resistance under corrosive conditions was determined through constant load tests. Tensile specimens, manufactured from electrolytically zinc galvanized DP-1000 steel, were immersed in a 5% sodium chloride solution. The impact of material properties, pre-damaging and specimen geometry are critically discussed. Furthermore a discussion is given about the material testing methods most suitable to indicate high or low hydrogen embrittlement resistance if constant load testing is performed.

Effect of Pre-Heating Temperature on Microstructure and Properties of 22MnB5 Steel Hot Stamping

The hot stamping process of ultra high strength steel(UHSS) sheet is an innovative way to manufacture the components with a ultra high tensile strength. The sufficiency of martensitic transformation in formed component is affected by pre-heating temperature of blank directly. In this paper, experiments of heating UHSS blanks to 700°C, 800°C, 900°C and 1000°C were implemented to investigate the effect of pre-heating temperature on the formed component’s microstructure and mechanical properties. The results indicate that 900°C is the best pre-heating temperature for hot stamping process. The microstructure of formed component is all fine and uniform martensite. Meanwhile, tensile strength and vickers hardness raise up to 1580MPa and 450HV, respectively.

Continuous Cooling Transformation Behavior and the Phase Transformation Model in Low Carbon High Strength Sheet Steel

Phase transformation of austenite continuous cooling process in low carbon high strength sheet steel has been researched by DIL805 thermal mechanical simulate. The Austenite continuous cooling transformation (CCT) diagram of steel has been determined by dilatometry and metallography. With the increase of cooling rate, ferritic transformation, perlitic transformation, bainite transformation and martensitic transformation have produced in the organization. Mathematical equations of phase transformation point-cooling rate and phase variable-cooling rate have been established and phase transformation model of high fit degree has been gained by regression calculation. The results show that calculated value and experimental value are nearly similar, so the phase transformation model is feasible.

Properties of newly–developed hot–rolled high–strength sheet steels with phosphorus

Two types of hot–rolled high–strength sheet steels were developed. C–Mn–P steel with phosphorus content more than 0.04% and processed at a coiling temperature of less than 200°C exhibits very low yield ratio less than 70%, and superior properties in ductility, stretch flangeability, fatigue and bake–hardenability. It does not show 2nd cold work embrittlement due to P and brittleness of the heat affected zone of the welded joint. This steel is suitable for components which require high formability, such as wheel discs. The other type is an Nb–containing high–strength sheet steel with P addition instead of increasing C, that is, low carbon equivalent value. It also shows good ductility and superior stretch flangeability. Less hardenability of the welded zone of this steel results in a superior formability and touchness of the welded joint and makes it possible to apply this steel to a wheel rim.

Temperature conditions during ‘cold’ sheet metal stamping

This paper investigates the friction and deformation-induced heating that occurs during the stamping of high strength sheet steels, under room temperature conditions. A thermo-mechanical finite element model of a typical plane strain stamping process was developed to understand the temperature conditions experienced within the die and blank material; and this was validated against experimental measurements. A high level of correlation was achieved between the finite element model and experimental data for a range of operating conditions and parameters. The model showed that the heat generated during realistic production conditions can result in high temperatures of up to 108°C and 181°C in the blank and die materials, respectively, for what was traditionally expected to be ‘cold’ forming conditions. It was identified that frictional heating was primarily responsible for the peak temperatures at the die surface, whilst the peak blank temperatures were caused by a combination of frictional and deformation induced heating. The results provide new insights into the local conditions within the blank and die, and are of direct relevance to sheet formability and tool wear performance during industrial stamping processes.

Influence of Fe Oxidation on Selective Oxidation Behavior of Si and Mn Added in High Strength Sheet Steel

In the process of hot dip galvanizing high tensile strength sheet steels containing Si and Mn, selective surface oxidation of Si and Mn causes coating defects. One promising method for overcoming this problem is an oxidation-reduction process. When the steel surface is exposed to an oxidizing atmosphere, it will react primarily by forming an Fe oxide, which can be reduced by hydrogen in a reduction process that follows. It has been explained that good wettability can be obtained due to the formation of pure iron. However, the mechanism of suppression of selective surface oxidation has not been clearly understood in detail yet. In order to reveal this mechanism, the present study focused on both Mn and Fe oxidation behavior during the oxidation-reduction process for a cold-rolled sheet steel containing 0.25mass%Si-1.8mass%Mn. Surface and cross-sectional analyses were performed by using secondary electron microscopy and transmission electron microscopy. Selective surface oxidation behavior was investigated by glow discharge optical emission spectroscopy. The main results obtained are as follows. First, selective surface oxidation of Mn was suppressed even if soaking was continued after completion of the reduction of the Fe oxide. Second, as the reduction process proceeded, Mn was trapped as an internal oxide under the Fe oxide layer. Moreover, depletion of solute Mn was observed in the matrix. From these results, depletion of solute Mn is supposed to suppress the outer diffusion of Mn during soaking. Therefore, selective surface oxidation of Mn is suppressed even after Fe oxide reduction is completed.

