Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: P
- PDF / 4,141,865 Bytes
- 12 Pages / 630 x 792 pts Page_size
- 97 Downloads / 261 Views
I.
INTRODUCTION
D U A L - p h a s e steel technology produces multiphase structures consisting of large amounts of ferrite, small amounts of martensite, and other phases in steels which have been intercritically annealed. [1'2,31 These microstructures have better combinations of high strengths and ductilities than do ferrite-carbide microstructures in coldrolled and annealed steels. In an effort to further improve ductility at high strength levels, isothermal transformation of austenite after intercritical annealing has been conducted. [4-1~ The isothermal treatments cause the intercritically formed austenite to transform to bainite, and frequently some retained austenite is incorporated into the transformed microstructures, t4'5'7-1~ The beneficial effect of deformation-induced transformation of austenite to martensite in highly alloyed steels is well known, m-18] However, in low-carbon sheet steels, it has been difficult to retain sufficient amounts of austenite to significantly enhance ductility. [19-32]In an effort to increase the amount of retained austenite and to extend the benefits of intercritical annealing and isothermal transformation, two steels of special compositions were selected for study. One steel contained 4 pct Ni, an alloying element which is well known for its ability to stabilize austenite. In addition to the fully austenitic AISI type 300 stainless steels which contain more than 6 pct Ni, m-:5] cryogenic steels with 5 to 9 pct Ni are produced to retain significant amounts of austenite in tempered martensitic microstructures. [33,34] The other steel contained 1.2 pet Si and
YASUHARU SAKUMA, Visiting Scientist with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, is Senior Researcher, Nippon Steel Corporation, Chiba-Ken 299-12, Japan. DAVID K. MATLOCK, Charles F. Fogarty Professor, and GEORGE KRAUSS, Professor, are with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted May 17, 1991. METALLURGICAL TRANSACTIONS A
1.5 pct Mn. These two alloying elements are commonly used in commercially produced dual-phase steels. Manganese behaves similarly to Ni as an austenite stabilizer and also enhances hardenability, t35] Silicon retards the formation of cementite [36] and thereby causes the retention of austenite in bainitic microstructures. [37-46] The purpose o f this part of the study is to evaluate the microstructures and room-temperature mechanical properties of steel sheets alloyed to incorporate retained austenite in microstructures produced by intercritical annealing and isothermal transformation. The second part of the study deals with the mechanical stability of austenite at testing temperatures above and below room temperature. [47]
II.
EXPERIMENTAL PROCEDURE
The chemical compositions of the vacuum-melted 4 pct Ni and 1.2 pet Si-l.5 pct Mn steels are given in Table I, together with Acl and Ac3 transformation temperatures calculated from Andrews empirical formulas.t4a] Soluble A1 w
Data Loading...
