Martensitic carbon steels: Cold & hot formable alloys

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1 Martensitic carbon steels: Cold & hot formable alloys Prof. Dr. Hardy Mohrbacher NiobelCon bvba

2 ! Fundamentals Some facts about martensite Heat treatments Fracture and HIC Martensite is the strongest microstructure in carbon steel. Martensite is formed from austenite when the cooling speed is fast enough to prevent diffusion-controlled transformation. Alloying elements (B, Cr, Mo, Ni) reduce the critical cooling speed for martensite transformation. As-quenched martensite has limited ductility and toughness. Martensite is a non-equilibrium phase that is sensitive to heat treatment (tempering, welding, paint baking, ). Properties of martensitic steels improve as the grain size is refined.

3 ! Fundamentals Heat treatments Fracture and HIC The strength of martensite in low carbon steel Hardness (HV) Martensite hardness Ms temperature HV = (%C) 22MnB5 MS self-tempering potential 30MnB5 Ms = (%C) 33(%Mn) Carbon content (wt.%) Ms temperature ( C) Hardness (strength) of fully quenched martensite increases nearly linearly with the carbon content. Start temperature of martensite transformation decreases with the carbon content. At lower carbon contents self-tempering occurs under industrial quenching conditions.

4 ! Fundamentals Heat treatments Fracture and HIC Hardenability by boron and boron-molybdenum synergy 22MnB5 (+0.25%Cr) / reheating to 950 C Temperature C K/s HV 05 A + B M F P Critical cooling rate for 90% martensite hardness ( C/s) %C 0.25%Si 1.3%Mn / reheating to 950 C no Mo 0.25% Mo 0.50% Mo Time (s) Boron content (wt. ppm) 30

5 ! Fundamentals Heat treatments Formability of martensitic carbon steel Fracture and HIC % Martensite 100% Min. elongation A80 (%) C% Major true strain DC04 600DP 800DP 1000DP 1200M 1400M Tensile strength (MPa) Minor true strain

6 ! Fundamentals Heat treatments Fracture and HIC Toughness behaviour of fully martensitic steel Crystallinity of fracture surface (%) Ductile-to-Brittle transition range 0.22%C, Mn, Si, Cr, B Impact energy (J/cm 2 ) Test temperature ( C)

7 ! Fundamentals Heat treatments Fracture and HIC Toughness behaviour of fully martensitic steel 120 Impact energy (J/cm 2 ) %C 0.40%C low Phosphorous low Sulphur low Nitrogen Niobium added Hardness (HV) Increase of carbon content always reduces toughness. Reduced levels of impurities (P, S, N) improve toughness. Structural refinement (Nb addition) improves toughness.

8 Fundamentals! Heat treatments Fracture and HIC Effect of tempering on hardness (strength) of quenched carbon steel Hardness (HV0.2) % C 0.18% C BH 0.10% C C segregation (C < 0.2%)!-carbide ppt. (C > 0.2%) rod shaped carbide ppt. Mo, Cr, V, Ti, Nb, W recovery spheroidal Fe 3 C ppt. secondary ppt. of special carbides recrystallisation carbide coarsening Tempering temperature for 1 h ( C) Tempering < 200 C: Medium carbon steels (C > 0.2%) show strength decrease by precipitation of!-carbides. Low carbon steels (C < 0.2%) are less sensitive due to: self-tempering during quenching, bake hardening. Tempering > 250 C: Precipitation of cementite, first needles then spheroidal shaped. Tempering C: Secondary precipitation of special carbide formers increasing strength. Tempering > 600 C: Coarsening of precipitates resulting in loss of strength.

