Inclusion Control in High-Performance

ELSEVIER Journal of Materials Processing Technology 53 (1995) 177-186

Journal of Materials Processing Technology

Inclusion Control in High-Performance Steels L.E.K Holappa and A.S. Helle Helsinki University of Technology, Laboratory of Metallurgy FIN-02150 Espoo, Finland

Progress of clean steel production, fundamentals of oxide and sulphide inclusions as well as inclusion morphology in normal and calcium treated steels are described. Effects of cleanliness and inclusion control on steel properties are discussed. In many demanding constructional and engineering applications the nonmetallic inclusions have a quite decisive role in steel performance. An example of combination of good mechanical properties and superior machinability by applying inclusion control is presented.

1. INTRODUCTION Controlling inclusions in steel is closely connected with the concept of "clean steel".

Cleanliness of steel is defined as a low content of harmful impurities such as oxygen, sulphur, phosphorus, hydrogen and nitrogen, in some cases even carbon /1/. Steelmaking technology has made great achievements during the last 25 years which has resulted in remarkable lowering of impurities in steel as shown in Fig. 1.

0.1 _-p..... 0.001 0 ~ _ 0.0001 lppm _. _ : : H . : _ ..... 0.00001 .... : .... : .... '. .... : .... 1970 1975 1980 1985 1990 1995 YEAR

Fig. 1. Progress of lowering impurities in steel on the basis of compiled literature data/2/. Elsevier Science S.A. SSDI 0924-0 t36(95)01974-J 178 L.E.K. Holappa, A.S. Helle / Journal of Materials Processing Technology 53 (1995) 177-186 Steel cleanliness can also be considered as a low content of nonmetallic inclusions, mainly oxide and sulphide inclusions, in the steel. Further, the idea of "clean steel" often includes special requirements to the inclusions with respect to their composition, morphology, type, size and distribution in the liquid steel, in solidified ingot and in the final product. Thus the term "inclusion control" first means avoiding of harmful effects of inclusions and, secondly, accomplishment of beneficial influences by inclusion modification either on the steehnaking process or on the properties of the steel during its further manufacturing or of the final product.

2. INCLUSION CONTROL IN STEELMAKING 2.1 Fundamentals of inclusion formation The purpose of steel deoxidation is to bind a major part of the dissolved oxygen in the steel melt in oxide form and to remove the so formed inclusions from the steel melt to the top slag or onto the refractory lining. The thermodynamic basis of deoxidation can be seen in Fig.2. Earlier, additions of silicon and manganese were often considered as a satisfactory way to deoxidise steel (or to kill i.e. to prevent CO-formation during casting). With typical Mn- and Si-rest contents in the steel (<1% / 0.5%) the residual oxygen content in the steel

I ~MnS

I % ..... ~ I NCeS FexMn 0 -~~ Y i0_ I Io -I \

- ~ .~o2 10-2 , ~ 10_ 2

_>10 -3 10 .3

~ 10 -4 10 -4- ~ o CoS m

10-5 10-5

10 -6 %'~ 10 -6 \

N

i0-7 ~, \ i0 -7 10 -5 10 -4 10 -3 10 -2 10 -I I a M {%)

Fig.2. Deoxidation and desulphurisation equilibria in steel with various dissolved metallic elements and carbon/3/. L.E.K. Holappa, A.S. Helle I Journal of Materials Processing Technology 53 (1995) 177-186 179 was 50... 100 ppm as can be estimated from Fig.2, too. Thus, when aiming at better oxide cleanliness stronger deoxidation means had to be adopted. Nowadays deoxidation with aluminium is the most common method. It leads to rest dissolved oxygen content below 5 ppm in the steel melt with 0.02...0.03 % A1. However, the total oxygen content in the steel ladle or in the final product greatly varies between 5...50...100 ppm depending on the steelmaking practice, and careful prevention from reoxidation during the ladle treatment and casting operation is needed. In aluminium deoxidised steels the inclusions are mostly pure aluminium oxides A120 3. In the case of low aluminium containing steel e.g. coarse grain steels the inclusions are, however, manganese silicates with less A120 3. Equilibrium of sulphur in steel melt with different sulphide forming elements is also shown in Fig.2. In normal steels sulphur is in dissolved form in liquid steel but during the casting and steel solidification it will react with manganese and form manganese sulphide inclusions in steel. They are readily deformable at hot rolling temperatures and are thus elongated to long stringers as shown later in Fig. 4a and c. Desulphurisation of the steel melt is, in general, based on sulphur transfer from the metal to a highly basic slag