METHOD OF PRODUCING STEEL SECTIONS OF IMPROVED QUALITY

METHODE DE PRODUCTION DE PROFILES D'ACIER DE QUALITE AMELIOREE

English Abstract

ABSTRACT
A steel section 6 to 40 mm thick, emerging from a
hot rolling mill at 1000 to 800°C is surface quenched for
2 to 6 seconds at a cooling rate of the order of 1.5 to
5 MW(megawatts)/m2. During subsequent self-tempering of the quenched
surface by heat from the remainder, the section tends to
an equalization temperature of 500 to 700°C.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of improving the quality of a steel
section having a thickness of 6 to 40 mm, emerging from a
hot rolling mill at a temperature of 1000 to 800°C, the
method comprising subjecting the section to surface
quenching for 2 to 5 seconds, wherein the cooling rate
during the surface quenching is of the order of 1.5 to
5 MW(megawatts)/m2, and the equalization temperature of the section
due to subsequent self-tempering is 500 to 700°C.
2. A method as claimed in claim 1, in which the
steel contains 0.11 to 0.20% carbon, 0.5 to 1.1% manganese,
0.20 to 0.40% silicon, and 0.018 to 0.040% aluminium.
3. A method as claimed in claim 1 or 2, in which the
steel is semi-killed steel containing 0.11 to 0.20% carbon,
0.5 to 1.1% manganese, 0.20 to 0.40% silicon, and 0.018 to
0.040% aluminium.
4. A method as claimed in claim 1, in which the
steel is aluminium killed steel containing 0.05 to 0. 20%
carbon, 0.20 to 0.40% silicon, 1 to 2% manganese, and 0.018
to 0.040% aluminium.
5. A method as claimed in claim 4, in which the
steel further contains 0.015 to 0.040% niobium.
6. A method as claimed in claim 1, in which the
steel contains 0.05 to 0.20% carbon, 0.20 to 0.40% silicon,
1 to 2% manganese, 0.018 to 0.040% aluminium, max. 0.06%
niobium, and 0.04 to 0.20% vanadium

Description

:
The present inven-tion relates to a method of improving
the qlali-ty of rolled steel products in the form of sections,
the method being applicable to any ~ind of steel, whether
killed, semi-killed, or rimming steel.
The main qualities required by users of steel sections
- are, among other things, as high as possible an elas-tic
limit for the grade of steel used, as well as weldability,
fatigue strength, and ductility satisfactory for -the use to
` which the sections are to be put.
` 10 On the other hand, in order to improve weldability
.~ and ductility of a steel, it is necessary to decrease its
carbon and manganese conten-t, which concurrently resul-ts in
a decrease of its tensile strength. To remedy this draw-
back, steel can be subjected to a sui-table cooling treatment,
15 preferably applied directly at -the outlet of a rolling mill,
which permits the elastic limit of the sec-tion to be raised.
When cooling is effected by convection or radiation,
the coolinæ law of a rolled section depends to a large
extent on its dimensions. Thus for a sec-tion having
20 predetermined dimensions it is necessary, in order to modify
its elastic limit, to adopt further procedures to complete
; -the effect of the coolinæ as such. ~ong these procedures
one should men-tion in particular increasing the content of
alloying elements. This is certainly effective but has the
.
25 drawback of beirg more costly the higher the elastic limit
desired.
~nat is required is a me-thod permitting the above-
,

;- mentioned drawback to be elimina-ted wi-thou-t increasing
-the carbon and manganese content of the s-teel in a way
which is inacceptable wi-th regard to weldability.
:
The present inven-tion provides a me-thod in whic~l,
at the ou-tlet of a hot rolling mil], the said section
while being at a -temperature of 1000 -to ~00C is subjected
to surface ~lenching for 2 to 6 seconds, the sec-tion whose
thickness is of 6 to 40 ~n being subjected during quenc~hing
~ J~SQ~V~
`~ ~ to a cooling rate of the order of 1.5 to 5 ~w~m', -the
equaliza-tion -temperature of the section due to selL-
tempering being be-tween 500 and 700C.
Self-temperin~ is tempering of the q-uenched surface b~y heat
conduc-ted -to -that surface froin the remainder of the section.
The equalization temperature is the tempera-ture -towards
which the quenched ~;urface and -the renainder of the sec-tion
converge during self-tempering.
~he application of -this me-thod -to steel containing
0.11 to 0.20% carbon, 0.5 -to 1.1% manganese, 0.20 to 0.40%
silicon, and 0.018 to 0.040~0 aluminium made i-t possible to
classify this steel as St.52 with an elastic limit higher -than
36 kg/mm2 and a Charpy V impact strength higher than 5 kg/cm2
at -20 C. ~he same method when applied -to steel con-taining
0.11 to 0.20% carbon and 0.5 -to 1.1% manganese but being of
a semi-killed kind made it possible -to ob-tain steel of the
same class but whose Charpy V impac-t strength was higher
than 3 kg/cm2 at -20C.
In a second test of the method according to the

invention, applied this time -to steel containing 0.05 -to
: 0.20% carbon, 0.20 to 0.40% silicon, 1 to 2% manganese, and
0.018 to 0.040% aluminium, -the s-teel being al~uminium killed
and optionally also con-taining 0.015 -to 0.040% niobium, it
was possible to obtain steel of class St.E47 whose elastic
limit was a-t least 47 kg/m.n2 and whose Charpy V impact
strength was 5 kg/cm2 at -20C.
: In a third test of the method according to the
inven-tion, applied to steel having the same composi-tion as
the preceding s-teel but con-taining up -to 0.06% niobium and
0.04 to 0.20% vanadium, it was possible to obtain s-teel of
class St.E 70 with an elastic limit of at leas-t 70 kg/mm2
and a Charpy V impact strength higher -than 5 kg/cm2 at -60Co