Formability

Formability - the capacity of steel to sustain transverse or longitudinal bends without failure - depends on strip ductility. This is developed through proper specification of analysis, hardness and microstructure. Also, formability is greatly influenced by the manufacturing conditions: fabricating method (e.g., stamping, piercing, coining), orientation of bending axis to direction of grain, and the bending radius relative to stock thickness and hardness. It is important to specify and understand all factors affecting formability.

Formability of untempered strip is determined by a bend test. A sample is slowly bent over 180° until its ends are parallel. The measured distance between ends (Nt) is the multiple of strip thickness (t) that should be specified, for
transverse (  |  ) and longitudinal (=) bends.

Formability test - Untempered strip

The first table lists mechanical properties for various thicknesses and grades of as-rolled, annealed and BarcoForm® steel; the second table adds formability values for the same annealed and Barco-Form grades. (Barco-Form is a spheroidized annealed product of Theis Precision Steel.) Obviously, the low-hardness, annealed materials have the highest ductility and will accept the most severe bends.

The second table indicates that Nt/t is 2, parallel to the grain (=), when t=0.040" (1.016 mm). Therefore, the measured distance between parallel ends Nt=2 X 0.040" (1.016 mm), or 0.080" (2.032 mm).


Mechanical property ranges of annealed and hard rolled carbon steel

TRM and range of carbon and manganese content
    TRM-14
C .47-.56
Mn .60-.90
TRM-31
C .65-.75
Mn .35-.50
TRM-34
C .68-.80
Mn .50-.80
TRM-54
C .90-1.05
Mn .30-.50
Strip
Thickness
HDNS
or UTS
As hard
rolled
Ann. Barco
Form
As hard
rolled
Ann. Barco
Form
As hard
rolled
Ann. Barco
Form
As hard
rolled
Ann. Barco
Form
.076" RC 14-29     16-30     16-30     16-32    
(1.93 mm) RB 95-106 85 80 97-107 87 85 97-107 89 87 95-107 92 90
and UTS (KSI) 95-135 85 80 105-105 87 84 105-105 88 85 105-160 95 92
over % EL-2 XX MIN   20 28   15 20   15 20   15 20
.036" RC 14-27     15-29     15-29     15-30    
(9.14 mm) RB 95-105 85 80 95-105 87 83 97-105 89 86 97-107 92 88
and UTS (KSI) 97-130 85 80 105-145 87 83 105-145 87 85 97-155 95 90
over % EL-2" Min   20 28   15 20   15 20   15 20
0.15" 30T 74-84 73 70 78-84 74 72 78-84 75 73 78-85 77 74
(0.381 mm) 30N 95-130 85 80 90-118 85 83 92-118 85 83 97-150 92 85
To .035" UTS (KSI)   20 28   20 25   20 25   15 20
(.889 MM) % EL-2" Min                        
.008" 15T 90-93 87 86 90-93 87 86 90-93 88 87 90-94 91 89
(0.203 mm) 15N 90-125   78 89-118   80 89-118   83 89-125   90
To .014" UTS                        
(0.356 mm) (KSI)                        
Under .008" DPH-KG 210-290   175 210-290   175 210-290   175 210-320   180
(.203 mm) 2KG                        


Formability values for annealed grades

Thickness, t Direction Nt/t Nt/t Nt/t
inches (mm) of bend Ann BarcoForm Ann BarcoForm Ann BarcoForm
.076" (1.93mm)   |   2 0 2 0 2 0
and over = 4 3 4 3 4 5
0.036-0.075"   |   1 0 1 0 2 0
(.914-1.905 mm) = 2 1 2 1 3 2
0.045-0.036"   |   0 0 1 0 1 0
(0.381-0.914 mm) = 1 0 1.5 1 2 1
0.008-0.014"   |   0 0 1 0 1 0
(0.2037-0.356 mm) = 0 0 1 0 1 .5

Formability of pretempered strip

Both bend and break tests are used to measure the formability of pretempered steel, with the choice usually based on relative strip hardness.

