U.S. patent application number 13/639805 was filed with the patent office on 2013-02-14 for mechanical part made of steel having high properties and process for manufacturing same.
This patent application is currently assigned to ASCOMETAL. The applicant listed for this patent is Thomas Sourmail. Invention is credited to Thomas Sourmail.
Application Number | 20130037182 13/639805 |
Document ID | / |
Family ID | 42942210 |
Filed Date | 2013-02-14 |
United States Patent
Application |
20130037182 |
Kind Code |
A1 |
Sourmail; Thomas |
February 14, 2013 |
MECHANICAL PART MADE OF STEEL HAVING HIGH PROPERTIES AND PROCESS
FOR MANUFACTURING SAME
Abstract
A mechanical steel part in steel with high characteristics,
characterized in that its composition, comprising in weight
percentages, is 0.05%.ltoreq.C.ltoreq.0.25%;
1.2%.ltoreq.Mn.ltoreq.2%; 1%.ltoreq.Cr.ltoreq.2.5%; wherein the
contents of C, Mn and Cr are such that (830-270C %-90 Mn %-70Cr
%).ltoreq.560; 0<Si.ltoreq.1.55; 0<Ni.ltoreq.1%;
0<Mo.ltoreq.0.5%; 0<Cu.ltoreq.1%; 0<V.ltoreq.0.3%;
0<Al.ltoreq.0.1%; 0<B.ltoreq.0.005%; 0<Ti.ltoreq.0.03%;
0<Nb.ltoreq.0.06%; 0<S.ltoreq.0.1%; 0<Ca.ltoreq.0.006%;
0<Te.ltoreq.0.03%; 0<Se.ltoreq.0.05%; 0<Bi.ltoreq.0.05%;
0<Pb.ltoreq.0.1%; the remainder of the steel part being iron and
impurities resulting from processing, and wherein the in that its
structure of the steel is bainitic and contains no more than a
total of 20% of martensite and/or pro-eutectoid ferrite and/or
pearlite.
Inventors: |
Sourmail; Thomas; (Metz,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sourmail; Thomas |
Metz |
|
FR |
|
|
Assignee: |
ASCOMETAL
Courbevoie
FR
|
Family ID: |
42942210 |
Appl. No.: |
13/639805 |
Filed: |
April 7, 2011 |
PCT Filed: |
April 7, 2011 |
PCT NO: |
PCT/FR2011/050781 |
371 Date: |
November 5, 2012 |
Current U.S.
Class: |
148/645 ;
148/330; 148/332; 148/333; 148/334; 148/649 |
Current CPC
Class: |
C21D 6/002 20130101;
C22C 38/04 20130101; C21D 1/18 20130101; C21D 6/005 20130101; C22C
38/02 20130101; C21D 1/19 20130101; C22C 38/44 20130101; C22C 38/46
20130101; C21D 9/00 20130101; C22C 38/58 20130101; C21D 2211/002
20130101; C22C 38/18 20130101; C21D 8/005 20130101 |
Class at
Publication: |
148/645 ;
148/649; 148/330; 148/332; 148/333; 148/334 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/38 20060101 C22C038/38; C22C 38/54 20060101
C22C038/54; C22C 38/46 20060101 C22C038/46; C22C 38/50 20060101
C22C038/50; C21D 8/00 20060101 C21D008/00; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2010 |
FR |
10 01433 |
Claims
1. A mechanical steel part in comprising in weight percentages:
0.05%.ltoreq.C.ltoreq.0.25%; 1.2%.ltoreq.Mn.ltoreq.2%;
1%.ltoreq.Cr.ltoreq.2.5%; wherein the contents of C, Mn and Cr are
such that (830-270C %-90Mn %-70Cr %).ltoreq.560;
0<Si.ltoreq.1.5%; 0<Ni.ltoreq.1%; 0<Mo.ltoreq.0.5%;
0<Cu.ltoreq.1%; 0<V.ltoreq.0.3%; 0<Al.ltoreq.0.1%;
0<B.ltoreq.0.005%; 0<Ti.ltoreq.0.03%; 0<Nb.ltoreq.0.06%;
0<S.ltoreq.0.1%; 0<Ca.ltoreq.0.006%; 0<Te.ltoreq.0.03%;
0<Se.ltoreq.0.05%; 0<Bi.ltoreq.0.05%; 0<Pb.ltoreq.0.1%;
the remainder of the steel part being iron and impurities resulting
from processing, and wherein the structure of the steel is bainitic
and contains no more than a total of 20% of martensite and/or
pro-eutectoid ferrite and/or pearlite.
2. The part according to claim 1, wherein
.ltoreq.0<Si.ltoreq.0.3%.
3. The part according to claim 1, wherein
0.8.ltoreq.Si.ltoreq.1.5%.
4. The part according to claim 1, wherein Ni.ltoreq.0.5%.
5. The part according to claim 1, wherein
0.04%.ltoreq.Mo.ltoreq.0.5%.
6. The part according to claim 1, wherein
0.05%.ltoreq.V.ltoreq.0.3%.
7. The part according to claim 1, wherein
0.005%.ltoreq.Al.ltoreq.0.1%.