9 Fundamentals! Heat treatments Fracture and HIC Tempering treatments on hot stamped components 22MnB5 tempered for 3600 s after Säglitz, Matlock, Krauss: AIST 2008

10 Fundamentals! Heat treatments Fracture and HIC Tempering treatments on hot stamped components after Säglitz, Matlock, Krauss: AIST 2008

11 Fundamentals! Heat treatments Fracture and HIC Tempering treatments on hot stamped components after Säglitz, Matlock, Krauss: AIST 2008

12 Fundamentals! Heat treatments Fracture and HIC Effect of low temperature tempering on strength and ductility Strength (MPa) paint baking temp. A 80 TS 0.21%C, Mn, Si Elongation (%) Impact energy (J/cm 2 ) paint baking temp. R min low Si high Si min. bending radius (xt) Tempering temperature ( C) Tempering temperature ( C) Low temperature tempering: reduces strength leads to temper embrittlement Paint baking temperature appears in this respect uncritical

13 Fundamentals! Heat treatments Fracture and HIC Tempering treatments on hot stamped components YR: Elongation is not the most important property. Ref.: Laumann, Pfestorf, BMW AG, WAMM 2008 Resistance against crack initiation and propagation is more relevant.

14 Fundamentals Heat treatments! Fracture and HIC Microstructural effects on properties " Yield strength " yield = " 0 + K yield 1 " Fracture resistance " fracture = K fracture 1 d eff d eff " Ductile-to-Brittle Transition temp. T DBT = T 0 + K DBT 1 d eff The effective size d eff : is differently defined for each of the properties, relates to microstructural details, is not easy to measure. The practical message is: keep the prior austenite grain size small (Nb addition), avoid large particles (like TiN), minimize inclusions (like MnS, Al 2 O 3, SiO 2 ), reduce P to low levels and/or add Mo.

15 Fundamentals Heat treatments! Fracture and HIC Structure and structural refinement in martensitic steel Martensitic microstructure High-angle martensite packet boundary Martensite lath boundary (low-angle) Austenite grain boundary before quenching Fracture proceeds along lath boundary Crack deflects at packet boundary critical fracture stress = C " Smaller prior austenite grain: " Smaller packet size " Higher fracture resistance 1 packet size Prior austenite grain size (PAGS) Std. Boron steel (22MnB5) Boron steel with Nb addition

16 Fundamentals Heat treatments! Fracture and HIC Failure avoidance in martensitic carbon steel TiN TiN 2 "m avoid large particles and inclusions reduce crack intiation MnS coarse carbides, nitrides impede crack propagation P segregation to PAGB refine PAGS (add Nb) avoid P segregation (add Mo)

17 Fundamentals Heat treatments! Fracture and HIC Microstructural defects leading to delayed cracking elongated inclusion (e.g. MnS) Surface surface or peripheral cracks G.B. oxide inclusion (e.g. Al 2 O 3, SiO) cracked or delaminated hard or brittle phases (e.g. cementite, TiN) Matrix 2 nd phase voids, micro-cracks, phase delamination, dislocation pile-ups no hydrogen applied + residual stress with hydrogen applied + residual stress + internal pressure existing defect fracture process zone hydrogen recombination = build-up of internal pressure H H H H H H H H H H H H H H H cohesive strength cohesive strength reduced due to adsorbed or trapped hydrogen

18 Fundamentals Heat treatments! Fracture and HIC Improving stress-corrosion cracking resistance in martensitic steel Bent-beam test in corrosive medium 15 : Without Nb PAGB Carbide distribution after tempering Performance value (Sc) 10 better (920) (1000) (920) (980) : With Nb ( C) : Austenitizing temp. Without Nb (1000) With Nb Sumitomo Steel 1988 Prior austenite grain size ("m)

19 Fundamentals Heat treatments! Fracture and HIC Improving stress-corrosion cracking resistance in martensitic steel Corrosion fatigue life (relative value) Strength (MPa) %C 1.8%Si, 1.0%Cr, 0.2%Cu, 0.5%Ni, V, Ti -(Cr, Ni) +Cu +(Si, Cr, Cu) 0.5%C Nb, Ti Ti -(Cr, Cu, Ni) C, Si increase strength lower C gives better toughness Cr, Cu, Ni reduce corrosion reaction Nb, Ti, V carbides provide hydrogen trapping Hardness after tempering (HRC) Ascometal, Kobe Steel 2008

