Ultra-high Strength Steel Sheets Research Articles

초고장력 강판을 적용한 Cowl Cross Member의 레이저 용접 적용에 관한 연구

A cowl cross is a part of the car's instrument panel used to maintain the rigidity of the vehicle body side. The application of laser welding has the benefits of speed and thermal deformation compared to arc welding. An ultra-high strength steel sheet is used to reduce the weight of the vehicle body parts. Generally, formability of such a steel sheet is poor because its elongation is very low. For this reason, a method for cold forming of an ultra-high strength steel sheet is required. This paper describes how to improve the formability and weldability of the ultra-high strength steel sheet. Mechanical tests of this material were also performed to evaluate the welding properties of <TEX>$CO_2$</TEX> (GMAW) and those of laser welding.

Direct Punching in Inclined Ultra-High Strength Steel Sheets

A direct punching of inclined ultra-high strength steel sheets having low ductility was carried out to improve the quality of the sheared edge. In the direct punching of an inclined sheet, the contact between the sheet and punch became gradual because of touch from the bottom edge of the punch, and thus the sheared portion tended to bend in the latter half of punching. As the strength of the sheet increased, defects such as burr and secondary burnished surface were occurred around the sheared edge due to the low ductility. The range of direct punching for sheared edge without defects was shown for the high strength steel sheets. To prevent the occurrence of defects, a punch having inclined bottom was used. In this punch, the contact with the sheet was changed, the uniform contact with the inclined bottom was given. In the punch having the inclined bottom, the secondary burnished surface in the edge of 980 MPa grade steel sheet was prevented.

Improvement of Joinability in Mechanical Clinching of Ultra-High Strength Steel Sheets Using Counter Pressure

A mechanical clinching using counter pressure of a rubber disk was developed to join the ultra-high strength steel sheets having low ductility. In the proposed process, the interlock was increased by the increment of metal flow with the counter pressure of rubber disk in the die cavity. The two kind of ultra-high strength steel sheets having different ductility were used in the mechanical clinching. The effect of the shape of rubber disk on the deforming behaviour of the sheets was investigated. The joinability was improved under the appropriate shape of rubber disk for both sheets, and then the sheets having 56% of reduction area were successfully joined whereas the sheets were not joined without the counter pressure. Although the joinability of the sheets having 43% of reduction area was improved, the cracks occurred in the upper sheet around the punch sidewall. The maximum static load and the fatigue limit of the joined sheets were measured in the tension-shearing and cross-tension tests. It was effective for the improvement of joinability in the mechanical clinching of ultra-high strength steel sheets to use the counter pressure of the rubber disk.

Application of mechanical trimming to hot stamped 22MnB5 parts for energy saving

An energy-saving mechanical trimming technology for hot stamped ultra high strength steel sheets is suggested in this study, in which the sheet thickness on the desired trim line is reduced by hot half-trimming at the same time as hot stamping, and the half-trimmed part is subsequently fully trimmed by mechanical trimming. Effectiveness of the proposed process has been investigated by FE-simulation and experiment. In this process, the hot half-trimming depth in hot stamping and the clearance between the punch and die in mechanical trimming were considered to be the main process parameters determining trimmed surface quality, trimming load, and tool wear. A high-quality trimmed surface, as compared with that produced by conventional cold trimming, can be achieved with a large hot half-trimming depth in hot stamping and small clearance in mechanical trimming. It was also shown that production time and cost can be reduced considerably by mechanical trimming using hot half-trimming, with low probability of tool chipping and reduced tool wear, as compared to conventional cold trimming and laser cutting processes. Therefore, mechanical trimming using hot half-trimming as proposed in this study can replace the high energy- and production time-consuming laser cutting process in the mass manufacturing of automotive parts.