8. The part according to claim 1, wherein
0.0005%.ltoreq.B.ltoreq.0.005% and 0<N.ltoreq.0.0080% and Ti
%.ltoreq.3.5%.
9. The part according to claim 1, wherein
0.005%.ltoreq.Ti.ltoreq.0.03%.
10. The part according to claim 1, wherein
0.005%.ltoreq.S.ltoreq.0.1%.
11. A process for manufacturing a steel part comprising the
following steps: preparing a steel blank or bar whose composition
conforms to claim 1; hot-working the blank or bar an austenitic
phase by forging or rolling; cooling the hot-worked blank or bar at
a rate that imparts a bainitic structure thereto, wherein the
cooled, hot-worked blank or bar contains a total of no more than
20% martensite and/or pearlite and/or pro-eutectoid ferrite; and
optionally carrying out one or more machining operation(s) to give
the part its final dimensions and surface condition.
12. The process according to claim 11 wherein, before or after the
machining operation(s), tempering is performed over a temperature
range of 200 to 350.degree. C. for 30 minutes to 4 hours.
13. The process according to claim 11, wherein the hot-worked blank
or bar is cooled naturally in still air.
14. The process according to claim 11, wherein the hot-worked blank
or bar is cooled in forced air.
Description
[0001] The invention concerns steels for mechanical parts having
high characteristics obtained by hot forging or bar machining.
[0002] Some steel grades allow the obtaining of high mechanical
characteristics for forged parts or as-rolled bars without the use
of controlled cooling or subsequent heat treatment. They are based
on the obtaining of a homogeneous bainitic microstructure.
[0003] Said grades have already been proposed such as those the
subject of documents EP-B1-0 787 812 or EP-A-1 426 453, which are
given industrial use for the production of forged parts for
internal combustion engines. However, to obtain high mechanical
characteristics on the parts described in these documents it is
necessary, unless being restricted to diameters of the order of 20
mm, to use carbon contents of 0.25% or higher.
[0004] While it is currently possible to guarantee a tensile
strength of the order of 1200 MPa after natural cooling, in
particular when using grades such as those described in EP-A-1 426
453, the obtaining of these mechanical characteristics is often
achieved at the expense of an impact strength of 30 Jcm.sup.-2 or
lower.
[0005] It is the objective of the invention to propose mechanical
parts made of a determined steel grade which, combined with
adequate heat and thermo-mechanical treatments, allow the
simultaneous obtaining of advantageous mechanical properties
(tensile strength Rm, yield strength Re, Re/Rm ratio, elongation A,
percentage reduction of area Z) and improved impact strength KCU
compared with known mechanical parts in steel for this same
use.
[0006] For this purpose the subject of the invention is a
mechanical part having high characteristics characterized in that
its weight percentage composition is as follows:
[0007] 0.05%.ltoreq.C.ltoreq.0.25%;
[0008] 1.2%.ltoreq.Mn.ltoreq.2%;
[0009] 1%.ltoreq.Cr.ltoreq.2.5%;
[0010] (830-270C %-90Mn %-70Cr %).ltoreq.560;
[0011] traces.ltoreq.Si.ltoreq.1.5%;
[0012] traces.ltoreq.Ni.ltoreq.1%;
[0013] traces.ltoreq.Mo.ltoreq.0.5%;
[0014] traces.ltoreq.Cu.ltoreq.1%;
[0015] traces.ltoreq.V.ltoreq.0.3%;
[0016] traces.ltoreq.AI.ltoreq.0.1%;
[0017] traces.ltoreq.B.ltoreq.0.005%;
[0018] traces.ltoreq.Ti.ltoreq.0.03%;
[0019] traces.ltoreq.Nb.ltoreq.0.06%;
[0020] traces.ltoreq.S.ltoreq.0.1%;
[0021] traces.ltoreq.Ca.ltoreq.0.006%;
[0022] traces.ltoreq.Te.ltoreq.0.03%;
[0023] traces.ltoreq.Se.ltoreq.0.05%;
[0024] traces.ltoreq.Bi.ltoreq.0.05%;
[0025] traces.ltoreq.Pb.ltoreq.0.1%;
the remainder being iron and impurities resulting from processing,
and in that the structure of the steel is bainitic and contains no
more than a total of 20% martensite and/or pro-eutectoid ferrite
and/or pearlite.
[0026] Preferably, traces.ltoreq.Si.ltoreq.0.3%.
[0027] Preferably, 0.8.ltoreq.Si.ltoreq.1.5%.
[0028] Preferably, Ni.ltoreq.0.5%.
[0029] Preferably, 0.04%.ltoreq.Mo.ltoreq.0.5%.
[0030] Preferably, 0.5%.ltoreq.V.ltoreq.0.3%.
[0031] Preferably, 0.005%.ltoreq.Al.ltoreq.0.1%.
[0032] Preferably, 0.0005%.ltoreq.B.ltoreq.0.005%, and
traces.ltoreq.N.ltoreq.0.0080% and Ti %.ltoreq.3.5N %.
[0033] Preferably, 0.005%.ltoreq.Ti.ltoreq.0.03%.
