Kari Mäntyjärvi

Bendability of Ultra-High-Strength Steel

Utilisation of ultra-high-strength (UHS) steels is rapidly spreading from the automotive industry into many other application areas. It is necessary to know how these materials behave in common production processes such as air bending. The bendability of UHS steels is much lower compared to normal and high-strength construction steels. In this work, experimental tests were carried out using complex phase (CP) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250) and S650MC HS steel in order to inspect material bendability and possible problems in the bending process. Mechanical and geometrical damages were registered and classified. The bending method used was air bending and press brake bending with an elastic lower die. The FE analysis was used to understand the stress state at different points in the material and build-up of failure. As UHS steels cannot stand large local strains, a large radius must be used in air bending. The results show that even when a large radius is used in air bending, the strain is not evenly distributed; there is a clear high strain area in the middle of the bend. It was also possible to simulate the other phenomena occurring in experimental tests, such as losing contact with the punch and ‘nut-like’ geometry, using FE analysis. Experimental test results also show that by using an elastic lower die, it is possible to avoid unwanted phenomena and obtain an almost 50% smaller punch radius, but the required force is 50% bigger than that required in air bending.

Laser-Assisted Bending

When sheets of high-strength (HS) and ultra-high-strength (UHS) steels are bent by a press brake the process suffers from large bending forces, considerable springback, and eventual cracks. Additionally, some unpredictable effects, such as lost contact to the punch, caused by strain hardening may occur producing a bend with erroneous radii. The strain hardening of the bending line may make further processes, such as forming or welding, more complex. One solution to these problems is to anneal the bending line with a laser in advance. Of course, it is also possible to utilise other types of heat sources, but the laser can offer the most precisely controlled heat treatment. The proper process parameters depend on the material, and it has been noticed that inadequate process parameters may harden the material instead of annealing. In this work some experiments on bending sheet metal samples of HS or UHS steel with previously laser-annealed bending lines have been carried out and the outcome analysed. The results show that the annealing produces better bending results compared to the conventional procedure. This includes lower springback, less hardening in the bending line and more precise geometry of the bend. It can be even suggested that proper annealing with strain hardening in bending will produce the original material structure. Obviously, more theoretical and experimental work is required to optimise the process parameters including the laser power and speed for each pair of material strength and thickness.

Identifying residual stresses in laser welds by fatigue crack growth acceleration measurement

During laser welding, residual stresses are thermally induced. They can have strong impact on the fatigue behavior and fatigue life. A standardized measurement method for the fatigue crack growth rate was expanded to identify residual stress along the cracking path. The second derivative of the measured crack opening and in turn the crack acceleration corresponded well with distinct acceleration maxima and minima and accordingly with tensile and compressive stress, as was basically proven by numerical simulation. The method is simple and extendable. It provides valuable information, as was demonstrated for various situations.

Cutting Edge and its Influence on the Fatigue Life of High Strength CrMn-Austenitic Stainless Steel

In the field of product design the fatigue behavior is one of the most challenging issues, especially in the case of high strength steels. It is well known that surface quality has significant influence on a steel component fatigue properties. Different cutting methods have dissimilar effects on cutting edge’s surface properties and quality. Thus different cutting methods have distinct influence on fatigue life of steels. For example, while thermal cutting is often fast and effective, it induces heat to work piece and thereby influences on microstructure. Unlike thermal cutting methods, waterjet cutting does not create any heat in the material, but it impairs the surface quality. In this work, the influence of laser-, high-definition plasmaand waterjet cutting on fatigue life has been studied. The results obtained by these cutting methods were compared to ones reached with machined specimens. The test material was 2.27 mm thick sheet of a temper rolled CrMnaustenitic stainless steel grade AISI 201 LN TR with the yield strength of 666 MPa and tensile strength of 814 MPa. Load-controlled low-cycle fatigue tests were performed using the R ratio (R=σmin/σmax) of -1 to flat type specimens. Specimens were stressed on transverse orientation at the stress amplitude of 70%, 80% and 90% of the yield strength. The qualities of cutting edge surfaces were evaluated by measuring surface roughness (Ra-value and Rz-value). Length of heat affected zone was measured from thermally cut edges and also hardness of cut edges was measured.

Mechanical Properties of Laser Heat Treated 6 mm Thick UHSS-Steel

In this work abrasion resistant (AR) steel with a sheet thickness of 6 mm was heat treated by a 4 kW Nd:YAG and a 4 kW Yb:Yag–laser, followed by self‐quenching. In the delivered condition, test material blank (B27S) is water quenched from 920° C. In this condition, fully martensitic microstructure provides excellent hardness of over 500 HB. The test material is referred to AR500 from now onwards. Laser heat treatment was carried out only on top surface of the AR500 sheet: the achieved maximum temperature in the cross‐section varies as a function of the depth. Consequently, the microstructure and mechanical properties differ between the surfaces and the centre of the cross‐section (layered microstructure). For better understanding, all layers were tested in tensile tests. For a wide heat treatment track, the laser beam was moved by scanning. Temperatures were measured using thermographic camera and thermocouples. Laser heat treated AR500 samples were tested in hardness tests and by air bending using a pres...