Another means of judging the formability of pretempered material is the vise break test, in which a sample strip is bent into a "U" shape in the jaws of a vise until it breaks. The vise break distance - space between jaws at fracture - is specified in terms of a "go" and "no-go" range which defines the required ductility. A large break distance indicates low ductility; a small distance represents a steel with better ductility but lower resistance to service stresses. (The specified vise break range must be compatible with the specified hardness range.)

The following tables shows formability values for 0.70 to 1.05% carbon strip, plotting the break ratio V/t (vise break distance divided by strip thickness) against hardness.

Hardness vs. approximate vise break of
hardened and tempered high carbon steel strip

The following are limited to strip with a width to thickness greater than 10:1.

EXAMPLE:
Specify hardness:
RC 48 to 50. Strip thickness:.045" (1.14 mm)
STEP 1: Determine average hardness in example RC 49
STEP 2: From table .035 to .070, RC 49 factors are 14.9 (low) and 20.8 (high)
STEP 3: Thickness .045 x low and high factors are .670" / .935" (17.02 / 23.75 mm) [range .265" (6.73 mm)]
STEP 4: For limits more restrictive than those determined in STEP 3, contact Theis Precision Steel.

Thickness: 0.035 - 0.070"
(.89 - 1.78 mm)
  Thickness: 0.015 - 0.030"
(.38 - .89 mm)
  Multiplying Factors  
RC Min Max   30N Min Max
43.0 10.0 15.8   64.0 10.0 15.8
43.5 10.4 16.2   64.5 10.9 16.7
44.0 10.9 16.6   65.0 11.5 17.2
44.5 11.1 16.9   65.5 11.8 17.4
45.0 11.6 17.3   66.0 12.2 18.0
45.5 11.9 17.8   66.5 12.8 18.6
46.0 12.2 18.1   67.0 13.1 19.1
46.5 12.8 18.6   67.5 13.9 19.7
47.0 13.1 19.1   68.0 14.0 20.1
47.5 13.6 19.5   68.5 14.8 20.7
48.0 14.0 20.1   69.0 15.0 21.0
48.5 14.4 20.3   69.5 15.8 21.7
49.0 14.9 20.8   70.0 16.0 22.0
49.5 15.3 21.3   70.5 16.4 22.4
50.0 15.8 21.8   71.0 16.8 22.7
50.5 16.2 22.1   71.5 17.2 23.1
51.0 16.8 22.7   72.0 17.8 23.7
51.5 17.2 23.1   72.5 18.1 24.0
52.0 17.8 23.7   73.0 18.4 24.3
52.5 18.1 24.1   73.5 18.7 24.6
53.0 18.8 24.5   74.0 19.1 25.2
53.5 19.1 25.1        
54.0 19.8 25.6        

Thickness: 0.010 - 0.014"
(.254 - .356 mm)
  Thickness: < 0.009"
(.23 mm)
  Multiplying Factors  
15N Min. Max. DPH Min. Max. KSI
82.0 10.0 15.7 423 10.0 14.0 200
82.5 11.1 16.9 434 11.2 15.2 210
83.0 12.1 18.1 458 12.2 16.0 220
83.5 13.1 19.0 484 13.5 17.4 230
84.0 13.5 19.4 498 14.5 18.5 240
84.5 14.2 20.0 525 15.6 19.6 250
85.0 15.0 21.0 544 16.6 20.7 260
85.5 15.8 21.8 560 17.5 21.6 270
86.0 16.8 22.6 577 18.5 22.6 280
86.5 17.2 23.1 600 19.5 23.5 290
87.0 18.2 24.1 615 20.5 24.6 300
87.5 19.1 25.1 645 21.5 25.4 310
88.0 19.8 25.6 675 22.2 26.4 320
88.5 20.8 26.7 697 23.5 27.3 330
89.0 21.2 27.0        
89.5 21.8 27.5        
90.0 22.1 28.0        


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