Effect of phosphorus on the mechanical properties of normalized 0.1 pct C, 1.0 pct Mn steels
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EVIOUS
work1,2 showed that the addition of up t o 0.2 pct phosphorus increased the yield strength of iron about 100 M P a (14.5 ksi) at room temperature and above without significantly affecting the notchimpact transition temperature. Below room temperature solid-solution softening occurs in Fe-P and F e - P - T i alloys, as it does in many other binary iron alloys. To investigate further the strengthening effects of phosphorus, a s e r i e s of 0.1C-1.0Mn steels containing up t o 0.2 pct phosphorus were rolled to 10 mm-thick (0.38 in.) plates and investigated in the normalized condition. This paper describes the p r o p e r t i e s of the resulting steels and discusses the appropriate strengthening mechanisms. MATERIALS
AND
PROCEDURES
T w o 14 kg (30 Ib) v a c u u m - m e l t e d heats with an a i m base composition of 0.I pct c a r b o n and 1.0 pct m a n g a n e s e were split t o obtain steels with four phosphorus levels (0.001, 0.05, 0.1 and 0.2 pct). The chemical compositions of the steels are shown in Table I. T h e s e steels were hot-rolled in s e v e r a l passes with reheats from the 89 m m diam (3.5 in.) ingot to 13 mm (0.5 in.) plate at a temperature of 1200°C (2192°F) and from 13 mm t o 10 mm (0.5 to 0.38 in.) in one pass at ll00°C (2012°F). The steels were then normalized at 925°C (1697°F) for 1.2 h. Tension specimens with a gage length of 25 mm (1 in.) and a d i a m e t e r of 6 mm (0.250 in.) and halfwidth Charpy V-notch specimens were machined from the plate. The long dimension of the Charpy specimens was parallel to the rolling direction, and
W. A. SPITZIG and R. J. SOBER are with the Research Laboratory, U.S. Steel Corporation, Monroeville, PA l5146. Manuscript submitted October22, 1976. METALLURGICAL TRANSACTIONS A
the axis of the notch was perpendicular t o the plate surface. The resulting microstructure was a mixture of f e r r i t e and pearlite and the g r a i n size was ASTM No. 6 in all the steels. Tension tests were done on a Instron machine at a s t r a i n rate of 1.25 × 10-4/s. In strain-rate change tests, the specimen s t r a i n r a t e s were increased from 2.5 × 10-s t o 1.25 x 10-4/s and from 1.25 × 10-4 t o 6.25 × 10-4/s. The specimens were i m m e r s e d in controlledtemperature liquid baths for testing below 22°C (72°F) and in silicon oil for testing above 22°C up to 200°C (392°F). At 250 to 400°C (482 to 752°F) a high-heatingrate quartz lamp furnace was used to heat the specimens during testing. RESULTS Fig. 1 shows the effect of phosphorus on the notchimpact properties of normalized 0.1C-1.0Mn steel. The phosphorus additions r e s u l t in a more g r a d u a l transition region between completely brittle and ductile fracture than that occurring in the base alloy. S i m i l a r behavior is observed with other alloy additions~'4 and with g r e a t e r additions of phosphorus t o i r o n .~ Because the transition mode changes from an abrupt transition t o a more gradual transition, it is more difficult to a s s e s s the transition temperature of the phosphorus-containing
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