MaX: Martensitic Stainless Steel for Hot Stamping

Transcription

1 MaX: Martensitic Stainless Steel for Hot Stamping

2 Aperam is a global stainless steel player offering a multitude of effective, innovative and environmentally friendly stainless steel solutions, tailored to meet our customer expectations. Aperam stainless: your key partner. We anticipate end-users new requirements and we support every customer, from technical assistance to product co-development, thanks to our global presence. Flat products by Aperam Stainless Europe and Aperam Stainless South America, welded tubes by Automotive Tube Europe, are your partners of choice. 2

3 4 Chemical composition 4 General properties 5 Applications 5 Product range 6 Tensile & Bending properties 7 Energy absorption 9 Oxidation & Corrosion resistance 9 Welding Aperam Stainless Europe, innovative stainless steel solutions to the automotive industry Innovation In order to meet new environmental regulations regarding CO 2 emissions, car manufacturers are focusing on vehicle weight reduction through product redesign and the extensive use of advanced lightweight materials including high-strength stainless steels. Therefore, car manufacturers are looking for new solutions to lighten chassis and structural parts in cars. High strength steels formed by hot stamping have proved to be good candidates for achieving better in-use performance together with a lighter structure. In particular, our martensitic stainless steel MaX fulfils the industrial targets for chassis parts in terms of mechanical and fatigue properties. It could potentially lead to a 15-5% weight saving. Competitiveness Due to their outstanding processability, making automotive parts with martensitic stainless steels guarantee excellent cost effectiveness. This family of stainless steels allows for significant process simplification and therefore reduces process costs. Price stability is excellent as those grades contain a limited amount of nickel and only a relatively small amount of chromium. The process route is known and mastered, ensuring that transformation costs are kept as low as possible. Proximity With 16 service centres (flat products, tubes and precision flat products) carrying out finishing operations on our stainless products, cutting and slitting to required dimensions, surface finishing and packing, we are committed to answering your individual needs and responding to your expectations. You thus benefit from the strength of a large organisation and the responsiveness of a human-scale operation. 3

4 MaX: Martensitic Stainless Steel for Hot Stamping The hot stamping process is now widely used in the automotive industry to produce complex parts with large in-use properties, allowing for significant vehicle mass reduction. This process presents numerous advantages: low material cost, high mechanical strength and a good geometry control during the forming process. Martensitic boron carbon steels are undeniably well adapted to the hot stamping process and constitute a landmark in the history of the automotive industry. However, the demand is now to develop materials that allow for process simplification to reduce production costs. In this regard, Martensitic Stainless Steels (MSS) have been considered as potential complementary solutions to existing press hardening steels. First MSS contain between 11% and 14% Cr which considerably enhances their oxidation and corrosion resistance. Therefore it is not necessary to add expensive coatings for oxidation protection. Contrary to coated boron carbon steels, the absence of coating makes MSS compatible with fast heating processes (joule, induction and flame). Moreover, the high chromium content of MSS provides them with excellent quench hardenability, making them insensitive to cooling rates between 1 C/s and 1 C/s. As a comparison, boron carbon steels must be cooled at a speed higher than 27 C/s to reach the right final properties. The low oxidation kinetics together with the high quench hardenability of MSS makes the hot stamping process very flexible. Finally, it must be added that MSS do not contain any nickel and are therefore the most inexpensive family among stainless steels. 1. Chemical composition Elements C Si P Max S max Mn Cr Nb % < < This grade is in accordance with: > > Parliament and Council Regulation (EC) N 197/26 concerning the safe use of chemicals by their registration and evaluation and, in some cases, authorization and trade restrictions (REACH) > > Aperam Material Safety Data Sheet: Stainless Steels (European Directive 21/58/EC) > > European Directive 21/58/EC related to end-of-life vehicles and later modifications 2. General properties The main features of MaX are: > > Its ability to be hardened by hot stamping up to a tensile strength of 1.2 GPa > > Its ability to be air quench hardenable > > Its oxidation resistance (no coating needed) > > Its outstanding fatigue behavior > > Its compatibility with fast heating (joule or induction) > > Its ability to make the hot stamping process more flexible (process cost savings) > > Its ability to be formed on either a single step or a multiple steps process (allowing in-die trimming, hole punching or very complex shapes...) This grade does not need further tempering treatment after austenitization. It is versatile and fulfils direct and indirect hot stamping process requirements with high flexibility compared to carbon steels. Higher Drawability Indirect process Calibration Fl Curves Epsilon DP6 MaX T MaX Coated boron carbon steel T Coated boron carbon steel MaX.1 -,2 -,1,1,2,3 Epsilon 2 t t 4

