[0001] The present invention relates to an Si-alloyed high carbon steel, which by isothermal
heat treatment obtains particularly advantageous strength and toughness properties
and which is useful especially in wearing parts subjected to heavy impacts.
[0002] For such wearing parts it is generally known to use Mn-alloyed austenitic steel,
so-called Hadfield steel, when, in addition to abrasion resistance, toughness is required
of the part.
[0003] If toughness is not necessary it is possible to use e.g. high carbon chromium alloyed
steels (1.0 % C, 12 % Cr). Both such steels have several drawbacks. 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.
[0004] High carbon chromium steels, on the other hand, are brittle and their workability
is poor. They are also expensive due to high alloy content.
[0005] The advantageous mechanical properties of the steel according to the present invention
are based on the bainitic-austenitic dual-phase microstructure obtained 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.
[0006] The chemical composition figures stated in the following are based on the weight.
[0007] 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 decomposition of
austenite at a suitable temperature.
[0008] The use of silicon as an alloying element is known e.g. in spring steels wherein
C- and Si-contents are generally C < 0.8 %, Si < 2.0 %. In these steels, Si-alloying
is generally used as an alloying element increasing hardenability and tempering resistance.
[0009] Commonly known are also low carbon high Si-alloyed steels (C < 0.1 %, Si- 2.0 - 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 350 - 450°C or to lower bainite within the temperature range of 280 - 350°C. Thus,
the obtained bainitic ferrite only contains ~0.01 % of carbon. With carbide formation
prevented, carbon must diffuse into the remaining 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 1.0 % 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 and holding time, it is possible to control the
bainite-austenite ratio obtained as a result of the decomposition of austenite.
[0010] The following examples illustrate mechanical properties obtained with the steel according
to the invention.
[0011] The chemical compositions of the example steels are presented in Table I.

[0012] The test steels were heat treated as follows: austenitizing 920 - 1030°C, 10 min.
+ isothermal bainitizing at 380°C, 350°C or 320°C, water cooling. The test specimen
were subjected to tensile tests performed with an 0 8 mm tensile test specimen, to
impact tests (KV) and residual austenite content was determined with X-ray measurements.
Test results are illustrated in Table II.

[0013] By comparing the strength and toughness values obtained with residual austenite contents,
it can be seen that the best combinations of properties are accomplished with the
residual austenite content between 30 and 40 %. Thus, the yield strength will be R
p 0.2 > 850 N/mm and the tensile strength R > 1300 N/mm2 when the isothermal bainitizing
temperature T
B is 380°C. Lowering the bainitizing temperature below 350°C increases the strength
of the steel considerably. The bainitizing time will then be longer and the microstructure
obtained is lower bainite. Elongation to fracture A
5 > 20 %. Too low a C + Si-content leads to a too small amount of residual austenite
with the stronger but more brittle bainite controlling the properties. This is the
case with the example steel 1. Thus, C + Si must be ≥ 2.80. Too high a C + Si-content,
on the other hand, leads to a too high residual austenite content. Thus, the residual
austenite is too much in control of mechanical properties, the strength thus remaining
lower. Thus, the residual austenite is also mechanically more unstable which impairs
the elongation to fracture. This is the case with the example steel 4. Thus, C + Si
must be ≤ 3.5.
[0014] 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 ≥ 2.0 %. Thus, the elongation to fracture
A
5 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 stainless
steels. However, in unworked condition, yield strength of both of these steels is
≤ 50 % of the yield strength of the steel of the present invention.
[0015] 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 on the stability of austenite. Also carbide
forming chromium and niobium can be used in alloying. The former improves hardenability
on large bar diameters and it can be used in amounts ≤ 1 %, preferably ≤ 0.5 %. Niobium,
on the other hand, can be used to control grain growth properties. The alloying amount
needed for this is ≤ 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 %.
[0016] 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.
1. High strength steel provided with a bainitic-austenitic microstructure which is
accomplished by isothermal heat treatment, characterized in that the steel contains
various alloying elements in the following amounts expressed as weight percentage:

the remainder consisting of iron and normal impurities.
2. Steel according to claim 1, wherein the alloying sum C + Si = 2.9 - 3.4 %.
3. Steel according to claim 1 or 2,
characterized in that it contains one or more of the following alloying elements with
the following contents:
4. Steel according to claim 1, 2 or 3,
characterized in that it has been provided with a dual-phase microstructure produced
by isothermal heat treatment carried out at a temperature of 350 - 450oC, the microstructure mainly consisting of upper bainite and residual austenite and
wherein the proportion of residual austenite is 30 - 40 % by volume.
5. Steel according to claim 1, 2 or 3,
characterized in that it contains a dual-phase microstructure obtained by isothermal
heat treatment carried out at a temperature of 280 - 350°C the microstructure mainly
consisting of lower bainite and wherein the proportion of residual austenite is 30
- 40 %.