수소주입된 고강도 DP 박강판의 소형펀치시험결과 분석

The small punch(SP) tests that can be applied to high strength sheet steel in automobile were carried out to evaluate the behavior of hydrogen embrittlement of DP sheet steels. In order to charge hydrogen at DP sheet steels, DP sheet steels were treated by the electrochemical hydrogen charging method under the charging conditions of current densities of 100, 150 and 200 <TEX>$mA/cm^2$</TEX> for charging times of 5, 10, 25 and 50 hrs. Respectively, After hydrogen charging with experimental conditions, SP tests were performed. From the SP results, the correlations between the variation of bulb diameters and bulb heights with the hydrogen charging conditions were analysed. It was shown that the variation of bulb diameters were not significant with the hydrogen embrittlement due to the amounts of hydrogen charging. On the other hand, the bulb heights were observed to decrease with increasing hydrogen contents. It was thought that these results of the variation of bulb shapes after SP tests would be estimated as the index of evaluation of hydrogen embrittlement.

성형공법에 따른 초고강도 모자형 박판부품 형상정밀도 평가

In this paper, the shape accuracy of the stamped hat-type product is quantified and analyzed with ultra high strength steel (UHSS) sheets. The shape of the hat-type product is designed in order to simplify the geometry of the side sill and the stamping methodology is proposed in order to verify the effect of the stamping procedure on the springback amount. Experiments and finite element analyses are conducted with four kinds of the forming sequences. The springback amounts are measured and compared according to the forming procedure with the embossing shape. Experimental result in company with analysis one illustrate that the springback amount is reduced with embossing in the padding operation. They also fully demonstrates the proposed forming procedure and the analysis method can be effectively applied to the process design for producing parts with ultra high strength steel.

Fundamentals for Controlling the Microstructure and Properties of Cold Rolled and Continuously Annealed Sheet Steels

The strict control of microstructure is indispensable for meeting the demands for sheet steels with superior properties. This paper reviews the state-of-the-art related to continuous annealing process from the viewpoint of physical metallurgy. For mild steel, control of texture is one of the most important issues ensuring deep drawability. Therefore, the fundamental guiding principles on texture control are reviewed. Heterogeneity in cold rolled structures is discussed, with emphasis placed on grain boundary, shear band and cementite in terms of preferential nucleation of recrystallized grains. Furthermore, C-X interactions in the recovery stage are discussed in conjunction with scavenging effects. The ND// texture usually evolves according to the grain growth, which is enhanced by reducing the pinning effect. The ductility in low-C steels is significantly influenced by the state of C, which implies the importance of C control in the continuous annealing process. As for high strength sheet steels, the basic guiding principles for dual phase and transformation induced plasticity (TRIP) steels are reviewed in terms of phase transformation and TRIP effect. Because the strength of steels currently in practical use is considered to be only a part of their potential, steels are materials with many advantages and technical problems that remain to be solved.

Fe酸化がSi,Mn選択外部酸化挙動に及ぼす影響

In the process of hot dip galvanizing high tensile strength sheet steels containing Si and Mn, selective surface oxidation of Si and Mn causes coating defects. One promising method for overcoming this problem is an oxidation-reduction process. When the steel surface is exposed to an oxidizing atmosphere, it will react primarily by forming an Fe oxide, which can be reduced by hydrogen in a reduction process that follows. It has been explained that good wettability can be obtained due to the formation of pure iron. However, the mechanism of suppression of selective surface oxidation has not been clearly understood in detail yet. In order to reveal this mechanism, the present study focused on both Mn and Fe oxidation behavior during the oxidation-reduction process for a cold-rolled sheet steel containing 0.25mass%Si-1.8mass%Mn. Surface and cross-sectional analyses were performed by using secondary electron microscopy and transmission electron microscopy. Selective surface oxidation behavior was investigated by glow discharge optical emission spectroscopy. The main results obtained are as follows. First, selective surface oxidation of Mn was suppressed even if soaking was continued after completion of the reduction of the Fe oxide. Second, as the reduction process proceeded, Mn was trapped as an internal oxide under the Fe oxide layer. Moreover, depletion of solute Mn was observed in the matrix. From these results, depletion of solute Mn is supposed to suppress the outer diffusion of Mn during soaking. Therefore, selective surface oxidation of Mn is suppressed even after Fe oxide reduction is completed.