5 Direct process > Standard hot stamping process Cutting line Standard roller earth furnace Strong water cooling press Laser cutting > MaX hot stamping process UTS 5 MPa; EI 3% Easy to cut or or Fast heating No C depletion Multistep is possible 3. Applications MaX is suitable for direct and indirect hot stamping processes. The excellent fatigue properties together with the 1.2 GPa tensile strength make the MaX best suited for automotive chassis parts such as lower control arms or engine cradles. The elevated crash performances make the MaX also suitable for body-in-white application where energy absorption is the key, such as the front and rear rails. 4. Product range > > Forms: Sheets, blanks, coils > > Thicknesses:.5 to 6 mm, depending on the finish > > Width: Maximum 125 mm > > Finish: Cold and hot rolled depending on the thickness Physical property Unity Measurement temperature Value Density 4 C 7.7 Melting point C 151 Specific heat J.kg -1.K -1 4 C 62 2 C 48 8 C 82 Thermal conductivity W.m -1.K -1 2 C 32 Thermal expansion 1-6 m.m -1.K C 2-4 C 2-6 C Electrical resistivity μω.cm 2 C 6 Curie point 725 Young s modulus GPa 2 C 25 Poisson ratio 2 C.24 Transformation temperature Value ( C) Experimental condition Ac1 915 Heating rate: 1 C/s Ac3 955 Heating rate: 1 C/s Ms 365 Cooling rate: 3 C/s Temperature ( C) % 99% Ferrite/Bainite Ferrite/Carbides Martensite Time (s) 5

6 5. Tensile & Bending properties 5.1 Tensile Annealed condition According to ISO , part 1, Specimen perpendicular to the rolling direction. Specimen Lo = 8 mm (thickness < 3 mm) Lo = 5,65 So (thickness 3 mm) T ( C) 11 1 YS, UTS (MPa) El% Holding time (s) 8 MPa 1 MPa 12 MPa 13 MPa Tempering temperature ( C) YS UTS El% Austenitizing temperature-time map for MaX. Iso-UTS lines are indicated. Markers represent experimental data and lines are extrapolations for non investigated time and temperature range Effect of tempering temperature (tempering time of 48 hours) on the tensile properties of MaX 5.2 Bending Bending tests done according to VDA standard on 1.5 mm thick specimens. Values reported are measured after spring back. Mechanical property As-delivered Standard heating (1 C/s) 95 C 5 min Fast heating (1 C/s) 12 C sec YS (MPa) UTS (MPa) TEl (%) Bending angle (t=1.5 mm) >1 >7 >8 6

7 6. Energy absorption 6.1 Toughness Fracture energy values from Charpy impact tests are reported in the figure below as a function of testing temperature. Whereas standard 1.46 exhibits low impact toughness and brittle fracture even at room temperature, MaX is fully ductile down to -4 C with a fracture energy above 5 J/cm². This improved ductility results from the composition optimization and more precisely from the Nb addition which prevents excessive grain growth during austenitization Impact toughness (J/cm ) Testing temperature ( C) 1.46 MaX Evolution of Charpy impact toughness as a function of test temperature, showing the improved ductility of MaX compared to Dynamic crash test Bending crash tests have also been performed on hot formed Ω-shape samples (4 kg projectile, 8 m/s speed). Hot forming operation consisted in a 95 C-5min austenitization and hot stamping with a quenching in the tool. Improvement brought by the optimized metallurgy of MaX is evidenced on the below figure. Standard 1.46 shows large brittle cracks in the bent zones whereas MaX folds without such cracks, consistently with Charpy tests. Standard 1.46 with large brittle cracks MaX without any brittle cracks Coated boron carbon steel Details of the crushing zone 6.3 Fatigue behavior Shot peening procedure The fatigue behavior was characterized on as-quenched MaX with and without surface preparation by shot peening. Shot peening was done according to French NF L standard (similar to the SAE AMS243S standard). Two different intensity levels of 25/3N and 15/2A were performed with 18 µm spherical steel shots at pressures of 2.5 bars and 5 bars, respectively. 7

