Note: Descriptions are shown in the official language in which they were submitted.
Silicon alloyed steel
The present invention relates to Si-alloyed high carbon steel,
which by isothermal heat treatment o~tains particularly advan-
tageous strength and toughness properties and which is useful
especially in wearing parts subjected to heavy impacts.
For such wearing parts it is generally known to use Mn-
alloyed austenitic steel, so~called Hadfield steel, when, in
addition to a~rasion resistance, toughness is required of the
part,
If toughness is not necessary~ it is possihle to use e.g.
high carbon chromium allo~ed steels (1,0 % C, 12 % Cr). Both
such steels have several dra~backs~ Hadfield steel (1,0 % C,
13 % Mn) is difficult to manufacture, it can only be formed
by casting and its corrosion resistance and weldability are
poor. Due to the high Mn~alloy content, this steel is also
expensive.
High carbon chromium steels, on the other hand, are brit-tle
and their workability is poor. They are also expensive due to
high alloy content.
The advantageous mechanical properties of the steel according
- 20 to the present invention are based on the bainitic-austenitic
dual-phase microstructure o~tained in the isothermal heat
treatment, The bainitic component of the microstructure gives
the steel good initial hardness and rich residual austenite
gives it strong strain hardening capacity.
- In the present steel, advantage has been taken of a known
effect of silicon to prevent carbide formation. By increasing
the silicon content of a high carbon steel up to 2,0 - 3,0 ~,
carbide formation can be prevented during isothermal decom-
position of austenite at a suitable temperature~
The use of silicon as an alloying element is known e.g. in
spring steels wherein C- and Si-contents are generally Cc0~8
Si~2,o %. In these steels, Si-alloying is generally used as
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~(1617
an alloying element increasing hardenability and tempering
resistance.
Generally known are also low carbon high Si-alloyed steels
(C < 0 ~1 % ~ Si~V 2 r - 4~0 %~ which are used as core plates of
electromagnets.
The purpose of Si~alloying is to prevent the formation of
carbide (cementite~,, when after the austenitizing the steel
is allowed to decompose isothermally to upper bainite within
a temperature range of 3sa - 45QC or to lower bainite within
the temperature range of 280 - 350C, Thus, the obtained
bainitic ferrite only contains ~0,01 % of carbon. With car-
bide formation prevented, carbon must diffuse into the re-
maining austenite as the bainite reaction proceeds. This,
on the other hand~ increases the stability of austenite with
increasing carbon content. If for example the carbon content
of a steel is 1, a ~ and it decomposes to 50 ~ bainite without
carbide formation, the carbon content of residual austenite
increases to appr. 2 %. Thus, by controlling the composition
(C- and Si-content) of the steel, decomposition temperature
20 and holdingtime, it is possible to control the bainite-
austenite ratio obtained as a result of the decomposition
of austenite.
The following examples illustrate mechanical properties
obtained with the steel according to the invention.
The chemical compositions of the example steels are presented
in table 1.
,
~,
Steel C Si Mn C~ Ni Cu ~ll C~Si
% %. % % % % % %
1 0~8Q 2~07 0,48 0,24 0,11 0,16 0,033 2,87
2 Q,78 2,45 0,55 0,07 0,07 0,01 0,034 3,23
3 Q,93 2,33 0,48 0,03 a,01 0,01 0,030 3,26
4 0,g8 2,49 0~51 0,02 0,01 Q,01 0,030 3,47
0,80 2,40 0,52 0,7g 0,01 0,01 0,035 2,91
The test steels were heat treated as .~ollows: austenitizing
92Q ~ 1030C~ lQ min ~ isothermal bainitizing at 380C, 350C
or 320C, water cooling, The test specimen were subjected to
tensile tests performed with an ~ 8 mm tensile test specimen,
to impact tests (KV~ and residual austenite content was
determined with X-ray measurements. Test results are lllus-
trated in table 2.
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13 ~30617
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36~7
By comparing the strength and toughness values obtained with
residual austenite contents~ it can be seen that the ~est
com,~inations of properties are accomplished with t~e residual
austenite content ~etween 30 - 40 %, Thus~ the yield strength
will be Rp o 2 7 850 N~mm2 and the tensile strength
Rm ~1300 N/~m2 when the isothermal bainitizing temperature
is 380C. Lowering the bainitizing temperature ~elow 350C
increases the strength of the steel considerably. The bain-
itizing time will be then longer and the microstructure ob-
tained is lower bainite, Elongation to fracture A5 ~ 20 ~.
; Too low a C + Si-content leads to too small an amount of
residual austenite, stronger but more brittle bainite con-
trolling the properties. This is the case with the example
steel 1, so C + Si must be ~2,80.
Too high a C + Si-content, on the other hand, leads to too
high a residual austenite content. Thus the residual austenite
is too much in control of mech'anical properties, the strength
thus remaining lower. Thus, the res,idual austenite is also
mechanically more unstable which impairs the elongation to
fracture. This is the case with the example steel 4, so C + Si
must be c 3,5.
According to the test results, the most suitable range for the
sum is C + Si = 2,90 - 3,40 ~, however with C ~ 0,8 % and Si 2
2,0 ~.Thus the elongation to fracture A5 is 30 -- 40 ~ and
consists mainly of uniform elongation which is an indication
of strain hardening capacity found only in austenitic Hadfield
manganese steel and staïnless steels. However, in unworked
condition, yield strength of ~oth of these steels is ~50 %
of the yield strength of the steel of the present invention.
In order to improve its heat treatment properties, the steel
according to the invention can be alloyed with austenite
stabilizing alloying elements, such as manganese and nickel,
up to appr. 1 ~. Thus for the C-content, it is necessary
to take into account the effect of the additional alloying
- : ~" ~ ~
~L~.30617
on the stability of austenite, ~lso carbide forming chromium
and niobium can be used in alloying~ The former improves
hardenabilit~ on large bar diameters and it can be used in
: amounts5~ 1 %r preferably ~a~s %~ Niobium, on the other hand,
can be used to control grain growth properties, The alloying
amount needed for this is C 0,1 ~Al~alloying is preferable
for binding of free nitrogen in ferritic bainite which is
advantageous for toughness, particularly at low temperatures.
The alloying amount needed for this is ~ 0,1 ~.
The steel according to the invention has produced a combination
of strength and toughness properties that has been impossible
to obtain with prior art steels. Moreover, since these
properties are achieved by simple isothermal heat treatment
and inexpensive alloying, the steel according to the invention
can be expected to receive wide acceptance and to be widely
used in applications requiring high strength and good abrasion
resistance.
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