Properties and aging tendency of sheet steel annealed in humid hydrogen

Conclusions

• 1. Annealing of cold rolled 08kp in humid hydrogen combined with recrystallization annealing lowers the yield strength and the ratio σ_T/σ_b, increases the ductility, and reduces the tendency to strain aging,

  TABLE 3

  Annealingcondition	Tangent of slopeof ADIF curveafter aging, tgαC	Change in slopeof ADIF curveafter aging, ΔtgαC	Critical ampli-tude of fracturein aged condi-tion, γcr.c· 106
  900°C 1 h	2,9	2,6	50
  800°C 5 h	3,7	14,9	45
  700°C 4 h	5,2	4,1	47
  700°C 8 h	10,5	6,5	35

  TABLE 4

  Annealingtemperature, °C	Annealingtime, h	Average graindiameter, d·103, mm	Grain size(grade) speci-fied (GOST5639-65)	Difference ingrain size,S·103
  700	1	15	9	0,18
  	3	28	7–8	0,19
  	5	27	7–8	0,32
  	8	35	6–7	0,29
  800	1	26	8	0,62
  	3	37	6–7	1,99
  	5	45	6	1,17
  	8	40	6	6,57
  850	1	30	7	1,17
  	3	47	5–6	8,02
  	5	45	6	20,8
  	8	67	5	68,9

  which is due to the lower concentration of carbon and nitrogen atoms in normal interstitial positions and also to the lower total concentration of these elements in the steel.

• 2. A minimal tendency to strain aging with a fairly low original value of the yield strength in combination with the optimal ferrite grain size for good formability and the minimal difference in ferrite grain size are provided by annealing at 700°C for 4–8 h. The dislocation mobility in the aged condition is highest in this case.