8 6.3.2 Specimen surface before testing The as-quenched MaX surface was smooth as highlighted by the low Ra value recorded (.4 µm). The specimen surface roughness increased significantly with increasing shot peening intensity, the Ra values in the 25/3N and 15/2N conditions being 1.2 and 1.6, respectively. Shot peening also introduced compressive residual stresses, the peak values being -69 MPa and -73 MPa for the 25/3N and 15/2A specimens, respectively. As a comparison the near surface residual stress state of the as-quenched specimen was tensile at 18 MPa. As-quenched 25/3N 15/2A Ra (µm) Rz (µm) Residual peak stress (MPa) Characterization of the specimen surface before fatigue test High cycle fatigue properties Specimens with different surface states were then tested in high cycle fatigue to determine the Wohler curves (S-N curves) at stress ratios of -1 and.1. The stair case method was used to assess the endurance limit. Results show that endurance limit to tensile strength ratios for R = -1 are in range of.49 to.54 depending on the residual stress state. Shot peening leads to a small increase of the endurance limit (from.49 to.54). In any case all endurance limit to tensile strength ratios recorded on MaX are far higher than those recorded on similar carbon steels (.2 to.3). Endurance limit at R =.1 obtained for MaX in the condition 25/3N confirmed the excellent fatigue properties obtained with R = s (MPa) 8 7 s (MPa) N (Cycles) N (Cycles) 25/3N 15/2A as Q Rσ=.1 Rσ=-1 S-N curves at R =-1 for MaX with different surface states S-N curves at R =-1 and R =.1 for MaX with 25-3N shot peening treatment As-quenched 25/3N 15/2A R =-1 R =-1 R =.1 R =-1 e (MPa) 58 (22) 64 (13) 867 (1) 65 (9) e /UTS This outstanding fatigue behavior can be explained at least in part by the absence of coating such as Zn or Al-Si. Those coatings induce the introduction of brittle intermetallic phases at the surface of the material which leads to a drastic reduction in the near-surface mechanical properties that control the endurance limit. In addition MaX does not suffer from any decarburization at austenitization temperatures lower than 11 C and time lower than 5 min. This ensures that the near-surface mechanical properties are unchanged after heat treatment which explains the high endurance limit recorded. Light intensity 2, 1,5 1,,5 MaX 5min - Heating treatment + Air quench Carbon GDOES profiles obtained on samples heat treated 5 min at C during 5 min. No carbon depletion is measured until 11 C, Sputtering depth (µm) MaX-95 C MaX-1 C MaX-11 C 8

9 7. Oxidation & Corrosion resistance Due to its high chromium content, MaX oxidation resistance is 1 times larger than that of coated boron carbon steel as shown in the following figure. This property means MaX does not need to be coated to resist to high temperature austenitization treatment. 2 Mass increase (g/cm ) Time (min) Coated boron carbon steel MaX Oxidation at 95 C under air: MaX compare to Coated boron carbon steel; Hot stamping test on B-Pillar shows a very low oxidation Wet corrosion resistance seems to be moderate regarding the chromium content. MaX needs to be coated to resist to aggressive environment particularly with salt media. The following figure shows corrosion product obtained on blistered samples after phosphatation and cataphoresis treatments. The VDA test has been carried out during 1 weeks. MaX shows an intermediate behavior between Zinc (sacrificial) and Aluminium coating. Blistering After scraping MaX Coated boron carbon steel EZ 14 VDA Test 1 weeks on blistered samples. 8. Welding 8.1 Resistance spot welding Welding range and pull out 1Hz > > MaX thickness = 1.5 mm > > Coated boron carbon steel thickness = 1.2 mm > > Frequency = 1 Hz (Delta Spot) Assembly Welding range (ka) Current min (ka) Current max (ka) Pull-out resistance (dan) MaX/MaX Coated boron carbon steel/ Coated boron carbon steel MaX/Coated boron carbon steel

10 6 45 Hardness (HV) Distance (mm) MaX/MaX Coated boron carbon steel/coated boron carbon steel Weld nugget of MaX/MaX assembly Welding range and pull out 5Hz Hardness measurements > > MaX thickness = 1.2 mm > > Coated boron carbon steel thickness = 1.2 mm > > Frequency = 5 Hz (Delta Spot) Assembly Welding range (ka) Current min (ka) Current max (ka) Pull-out resistance (dan) MaX/MaX Coated boron carbon steel/ Coated boron carbon steel MaX/Coated boron carbon steel Laser welding > > MaX thickness = 1.2 mm > > Coated boron carbon steel thickness = 1.2 mm Laser welding of MaX/MaX Hardness (Hv) Ultimate strength (kn) Position (mm) Coated boron carbon steel MaX Comparison of hardness profiles MaX/Coated boron carbon steel Welding length (mm) Coated boron carbon steel MaX Tensile resistance of homogeneous assemblies Coated boron carbon steel/ Coated boron carbon steel and MaX/MaX as a function of welding length 1

11 8.3 MIG/MAG welding > > MaX thickness = 1.5 mm > > Wire = 41 stainless steel > > Tensile resistance of butt weld assembly = 178 MPa (91% of MaX tensile resistance) > > Endurance limit of butt weld assembly = 3 MPa (3 times higher than carbon steel assembly) Element C Mn Si S P Cr Ni Mo Cu N Wt% Typical chemical composition of 41 wire MIG/MAG welding micrograph of MaX with 41 wire 6 1 Vickers hardness (HV) Smax (MPa) Molten Zone Heat Affected Zone Base Metal 2,5 5 7,5 1 Distance (mm) Number of cycles Micro-hardness across the weld Wholer curve of weld butt configuration 11

12 215, Aperam Stainless Europe - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific authorisation of Aperam Stainless Europe. Photo credits: Fotolia (cover) - Others: Aperam Stainless Europe. Contact us Jean-Marc Herbelin T: +33() M: +33() E: jeanmarc.herbelin@aperam.com Aperam Stainless Europe contact@aperam.com Printed on FSC certified paper with vegetal based inks -