Microstructure of a quenched and tempered Cu-bearing high-strength low-alloy steel
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I.
INTRODUCTION
HIGH-strength low-alloy (HSLA) steels were developed to yield high strength at low cost. Some newer grades, such as the one used in this study, provide good low-temperature toughness and field weldability to meet the material demands for natural gas pipelines, ships, and off-shore drilling platforms exposed to arctic environments. Costs are kept low by microalloying and controlling the austenite-to-ferrite transformation to avoid the need for tempering. Some HSLA steels are controlled-rolled to achieve the desired ferrite grain structure, and precipitation-strengthened to reduce the amount of pearlite necessary for strengthening. 1.2,3 Recently the applications of HSLA steels have been expanding, and some attempts have been made to broaden their range of mechanical properties by heat treatment. 4'5 This paper describes the microstructures produced in a Cu-bearing HSLA steel similar to ASTM A710 Grade A (Armco Ni-Cop steel) by several heat treatments. It complements an earlier paper, which reported the mechanical properties of this steel. 5 The composition of this steel is given in Table I. It can be fabricated by controlled rolling to produce a fine-grained ferrite-pearlite microstructure (Figure 1), and aged to precipitate copper for added strength. Niobium precipitates as Table I.
Composition of Ni-COP Steel
Element Carbon Manganese Phosphorus Sulfur Silicon Copper Nickel Chromium Molybdenum Niobium Nitrogen
Content (Wt Pct) 0.04 0.49 0.010 0.009 0.27 1.17 0.90 0.70 0.20 0.040 0.007
M. T. MIGLIN is Senior Research Engineer, Babcock and Wilcox, Alliance, OH 44601. J. P. HIRTH, Professor, and W. A. T. CLARK, Associate Professor, are with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. A. R. ROSENFIELD is Research Leader, Battelle's Columbus Laboratories, Columbus, OH 43201. Manuscript submitted September 27, 1984. METALLURGICALTRANSACTIONS A
Fig. 1 --Optical micrograph of polished-and-etched (2 pct nital) section of the plane normal to the longitudinal direction in controlled-rolled plate.
carbonitrides in the austenite during hot rolling, retarding recrystallization and grain growth, and thereby reducing the final ferrite grain size. It also precipitates in the ferrite during and after transformation, which provides further grain refinement and precipitation strengthening. 6 Refinement of grain size strengthens the material without increasing the ductile-to-brittle transition temperature (DBTT) 5 Finish-rolling at a high temperature or cooling at a faster rate produces an acicular ferrite microstructure which is stronger, but has a higher DBTT and lower toughness. 1,5,7 After transformation the copper remains in solid solution. Aging between 500 ~ and 600 ~ allows the copper atoms to form clusters which increase the strength to a peak value within several hours or less. 9 Grain refinement by niobium additions has been studied in great detail. 6,9-16 The niobium carbonitrides are facecentered cubic (fcc), and those that form in austenite possess th
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