成形性向上を目指した高強度鋼板の最新動向

成形性向上を目指した高強度鋼板の最新動向

Development of Ti and Mo micro-alloyed hot-rolled high strength sheet steel by controlling thermomechanical controlled processing schedule

A new Ti and Mo micro-alloyed hot-rolled high strength sheet steel was developed by controlling thermomechanical controlled processing (TMCP) schedule, in particular focusing on rolling and coiling temperatures. The results revealed that the steels developed were strengthened mostly by a combined effect of ferrite (α) grain refinement and precipitation hardening. The rolling at austenite (γ) non-recrystallization region provided desirable features by introducing an accumulation of local strains (i.e., dislocations), which served as nucleation sites for γ→α transformation during coiling process, making ferrite grain more fine and homogeneous, although the precipitation hardening was somewhat weakened due to the precipitation of large (Ti, Mo)C carbides (i.e., γ region precipitation). Moreover, these fine and homogeneous ferrite grain, and well developed dislocation structure improved impact properties. The TMCP schedule of the rolling at γ non-recrystallization region (final rolling temperature of 880°C) and coiling at 620°C was found to provide an attractive combination of tensile and impact properties.

고강도 DP강과 TRIP강의 표면 수소 주입량에 따른 수소취성평가

Hydrogen charging was electrochemically conducted at high strength DP steels and TRIP steel with varying charging time. The penetration depths and the mechanical properties with charging conditions were investigated through the distribution of micro-hardness and the microstructural observation of the subsurface zone. The micro-Vickers hardness was measured to evaluate the hydrogen embrittlement of subsurface zone in addition to the microscope investigation. It was shown that the hydrogen amounts decreased in DP steels and TRIP steel with increasing hydrogen charging time. As shown by micro-Vickers hardness test and small punch test results, micro-Vickers hardness and SP energy for DP steels and TRIP steel decreased with increasing hydrogen charging time, for constant value of charging current density. SEM investigation results for the hydrogen contained samples showed that the major fracture behavior was brittle fracture which results in dimples on fractured surface and the size of dimples were decreased with increasing hydrogen charging time. These results indicate that hydrogen embrittlement is the major cause for the fracture of high strength steels and also micro-Vickers hardness test and small punch test is a valuable test method for hydrogen embrittlement of high strength sheet steels.

Improvement of the Hydrogen Microprint Technique on AHSS Steels

Preferential hydrogen diffusion paths in low-carbon ferritic steel and transformation-induced plasticity (TRIP) steel characteristic of new advanced high strength sheet steels (AHSS) were evaluated via the hydrogen microprint technique (HMT). As a consequence of hydrogen diffusion and evolution from the sample after charging, the technique involves reduction of AgBr emulsion applied to the surface of a charged sample to produce discrete Ag grains which identify, via scanning electron microscope (SEM) imaging, locations where hydrogen evolved. Enhancements to the results are revealed through a newly developed image-before-etch HMT method, consisting of SEM imaging before and then again after etching. The modified technique was implemented to facilitate direct comparison of silver grains with the features of an etched multi-phase TRIP steel microstructure. Results from SEM imaging demonstrated the effectiveness of the image-before-etch method to avoid artifacts and to positively identify hydrogen evolution sites in AHSS TRIP steel microstructures.

Application of the High Strength Steel in the Automobile

With the development of automotive industry, new high strength and high performance steel sheets are used more and more widely, which are the basis of weight reduction and safety improvement for automobiles. Taking examples of the design of car body, this paper focused on the remarkable effects of the application of AHSS in automobile lightweight, car safety and environment. It was also pointed out that the key topics will fall in the forming technique of AHSS and the applications of AHSS to new car body design to enhance automobile safety.

Evaluation of Hydrogen Embrittlement of High Strength Steel for Automobiles by Small Punch Test

The hydrogen embrittlement of high strength steel for automobiles was evaluated by small punch (SP) test. The test specimens were fabricated to be 5 series, having various chemical compositions according to the processes of heat treatment and working. Hydrogen charging was electrochemically conducted for each specimen with varying of current density and charging time. It was shown that the SP energy and the maximum load decreased with increasing hydrogen charging time in every specimen. SEM investigation results for the hydrogen containing samples showed that the fracture behavior was a mixed fracture mode having 50% dimples and 50% cleavages. However, the fracture mode of specimens with charging hydrogen changed gradually to the brittle fracture mode, compared to the mode of other materials. All sizes and numbers of dimples decreased with increasing hydrogen charging time. These results indicate that hydrogen embrittlement is the major cause of fracture for high strength steels for automobiles; also, it is shown that the small punch test is a valuable test method for hydrogen embrittlement of high strength sheet steels for automobiles.