20 Fundamentals Heat treatments! Fracture and HIC Effects of grain boundary phosphorous segregation % C, 1.3% Mn, 0.03% P Corrosive solution (NACE) YS 720 MPa 0.007% P, 0.15% Mo PAGB phophorous concentration (w.t.%) no Mo +0.15% Mo Applied stress (MPa) tempered at 700 C for 30 sec % P, 0.15% Mo 0.015% P, 0% Mo Tempering for 30 sec. at T ( C) Time to failure (h)

21 Fundamentals Heat treatments Fracture and HIC Managing crack initiation and propagation! Tailored mix of hard (martensite) and ductile phases: retard crack initiation stop crack propagation. Dispersion of ductile islands in martensitic matrix. Stops crack propagation. Possible process: Quench & Partition (Q&P) Ductile surface layer on martensitic core. Stops crack initiation under bending condition. Possible process: surface decarburization, cladding Hybride blank with martensitic and ductile sectors. Programmed deformation zone outside martensite. Possible process: laser blank welding Tailored tempering. Possible process: laser or induction heating

22 Fundamentals Heat treatments Fracture and HIC Martensite phase morphology in DP steel! Bad Good M F 0.15%C-Mn-Cr 10 "m 0.15%C-Mn-Cr-Nb 10 "m Crack can propagate along martensite line to macroscopic length. Crack is blunted in soft ferrite and cannot propagate.

23 Fundamentals Heat treatments Fracture and HIC! Surface decarburization treatment for improved bendability 0.5%C-steel / 2.0 mm gage 300 C 180 Hardness (HV) C after Q&T treatment 500 C 600 C Tempering temperature Bending angle (deg) r/t = Decarburized layer before Q&T treatment C 500 C Bending radius (mm) Distance from surface ("m) after Wada, Shiraishi and Onishi KAWASAKI STEEL GIHO Vol.13 (1981) No.2

24 Fundamentals Mutli-layer steel composite Heat treatments Fracture and HIC! Compose package of different steel grades into an assembled slab ductile steel grade hardenable steel grade Consolidate individual layers by slab reheating and hot rolling Result: Hot rolled or cold rolled sheet with different strength and ductility across thickness according to required function in final application.

25 Fundamentals Heat treatments Welding of martensitic carbon steel Fracture and HIC! Laser butt weld Welding of martensitic steel always leads to tempering effects in the HAZ. Hardness (HV) mm Position transverse to weld Consequences: Local loss of strength, One-step temper embrittlement ( C), Phosphorous segregation to PAGB ( C). Keep heat input as small as possible (laser welding preferrable). Post-weld heat treatments can do more harm than good.

26 Fundamentals Inverted process sequence Heat treatments Fracture and HIC! First welding... ThyssenKrupp Tailored Blanks GmbH then stamping and hardening No hardness sinks next to weld seam 600 Tailored hybrid blank PH steel C: 0.22% 600 Press hardened component Laser seam Hardness HV HSLA C: <0.10% Hardness HV HSLA PH Heating (950 C) & die quenching HSLA PH

27 Conclusions Martensite is the strongest type of steel. Using it in significant amounts will reach the limits of body weight saving potential. Martensite is sensitive to crack initiation and propagation. Microstructural design can be used to optimize the performance. Martensite is sensitive to heat changing properties even after treatments at low temperatures. Manufacturing processes and sequences need to be adapted. The alloying concept has a relatively big influence on martensite properties. Nb and Mo alloying is particularly beneficial. The cheapest alloying concept is not necessarily the best solution. Substantial experience with martensite is available outside the automotive sheet steel business, which can be readily used.