Experimental investigation on galling performance of tool steel in stamping of UHSS sheets

The objective of this study is to experimentally investigate the effect of surface roughness parameters on the galling performance of tool steel used in the stamping of ultra-high-strength steel (UHSS) sheets. The galling performance of uncoated and TiAlCrN coated AISI D2 tool steel was evaluated against a 1.4 mm-thick dual phase (DP) 980 UHSS sheet, using pin-on-flat surface test. For evaluating the galling performance of tool steel, the surface roughness parameters and topography were tested under three surface conditions: as-received (AS), as-received and post-polished (AS+P), and as-received coating (AS+C), whereas the sheet surface was tested under as-received and no post-treatment surface condition. The variation in the coefficient of friction was monitored as a function of sliding distance. The sliding distance at which severe galling began to occur owing to the transfer of sheet material to the tool surface was determined as the critical sliding distance for a sudden increase in the friction coefficient. Experimental results showed that a plateau-like topography, with low average surface roughness, negative skewness and high kurtosis value, improved the galling performance of tool steel. Additionally, the hard TiAlCrN coating of tool surface also effectively reduced galling tendency.

Toughness of Ultra High Strength Steel Sheets &lt;sup&gt;&lt;/sup&gt;

Currently only few methods exist for thin steel sheet testing, especially based on fracture mechanics concept. Charpy impact test is one of the most used method for testing notch toughness and fracture behaviors because of the simplicity and the other advantages [. This article deals with toughness testing of automotive ultra high strength steel sheets (UHSS). Several standard types of toughness test that generate data for specific loading conditions and/or component design approaches exist. Two definition of toughness will be discussed: i) Charpy V-notch toughness, method includes joining of thin steel sheets to one compact unit and ii) material (tensile) toughness [. Two steels were used, DP1000 and 1400M of 1,8 mm thickness and two joining techniques: bonding with adhesives and joining with holders. Effect of material, joining technology, structural adhesives, and number of joined plates on the toughness values was quantified at the room temperature. Toughness of steels by the tensile test was added for comparison. Fracture surface was observed using scanning electron microscope analysis.

Improvement of formability in deep drawing of ultra-high strength steel sheets by coating of die

The formability of deep drawn ultra-high strength steel sheets in dies coated with either titanium nitride (TiN) or Vanadium carbide (VC) at different drawing speeds and ironing ratios was investigated. TiN was deposited via chemical vapour deposition and physical vapour deposition (PVD) while thermal diffusion was used for VC deposition. In non-coated dies, seizure occurred on both surfaces of the die and the side wall of the drawn cup irrespective of the deep drawing conditions. The deep drawability is improved with coating of die. Whereas in coated dies, seizure became significant only during deep drawing extreme conditions of 120mm/s for TiN-coated dies; and this was prevented in VC-coated dies across all drawing conditions. The VC-coated die was suitable for deep drawing of ultra-high strength steel sheets. The delayed fractured observed in the ultra-high strength steel cups occurred for a large amount of ironing ratio and drawing speeds; and this can be prevented by appropriate heat treatment.

Mechanical clinching of ultra-high strength steel sheets and strength of joints

Ultra-high strength steel sheets having low ductility were joined by mechanical clinching with dies for control of metal flow. The diameter and depth of the die were modified to relieve concentration of deformation of the sheets for avoidance of the occurrence of sheet fracture. As the tensile strength of the steel sheets increased, the interlock decreased due to small metal flow. Two kinds of the ultra-high strength steel sheets having different ductility were used. The ultra-high strength steel sheets having large ductility were successfully joined using die having modified shape, whereas the sheets having small ductility were not joined. The static and fatigue strengths of the mechanically clinched joint were compared with those of the resistance spot welded joint. Although the static load of the mechanically clinched joint was smaller than that of the resistance spot welded joint in both tension-shearing and cross-tension tests, the fatigue load of the clinched joint was larger in the large number of cycles. It was found that mechanical clinching has superior fatigue strength due to the large yield stress of the sheets and relaxation of the stress concentration.

Improvement of Joinability in Mechanical Clinching of Ultra-high Strength Steel Sheets Using Counter Pressure with Ring Rubber

A mechanical clinching using counter pressure of a rubber ring was developed to join the galvanised ultra-high strength steel sheets having low ductility. In the proposed process, the interlock was increased by the increment of metal flow with the counter pressure of rubber ring in the die cavity. The ultra-high strength steel sheets having 45% of reduction area was used in the mechanical clinching. The effect of the shape of rubber ring on the deforming behaviour of the sheets was investigated. The joinability was improved under the appropriate shape of rubber ring, and then the sheets were successfully joined with the counter pressure. The maximum static load and the fatigue limit of the joined sheets were measured in the tension-shearing and cross-tension tests. It was effective for the improvement of joinability in the mechanical clinching of ultra-high strength steel sheets to use the counter pressure of the rubber ring.

