Cleavage delamination in impact tested warm-rolled steel
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I.
INTRODUCTION
W A R M working of steels by rolling in the subcritical and intercritical temperature range is considered to be a method for producing high-strength, ductile metals. Subcritically rolled steels show improved strength-ductility relationships when compared to cold-rolled steels of identical composition.~ Intercritical rolling is a step in certain controlled rolling procedures. Cleavage delaminations, or separations, have been observed on tension and notched impact fracture surfaces of warm-rolled steels. They appear as smooth splits propagating on internal planes parallel to the rolling plane usually separated by ductile, dimpled fracture in the main crack plane. 2-~6'~8'z~These studies show cleavage delaminations to be present in notched impact specimens over a range of test temperatures between 40 ~ ~6 and -157 ~ 2 The occurrence of cleavage delaminations is affected not only by test temperature but also by rolling temperature 2'2~ and rolling strain. 3'15'2~ Numerous mechanisms have been proposed to describe the cleavage delamination phenomenon. They include elongation of inclusions,4-8't6'~7deformation of austenite without recrystallization,5 grain boundary separation, 2'~~ and texture. 3,10,t2,t4'lS'20,32 The purpose of the present study is to delineate further the effect of warm-rolling temperature and strain on the crystallographic texture and cleavage delamination behavior of low carbon steels roiled in multiple passes below the At temperature. Additionally, by extending the existing texture based cleavage delamination theory, it will be possible to explain the observed fracture behavior of warm-rolled steel.
II.
EXPERIMENTAL
Billets of two compositions of 0.2 pct plain carbon steel were warm rolled at constant preheat temperatures T~ of 540 ~ 590 ~ and 650 ~ to true thickness strains e~ ranging between -0.61 (46 pct thickness reduction) and - 4 . 0 (98 pct thickness reduction). The details of the warm rolling are described elsewhere. 19.20Compositions of the two steels appear in Table I. A summary of the warm-rolling processing is given in Table II. D.L. BOURELL is Assistant Professor, Materials Science Program, University of Texas at Austin, Austin, TX 78712. Manuscript submitted January 13, 1983. METALLURGICALTRANSACTIONS A
Table I.
Compositions of Steel Used in This Investigation (Wt Pct)
PctC PctMn PctP PctS PctSi PctA1 PctN Steel A 0.20 0.46 0.011 0.043 - 0 . 0 0 6 0.008 Steel B 0.19 0.81 0.010 0.028 0.10 --Subsize Charpy impact specimens (2.5 mm • 10 mm) were machined to dimensions compliant with ASTM Standard E23-72. The test axis of all specimens was parallel to the longitudinal direction with the base of the notch parallel to the short transverse direction. The Charpy specimens were tested according to the procedure outlined in ASTM E23-72. The fracture surfaces of impact tested specimens were analyzed using optical microscopy. Selected {200} pole diagrams normal to the rolling plane were obtained from the Metals Division of the Army Materials and Mechanics Research Center, W
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