Steels_ Metallurgy and Applications, Third Edition

Engineering steels 285

be considered:

1. Lower billet reheating temperatures.


2. Lower bar finishing temperatures.


3. Faster cooling rates after rolling.

In rolling mills that do not have inter-stand cooling or delayed rolling facilities,

options (1) and (2) are interdependent and both are aimed at the refinement of

the austenite grain size. Whereas the austenite grain size will affect the pearlite

content and ferrite grain size, these microstructural parameters are also influenced

significantly by the rate of cooling from the finishing temperature.

Japanese steelmakers have introduced the term normalize-free for controlled

rolled products that involve billet reheating temperatures of around 1050~ and

finishing at or below 900~ in order to simulate the properties of normalized

bars. Whereas such finishing temperatures can be achieved by introducing delays

into the rolling schedule, economic rates of production can only be sustained

with the adoption of inter-stand cooling.

Quenched and tempered steels

As indicated previously, the tensile properties of quenched and tempered alloy

steel forgings can be reproduced in air-cooled, micro-alloy steels but the latter

tend to produce inferior impact properties. However, rolling offers a greater

opportunity than forging for low-temperature finishing and therefore a greater

potential for improved toughness via grain refinement.

The relative effects of vanadium and niobium on strength-toughness

relationships are shown in Figure 3.44. These additions were made to base

compositions containing 0.33% C but with varying manganese. Under normal

120

80

40

-40

-80

600

eheat temp. 1200 ~

0.025% N~ ~" Finishing tamp. 1150 ~

,/

_ .7 .7

v

. / /"

O.O5O% V

. ~'" ~, -~"' Reheat temp. 1100 ~
Finishing temp. 800 ~
,,,,,,, ,,, ,,,, i
0.025% Nb
.... ~ n I , ,I
700 800 900 1000
TS (N/ram 2)

Figure 3.44 Strength-toughness relationships in 0.33% C, micro-alloy steels