Passive Laser Assisted Bending of Ultra-High Strength Steels

Formability of ultra-high strength steels is poor causing problems in bending and stretch forming. The target of this work was to improve the formability of ultra-high strength steel sheets by controlled local laser heat treatments. Three steel grades, a bainitic-martensitic 4 mm DQ960 and two martensitic WR500 with 6 mm and 10 mm thicknesses were heated by controlled thermal cycles using a 4 kW Yb:Yag –laser, followed by self-cooling. Sheets with the thicknesses of 4 and 6 mm were treated on one side only by heating up to the austenitizing temperature. The 10 mm thick WR500 sheet was heat treated separately on the both surfaces by heating to a lower temperature range to produce a shallow tempered layers. The tensile and bendability tests as well as hardness measurements indicated that laser heat treatment can be used to highly improve the bendability locally without significant strength losses.

Bendability and Microstructure of Direct Quenched Optim® 960QC

Use of ultra-high-strength steels (UHSS) in weight critical constructions is an effective way to save energy and minimize carbon footprint in the end use. On the other hand, the demands for reducing manufacturing costs and energy consumption of the steelmaker are increasing. This has led to development of energy efficient direct quenching (DQ) steelmaking process as an alternative to the conventional quenched and tempered or thermomechanical rolling and accelerate cooled processes. Ruukki has employed thermomechanical rolling and direct quenching process (TM + DQ) for a novel type of ultra-high-strength strip and plate steels since 2001. Advantages of the ultra-high-strength level (>900MPa) can be fully utilized only if fabricated properties are on a sufficient level. Bending is one of the most important workshop processes and a good bendability is essential for a structural steel. Hence, the metallurgy and bendability of Ruukki ́s TM + DQ strip steel Optim® 960QC have been investigated closely. It was found that by optimizing process parameters and chemical composition, a good combination of strength and ductility can be achieved by a modification of martensitic-bainitic microstructure. Despite of smaller total elongation, the bendability of Optim® 960QC is at least on the same level as on conventionally manufactured 960MPa steels. However, it is important to pay special attention to bending process (tool parameters, springback, bending force, material handling) when bending UHSS. It was also found that the bendability of Optim® 960QC can be significantly enhanced by local laser heat treatments or roll forming.

A Novel Heat Treatment Line for Processing of Tailored Small Batch Steels

This study describes design and construction of a novel flexible heat treatment line for processing customer-oriented small batch steels. The induction heater (600 kW) developed is suitable for the sheet thickness in the range 3.2 30 mm and the width of 85 1250 mm. Sheets are fed using an electrical motor (1.5 kW) and a chain drive, the speed being in the range 0.3 7 m/min, depending on the power and the sheet dimensions. At this study, 4.5 (WR-1) and 10 mm (WR-2) thick wear resistant steels were tempered at different peak temperatures to compare the effect of rapid tempering on mechanical properties. Results showed that the heat treatment line is capable of producing tempered steel grades with adequate properties at industrial product rate. For example, 4.5 thick WR-1 tempered at 550 oC provided a yield strength (YS) over 1000 MPa with minimum bending radius of 6 mm (in the delivered condition YS = 1605 MPa and Rmin = 12). Tempering of WR-1 at 700 oC provided YS of 762 MPa and Rmin of 1 mm. Results were similar between two test materials, but the enhancement in bendability was slightly more effective with the thinner sheet.

Designing and Manufacturing of a Flexible Longitudinally Laminated Sandwich Panel Forming Tool

The main aim of the study was to develop forming tools for wide (over 1.2 meter) sandwich panels. Longitudinal laminating technology was selected for tool manufacturing due to its flexibility and cost efficiency. Laminating technology enables easy modification of the tool dimensions afterwards. The function to optimize or vary the dimensions of the tool was set as a secondary objective for the study. Forming tools for sandwich panels are usually complicated structures and joining of the plates can be difficult in some cases. Typically sandwich forming tools are capable to produce only narrow panels (less than 1 meter) and optimization must be done during designing of the tool. In this study, a rapid designing and manufacturing of a flexible sandwich panel forming tool was investigated. Sandwich panels are usually applied in light structures or voice covers due to their very low weight, high stiffness, durability and production cost savings. Designing of the forming tool was made by using a 3D CAD program. Conventional steel plates were used for the forming tool and the assembly was done by fixing the plate parts longitudinally together (laminating). Most important criterion for the forming tool was its capability to produce high quality geometry for the core. Laser welding assembly showed that the quality of the core was good enough for welding the lap joints properly. Both of the objectives were fulfilled: 1) forming tools were suitable for forming of wide cores (1.2 meter) and 2) the structure of the laminated tool enables to change or add new plate parts to change the dimensions of the final product.

Mechanical Properties of a Metal Sandwich Panel Manufactured Using Longitudinally Laminated Forming Tools

Sandwich panel structures are increasingly used in applications where the most important demands are the weight saving and long service life. Utilizing sandwich panels, extremely light-weight, stiff and robust structures can be manufactured. In this study, sandwich panels were produced by specially designed cost-effective forming tools. Various kind of test materials were used for corrugated cores and skin plates: conventional low-carbon steel grade EN 10130 and ferritic stainless steel grade 1.4509 with plate thicknesses of 0.6 and 0.75 mm. A common S355 structural steel was used as a reference for bending strength comparison.For measuring the stiffness, MTS tensile and fatigue testing machine was selected to determine the bending resistance of the sandwich panels. The bending force, needed for yielding and fracture, related to the bending length and intensity was compared with the results from bending of the reference plates. Results showed that the bending force of the panels is significantly higher than that of a plate having similar intensity. The best results were obtained with the stainless steel (SS) panel that had 27% higher bending force at the yield point than 5 mm thick S355 plate having 3 times larger intensity. The carbon steel panel was approximately 40% weaker than the SS-panel and both panel types lost strength when loading direction was changed from transverse to 45 degree and further to 90 degree load (longitudinal).