AHS Steels for Automotive Parts

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1 20-21June 2013 BITEC, Bangkok -Thailand AHS Steels for Automotive Parts Asst. Prof. Vitoon Uthaisangsuk, Dr.-Ing. Department of Mechanical Engineering, Faculty of Engineering King Mongkut s University of Technology Thonburi (KMUTT) 1

2 Outline Automotive evolution Car body structures Automotive steels Mild steels High strength steels Multiphase steels Press hardened steels 2

3 Outline Automotive evolution Car body structures Automotive steels Mild steels High strength steels Multiphase steels Press hardened steels 3

4 Materials used for car components Average 1975 vehicle weight 4 Source: Ducker Worldwide

5 Materials used for car components Average 2007 vehicle weight 5 Source: Ducker Worldwide

6 Trend of materials usage 6 Source: Ducker Worldwide

7 Aim of automotive industries (mass reduction) Source: WorldAutoSteel 7

8 Aim of automotive industries (mass reduction) Source: WorldAutoSteel 8

9 Aim of automotive industries (mass reduction) Source: WorldAutoSteel 9

10 Role of HSS and AHSS for vehicle mass reduction 10

11 Growth of AHSS 11

12 Increasing of safety regulations Evolution of North America crash tests 12

13 Safety concept 13

14 Safety concept Source : 14

15 Safety concept 15

16 Safety concept 16

17 Strength comparison AHSS vs conventional HSS Component material strength HSLA 350/450 DP 350/600 Yield strength (MPa) 17

18 Conflict between mass reduction & safety regulations Fuel economy / CO 2 emissions Lower weight / lighter gauges Safety Higher strength / heavier gauges Conflict between safety & fuel economy / CO 2 emissions Seeking of new challenging materials and manufacturing concept 18

19 Outline Automotive evolution Car body structures Automotive steels Mild steels High strength steels Multiphase steels Press hardened steels 19

20 Steel sheet concept for automotive parts Source : 20

21 Steel sheet concept for automotive parts 2010 The new Mercedes-Benz E-Class Source : 21

22 Steel sheet concept for automotive parts 2011 Volvo V70 Body Structure Source : 22

23 Steel sheet concept for automotive parts 2012 Audi A3 body Structure Source: 23

24 Steel sheet concept for automotive parts Source : 24

25 Steel sheet concept for automotive parts Source : 25

26 Cold rolled steels 26

27 Overview of cold rolled steel sheets 27

28 Strength evolutions of automotive steels Source: 28

29 Outline Automotive evolution Car body structures Automotive steels Mild steels High strength steels Multiphase steels Press hardened steels 29

30 Mild steels Best formability High and Ultra high strength steels 30

31 Mild steels 31

32 Mild steels for cold forming Specified value Typical value max. C P S Mn Ti Al Si N DC04 0,08 0,030 0,030 0,40-0,040 0,010 0,005 Microstructure: ferritic matrix Carbides (few) and AlN Mechanical properties: Low yield strength and tensile strength Low hardness Very high total elongation Great ductility and draw-ability YS MPa UTS MPa A 80% r 90 value n 90 value A g % >38% >1,6 >0,18 ~22 32

33 Mild steels for cold forming Specified value Typical value max. C P S Mn Ti Al Si N DC06 0,02 0,020 0,020 0,25 0,3 0,040 0,010 0,002 Microstructure: Ferritic matrix Precipitates (few) of micro-alloyed elements Mechanical properties: Low yield strength and tensile strength High n-value High r-value High uniform elongation Great ductility and draw-ability YS MPa UTS MPa A 80% r 90 value n 90 value A g % >41% >2,1 >0,22 ~25 33

34 Outline Automotive evolution Car body structures Automotive steels Mild steels High strength steels Multiphase steels Press hardened steels 34

35 High strength steels 35

36 Increasing strength of steel sheets 36

37 High strength steel by solid solution strengthening 37

38 Solid solution strengthening (mixed crystal strengthening) 38

39 Influence of alloying elements on the yield point substitutionally dissolved: P, Si, Cu, Mn, Mo, Ni, Al, Cr and interstitially dissolved: C, N 39

40 High strength steels by precipitation strengthening Formation 40

41 Increasing the yield point of a-iron through different types of precipitations 41

42 HS steels by work hardening (dislocation strengthening) 42

43 Dislocation density as a function of amount of deformation in pure iron Degree of deformation 43

44 Evolution of dislocation structure in deformed tensile samples The area next to the Lüders band is nearly without dislocation Dislocation in the area of uniform elongation Cell structure nearby fracture 44

