Development of Hot Rolled Thin Steel and High Strength PosMAC Steel for Steel Structures

Transcription

1 Development of Hot Rolled Thin Steel and High Strength PosMAC Steel for Steel Structures Part 1. PO Martensite Steel Part 2. PosMAC Martensite Steel Kyong Su Park, Jaeyong Chae, Chul Min Bae POSCO Technical Research Lab. POSCO the Great POSCO Research Laboratories

2 Part 1. PO Martensite Steel Why have to use the martensitic steel? For the protection of passengers in accordance with the enhancement of crash regulations Application parts: Possible to apply the martensitic steel to requiring anti-intrusion parts Bumper beam, Sill Side, Door Impact Beam, Roof Cross Member etc. Forming method: Roll forming, Crash forming etc. Benefits: Possible to produce the parts at low cost compared to HPF [Schematic of application parts]

3 PO Martensitic Steel: Endless Hot-Strip Rolling Technology Concept Continuous hot rolling technology by joining bars after a rough rolling process ( 06~) Unique joining method by shear deformation at high temperatures Roughing Mill Coil Box Crop Shear Leveler Joining Machine Finishing Mill ~ Laminar Flow Down Coiler Partial Descaler Crop Remover F1~7 High Speed Shear [ Pohang #2 HR Mill with endless rolling joining equipment ] Benefits Minimizing the quality deviation in the longitudinal direction of the strip (Material properties, flatness and size deviations) Thinner thickness compared to general hot-strip rolling processes Grade THK. [mm] Chemical Composition Mechanical Properties C [wt.%] Mn [wt.%] C eq. TS [Mpa] YS [Mpa] El. [%] R/t THK. Tol. PO1300M 1.2~3.2 ~0.2 ~1.5 ~ ~ ~1300 3~ ~ m PO1500M 1.2~3.2 ~0.25 ~1.5 ~ ~ ~1500 3~ ~ m 1) Bendability = Bending radius / Thickness 2) Carbon Equivalent(Ceq) = C +Mn/6 +Si/30 +(Cr +Mo +V +Nb)/5 +(Cu +Ni)/15

4 Strength (MPa) Strength (MPa) PO Martensitic Steel by Endless rolling Product Quality Good surface quality and good flatness D/S Center 50 m W/S [1.2mmt HR Coil] [Trimming process of PO product ] Microstructure and Mechanical properties variation Cooling Rate 70 C/s 430 Hv, Fully Martensite TS YS [Thickness variation of PO Coil] TS YS Head Middle Length Tail 1000 W/S Center Width D/S

5 Material Properties Tensile/ Bending Typical Tensile/Bending Properties (ASTM Standard) Grade Direction YS [Mpa] TS [Mpa] PO1300M PO1500M Uniform El. [%] Total El. [%] r-value Bending Test Result Grade Direction Min R/t C-dir 2.9 PO1300M L-dir t Not Good Good PO1500M C-dir 3.8 L-dir 3.8 Bendability = Bending radius / Thickness

6 Material Properties Hydrogen Delayed Fracture Low Delayed Fracture Risks under high applied stress and corrosion environment U-bending tests Diffusible hydrogen content [Applied high stress level] [Applied low stress level] Post test photos (300hr, 0.1N HCl) 1.1UTS 1.0YS Changes in Thickness 0.8YS 0.6YS Thickness loss of about 0.32 mm No HDF under 1.1 UTS and diffusible hydrogen content of 0.25 ppm in PO1300Mart

7 Review of mass production applicability Try-out for Automotive Part Application Part Name Bumper Beam Process Material Thickness Roll Forming PO1500Mart 1.4 mm [ Flatness, PO 1500MART ] [ Bumper Beam, PO 1500MART ] [ Roll Forming, PO 1500MART]

8 Part 2. PosMAC Martensite Steel What is PosMAC? (POSCO Magnesium Aluminum alloy Coating product) Ternary alloy coated steel - Zn - 3%Mg - 2.5%Al High Corrosion resistance - 5~10 times stronger resistance than GI steel

9 Banana Diagram [Ref.] Max Planck Paper

10 Hot-rolled Plating Steel with High Strength [Ref.] Catalogs Trend : High strength and light weight However, very difficult to reduce in thickness - Increase of hot rolling load - Problem of hot-rolled strip control Company Grade Thickness TS YS EL A B C D YS t ~ YS t ~ YS t ~ YS t ~ TS t ~ TS t ~ TS t ~ TS t ~

11 Thin PosMAC Martensite steel Applied two technologies Endless Hot Rolling Technology - Solve rolling problem of coil front and rear end - Very advantageous for thin hot-rolled strip Transformation strengthening - Low strength in the hot rolling stage : Thinner strip - High strength after cooling and tempering Trial 1st 2nd 3rd Condition TS YS EL (MPa) (MPa) (%) Surface Quality Defects OK Weak defects OK OK OK Weak defects

12 Thin PosMAC Martensite steel

13 Thin PosMAC Martensite steel Manufacturing Process PO PosMAC Iron Making Steel Making Hot Rolling Omitting Cold Rolling & Annealing PO/PosMAC Steel Strip CR Iron Making Steel Making Hot Rolling Cold Rolling Continuous Annealing CR/GI Steel Strip

14 POSCO the Great POSCO Research Laboratories