種々の試験法で得られる薄鋼板の水素脆化破壊限界の比較

Applications of ultra high strength steel sheets to automotive body have been expanding year after year. Hydrogen embrittlement (HE) is one of the problems of ultra high strength steels. Various methods are used as the evaluation method of HE resistance. In this study, the critical HE conditions obtained by SSRT, CSRT and 4-point bending test were compared by using the same materials, which have tempered martensite microstructure with the composition of a SCM435 or an V added steel with many hydrogen trapping precipitations. Specimens were charged with hydrogen by the cathodic charging method. The specimen used in the SSRT and the CSRT was machined with notches on the both sides of a parallel part and the stress concentration factor (Kt) of the specimens was 4.26 or 1.76. The specimen used in the 4-point bending test was coupon shape. The critical HE conditions evaluated with the average applied stress and the average hydrogen content of the specimen were different depending on the test methods. HE conditions were also evaluated with the local stress and the local accumulated diffusible hydrogen content at the fracture initiation point. The critical condition evaluated by the 4-point bending test was located in the higher stress and higher hydrogen content region compared with the critical conditions obtained by the CSRT and the SSRT.

Punching process including thickening of hole edge for improvement of fatigue strength of ultra-high strength steel sheet

A punching process including thickening of a hole edge of an ultra-high strength steel sheet with a taper punch and step die was developed to improve fatigue strength of the punched sheet. In this process, the sheet was punched by the bottom of the punch, and subsequently the hole edge was thickened by the taper of the punch and the corner step of the die. The taper angle of the punch and the step height of the die were optimised to increase the amount of thickening. The quality of the sheared edge for the thickened punched sheet was improved by ironing with the taper of the punch, and compressive residual stress was generated around the hole edge. Seizure on the surface of the punch was prevented by VC-coating for repeated punching and realistic punching speed. It was found that the present punching process including thickening is effective in improving the fatigue strength of the punched ultra-high strength steel sheet and in preventing occurrence of delayed fracture.

超高張力鋼板の穴縁部増肉加工による疲労強度の向上

The edge of a punched hole was thickened by burring processes to improve the fatigue strength of punched ultra-high strength steel sheets. In this process, the sheet was punched by the bottom of the punch, and subsequently the hole edge was thickened by the taper of the punch and the corner step of the die. The fatigue strength of the hole edge thickened by burring was higher than that of the punched hole edge. The punching and the burring were combined into the punching process including thickening. The step height of the die was optimised to increase the height of the hole edge. The quality of the sheared edge for the thickened punched sheet was improved by ironing with the taper of the punch, and compressive residual stress was generated around the hole edge. It was found that the punching process including thickening is effective in improving the fatigue strength of the hole edge in ultra-high strength steel sheets and in preventing the occurrence of delayed fracture.

超高張力鋼板の縮みフランジ変形における逐次接触パンチによるしわの抑制

To prevent the occurrence of wrinkling in the shrink flanging of ultra-high strength steel sheets, a gradually contacting punch was developed. Compressive stress around the convex corner of the sheet in shrink flanging was reduced by gradually bending the edge from the center to the outside of the sheet using the gradually contacting punch. Compressive strain around the convex corner edge decreased as the contact angle of the punch increased. Although a small camber occurred on the upper surface of the flanged sheet using the conventional flat bottom punch, the camber was large using the gradually contacting punch because of the gradual bending. The limiting flange height of the flanged sheet without wrinkling showed an increase of 26% for the 980 MPa grade ultra-high strength steel sheet.