45 Conventional high strength steels: Micro alloyed steels 45

46 Micro alloyed steels: alloying concept 46

47 Metallurgical process during hot rolling 47

48 Micro alloyed steels: properties, typical applications 48

49 HSLA steels Specified value Typical value max. C P S Mn Ti Al (min) Si Nb HX340LAD 0,11 0,030 0,025 1,00 0,15 0,015 0,50 0,09 Microstructure: Ferritic microstructure with a little pearlite Precipitates of micro-alloyed elements Mechanical properties: High yield strength High tensile strength Limited draw-ability and stretch-ability YS MPa UTS MPa A 80% r 90 value n 90 value A g % >21% ~1,0 ~0,14 ~14 49

50 Conventional high strength steel: High strength IF steels 50

51 High strength IF steels: alloying concept 51

52 High strength IF steels: properties, typical applications 52

53 Conventional high strength steels: Bake hardening (BH) steels 53

54 Schematic diagram of the bake hardening effect, by example of the production of a car door 54

55 Increasing strength by bake-hardening 55

56 Bake-hardening behavior of different steel grades 56

57 Bake-hardening steels: properties, typical application 57

58 Outline Automotive evolution Car body structures Automotive steels Mild steels High strength steels Multiphase steels Press hardened steels 58

59 Temperature in C Metastable iron - carbon (Fe-C) phase diagram H A I N B L + L + E L C L + Fe C 3 Fe 3 C (Cementite) D F a 800 M 700 a 600 G P O S a + Fe C 3 + Fe C 3 K 500 Q L 7 Carbon content in mass % Source: Pitsch, W.; Sauthoff, G.; Hougardy, H.P. in: Werkstoffkunde Stahl, Band 1: Grundlagen, Verein Deutscher Eisenhüttenleute, Springer-Verlag Cementite content in mass % 59

60 Transformation-Temperature-Time (TTT) diagram 60

61 Austenite 61

62 Bainite Ferrite - Martensite 62

63 Polymorphism of iron 63

64 Mechanical properties of different phases in steels Phase Re, MPa Rm, MPa A, % Hardening, HV Interstitial free condition (Ferrite) Ferrite (unalloyed) Ferrite (0,7% Ni, 0,6% Cr) about 180 Ferrite (13% Cr) Pearlite Cementite Bainite ( 0,1% C) about 320 Martensite ( 0,1% C) about 380 Martensite ( 0,4% C) Austenite (18% Cr, 8% Ni)

65 Homogeneity of grain structure - properties 65

66 Characteristics of AHSS 66

67 Microstructures of modern multiphase steels Higher strength 67

68 strain in % Microstructures vs mechanical properties of modern multi-phase steels F+P DP TRIP 10 CP MS Tensile strength in MPa ferrite perlite retained austenite bainite martensite 68

69 Adjustment of multiphase structures of hot-rolled strip 69

70 Multiphase steels 70

71 Characteristics of Dual Phase (DP) steels 71

72 Dual Phase steels: materials concept 72

73 Multiphase steels 73

74 Characteristics of TRIP steels 74

75 TRIP steels: TRansformation Induced Plasticity 75

76 Stress TRIP steels: The TRIP-effect Strain 76

77 Multiphase steels 77

78 Characteristics of complex phase steels 78

79 Complex phase steels: materials concept 79

80 Stress-strain behavior of automotive steels 80

81 Outline Automotive evolution Car body structures Automotive steels Mild steels High strength steels Multiphase steels Press hardened steels 81

82 Direct - indirect press hardening 82

83 Properties of press-hardened steel sheet 83

84 Temperature gradient during press hardening 84

85 Protection of blank surface 85

86 Demands on the surface coating 86

87 Changing of surface and microstructure 87

88 Trend of hot forming of steel 88

89 Hot stamping 89

90 Hot stamped of tailored blanks 90

91 Innovation Tailored Tempering 91

92 Press-hardened B-pillar 92

93 Body of Audi A4 93

94 Press-hardened parts in VW Passat 94

95 Example of a press-hardening plant 95

96 Heating of the blank 96

97 Blank B-Pillar after heating just before hot stamping 97

98 Hot stamping 98

99 Application of advanced high strength steels 99

100 Trend of advanced high strength steels World Japan 100

101 References - Jody N. Hall, Evolution of advanced high strength steels in automotive applications, General Motors Company, Chair, Joint Policy Council, Auto/Steel Partnership, Wagner S., Schmid P., Forming of high strength steel sheet, IFU, Stuttgart, Smith, W.F. and Hashemi, J., Foundation of metarials science and engineering, 5th edition, McGraw-Hill, ISBN Bleck, W., Material Science of Steel, RWTH Aachen University, 2007, ISBN:

102 Thank You Thank You 102