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Lecture 6:Strengthening of Plain Carbon Steels

MMat 380

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Contributions to strength in steels

Base material

Solid solution strengthening (i.e., %Mn)

Grain size (ferrite)

Precipitates (distance between ppts)

Cold work (dislocation density)

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Steels - Fe-C-Mn alloys

Mn added as ferromanganese Helps to de-sulpherize steel MnS

(Fe-S brittleness)

Powerful solid solution strengthener Powerful effect on heat treating med. %C

(0.3%C ) and high %C steels which are

usually Q&T

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1xxx - %Cin steel

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Strengthening low C steels(0-0.3%C)

2 major ways: increase carbon content

sacrifice % elongation; toughness because of Fe 3C

decrease grain size increase strength but doesnt affect ductility Hall-Petch Equation d = grain size (mm)

better strength without sacrificing ductility and

toughnesstrend: use fine grained steels and lower C content

2/1+= kd o y

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Examples

500Very fine12-13

300Fine8

250Coarse5

y (MPa)GrainASTMNo

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Grain size and strength

335 MPa 540 MPa

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ASTM grain size

Grain boundaries act as barriers todislocation motion

n=2 N-1 n = # grains/in 2 @ 100x magN = ASTM grain size No

Note: Table 2.1 pg. 74 Smith

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ASTM grain size

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Unit conversions

ASTM # 116 grains/mm 2 x (25.4) 2 = 10 323 gr/in 2

= 1.03 gr/in 2 @ 100x

ASTM # 82048 grains/mm 2x(25.4) 2 =1 321 287 gr/in 2

= 132 gr/in 2 @ 100x not 128

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ASTM grain size

1 3 6 8 10

Same magnification

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Effect of grain size on strength

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Applications

Heattreated0.6-0.91030

Structural0.3-0.61010-1025

Sheet0.25-0.41006

Product%MnGrade

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Low carbon steels (

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Low carbon steels (

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HSLA steels

Solid solution hardening (Mn) Decrease ferrite grain size by

Controlled rolling Controlled cooling

Precipitation hardening Nb (C,N) VC

Typical x70 pipeline steel%C = 0.06; %Mn = 1.50; %Nb and/or V ~0.04Controlled rolling to produce very fine grain size

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Strengthening HSLA steels

Obtain fine grain size(ASTM 10-13) by: Controlled rolling Controlled cooling

Can increase yieldstrength by 100-134MPa (i.e. 300-440 MPatotal)

670P

600Be

100Si

94Mn

80Ni

67Ti

40Al27Mo

20W

13.4V

13.4Co

6.7Cr

y increase of

per 1% additionElement

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Controlled rolling

Normal finish T in hot rolling: 900-1000 C Finish at temperatures: 750-800 C

Lower temperature R x & grain growth after hot rolling takes

longer end up with smaller g grain size\ smaller

grain size

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Controlled rolling

transformation of deformed gives finest grainsize

want to T of R x and make R x more difficult Nb in small amounts does this (~0.04%Nb added)

Need massive roll force to give required deformation Controlled rolled plate typically < 1 in thickness Thick plate usually has larger grain size

because it is finished at a higher T and has a larger grain

size

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Controlled rolling schematic

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Controlled cooling and grain size

Slow coolingHigher transformation

temperature

Fast cooling

Lower transformationtemperature

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Thick sections

Why do thick sections have lower yield stress?

When finish rolling at higher T (larger grain size) thick sections cool slower

raises T transf fewer nuclei grow to larger grain

size lower y

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Precipitation hardening

Microalloying Nb, V, Ti Nb (C,N) precipitate during hot rolling in

restricts grain growth

refines grain size retards R x & raises T transf Pancake grains

nuclei closer together therefore finer R x grain size

V VC precipitate on cooling after rolling

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Strength in HSLA steels

+ Standard C-Mn Steel 200-300 MPa+ Decrease grain size 100-134 MPa+ Increase Mn 67 MPa+ Increase Nb, V, Ti ppt hardening 67-100 MPa

Total: 434-600 MPa Can now afford to lower the C content and still

have 470-500 MPa steel

Can have any strength level wanted by varyingthe degree of strengthening components

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Effect of austenite grain size

Deformed

Small

Large

Ferrite grainsAustenite grains TRANSFORM TO