薄板技術の100年 −自動車産業と共に歩んだ薄鋼板と製造技術−

Development in sheet steels has progressed with a strong relation with automotive industry in Japan. To meet with the requirements from automotive industry, various types of sheet steels including high and ultra-high strength steel sheets have been developed. Progresses in three types of steel series will be discussed by checking the historical facts and technologies and their contributions. Introductions of interstitial free (IF) steel and continuous annealing system are the important events in mild steel developments for panels. Extensive work on finding the optimum mixtures of hard and soft phases to improve elongation of steels contributed to improve the crashworthiness of autobodies. Continuous annealing system also played an important role in producing these advanced high strength steels. Precipitation is used in characteristic ways which is to scavenge solute carbon and nitrogen and to prevent coarse cementite particle precipitation. It is also worth to point out that the strong collaborative activities particularly characteristic in Japan between steel manufacturers and auto companies have affected on the progress in advanced sheet steels.

Processing, Microstructures and Mechanical Properties of Ultra-high Strength Steel Sheet

An ultra-high strength steel sheet with a tensile strength over 1500MPa and excellent low-temperature impact toughness was obtained by either using controlled rolling plus air cooling (CR+AC) or laboratorial thermo-mechanical controlled processing (TMCP). The effects of CR+AC and the TMCP processes were then examined on the microstructures and mechanical properties of this steel sheet. The results showed that by using CR+AC technology a uniformly distributed bainitic ferrite microstructure could be produced in this steel sheet under the appropriate reheating and rolling condition. By using TMCP technology, a lath bainite and martensitic microstructure with superior mechanical properties could also be generated. A microstructural analysis was made to interprete the strengthening-toughening mechanisms in association with the processes.

Correction of Eccentricity between Punch and Die in Slight Clearance Punching of Ultra-high Strength Steel Sheets

An approach for correcting the eccentricity in slight clearance punching of ultra-high strength steel sheets having low ductility was developed to improve the quality of sheared edges of punched holes. In this process, a moving die was employed in order to correct the eccentricity between the punch and die and to ease the setting of tools. The gap between the die and holder allowed the eccentric die to move, and thus the die became concentric after several strikes. For the moving die, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased. It was found that the application of moving die in slight clearance is indispensable for high quality punching of ultra-high strength steel sheets.

Precision Piercing and Blanking of Ultrahigh-strength Steel Sheets

The paper proposes new technologies able to improve the surface quality of sheared products by means of a combined process of finish blanking and press shaving applied to materials having very high strength. Experimental researches were developed and the best combination of process conditions was identified. Furthermore, the technology was checked of so-called delayed fracture properties of the products by subjecting them into a solution of hydrochloric acid. They also have proved to possess good anti-delayed fracture properties.

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.

Self-pierce riveting of multiple steel and aluminium alloy sheets

Multiple steel and aluminium alloy sheets were joined by self-pierce riveting. Self-pierce riveting is attractive for joining multiple and dissimilar sheets, because joining of individual interfaces is not necessary unlike the conventional joining processes, i.e. sheets except for a lower sheet are merely pieced with a rivet shirt. The steel sheets ranged from mild steel to ultra-high strength one having 980MPa in tensile strength, and the joinability for three steel and aluminium alloy sheets for various combinations was examined from both experiments and finite element simulation. The joinability was improved by setting a softer sheet uppermost due to smooth piercing. In addition, the joining range for self-piece riveting of three high strength steel and aluminium alloy sheets was extended by optimising a shape of the die, and the ultra-high strength steel, mild steel and aluminium alloy sheets were successfully joined.

Effect of Pulsed Electric Current on the Tensile Properties of DP Type Ultra-High Strength Steel Sheets

Although ultra-high strength steel (UHSS) sheets with tensile stress above 1.0 GPa have a great attention in the automotive industries for reducing the weight and improving fuel efficiency of auto-body, the poor formability of UHSS sheets is one of the main factors hindering their industrial application. Enhancing the formability is a very important and challenging problem to both automotive and manufacturing engineers. In this study, the electro-plastic effect, which is first discovered in 1960s, of ultra-high strength steel sheets is investigated to improve the formability. To begin with, a test system is built up to carry out the tensile test with heavy electric current flowing through the specimen. The evolutions of the flow stress and the electric resistivity are obtained using this test system. The significant decrease in the flow stress caused by the heavy flowing through current is observed.