[0034] Preferably, 0.005%.ltoreq.S.ltoreq.0.1%.
[0035] A further subject of the invention is a process for
manufacturing a part in steel such as a mechanical part having high
characteristics, characterized in that it consists of the following
steps: [0036] a steel blank or bar is prepared whose composition
conforms to the above; [0037] the blank or bar is hot worked in the
austenitic region by forging or rolling; [0038] the hot-worked
blank or bar is cooled at a rate imparting a bainitic structure
thereto containing a total of no more than 20% martensite and/or
pearlite and/or pro-eutectoid ferrite; [0039] and optionally one or
more machining operations to impart the final dimensions and
surface condition to the part.
[0040] Before or after the machining(s), tempering can be carried
out at between 200 and 350.degree. C. for 30 minutes to 4
hours.
[0041] The hot-worked blank or bar can be cooled naturally in still
air.
[0042] The hot-worked blank or bar can be cooled under forced
air.
[0043] As will have been appreciated, the invention is based on a
composition of the part and its combination with a metallurgical
structure that is 80% bainitic or more, able to be obtained using
simple means such as cooling in still or forced air.
[0044] By <<bainite>> is meant both pure bainite and
the bainite+residual austenite mixture if austenite is present as
is frequently the case, and all possible bainite morphologies are
included including intragranular bainite (also called acicular
ferrite). The other phases which may be present, namely martensite,
pro-eutectoid ferrite and pearlite, must not represent more than
20% of the structure.
[0045] The grades used in the present invention, by having recourse
to so-called <<low-medium>> carbon and by lowering the
point of transformation onset chiefly through the incorporation of
chromium and manganese, allow the obtaining of tensile strengths in
the region of 1200 MPa or higher, with impact strengths of at least
40 Jcm.sup.-2, possibly even reaching 70 J.sup.SMcm.sup.-2.
[0046] However, these grades in the hot forged or as-rolled state
exhibit Re/Rm ratios of the order of 0.6 and hence significantly
lower yield strengths than those obtained with quenched-tempered
grades having the same mechanical strength.
[0047] Yet as will be shown, it is also possible according to the
invention, using subsequent low temperature tempering, to increase
the yield strength most significantly (by approximately 25%)
without however increasing mechanical strength. This type of
tempering is to be distinguished from the tempering sometimes used
on micro-alloy steels conducted in the region of 550-650.degree. C.
which allow the precipitation of alloy carbides. Whereas this type
of tempering is often accompanied by a major loss of impact
strength, tempering at low temperature as performed in the
invention has a beneficial effect on impact strength (being
increased by up to about 30%).
[0048] Support for the choice of composition ranges will now be
given for the various elements of the grade of the parts according
to the invention. All contents are given in weight percentages.
[0049] The C content is between 0.05 and 0.25%. This range called
the <<low-medium carbon>> range since its upper limit
lies in the lower region of the contents considered to be medium
carbons and its lower limit belongs to the region of low carbons,
provides for a very homogenous microstructure and hardness even in
the presence of segregations. In particular, for carbon contents of
less than 0.2% the hardness of martensitic microstructures
optionally present in the segregated regions is only slightly
higher than that of the bainitic microstructure. In addition, these
carbon contents allow ductility and impact strengths to be obtained
that are higher than those obtained, for the same level of
mechanical strength, with contents higher than 0.25%.
[0050] The Mn content is between 1.2 and 2%. Manganese is used
jointly with chromium as main element to lower the bainite
formation onset temperature (Bs) during continuous cooling. Since a
relatively low carbon content is used, relatively high contents of
Mn are required which additionally must contribute to meeting the
condition regarding C, Mn, Cr contents for calculating Bs (see
below). Manganese is limited to 2% to avoid excessive segregation
problems.
[0051] The Cr content is between 1.2 and 2.5%. In the present
invention, Cr is used for the same reason as Mn to lower the onset
temperature of bainitic transformation Bs.
[0052] The contents of C, Mn and Cr must additionally be such that
830-270 C %-90Mn %-70Cr %.ltoreq.560.
[0053] The onset temperature Bs of bainitic transformation can be
conventionally estimated using the following formula:
Bs=830-270C %-90Mn %-70Cr %-37N %-83Mo %
where the contents are expressed in weight percentages (see for
example Bhadeshia, Bainite in Steels, IOM 2001). In the invention,
having regard to the relatively low contents of Ni and Mo in the
steel, it is possible only to give consideration to the
contributions made by C, Mn and Cr. At all events, if Ni and Mo are
present in the upper content ranges examined below they will
contribute towards lowering Bs. It is therefore ensured that in any
case a Bs will be obtained of 560.degree. C. or lower.
[0054] Si is contained between traces and 1.5%. Silicon can be used
to avoid the formation of carbides which would deteriorate impact
strength during bainitic transformation. With carbon contents of
less than 0.2% however this formation of carbides remains small and
the addition of Si loses its advantage from this viewpoint. Also,
by promoting the formation of residual austenite, Si allows an
improvement in the fatigue resistance for some applications. In
some cases, however, its use may also be excluded through the need
to avoid excessive surface decarburization. Two variants of the
invention can therefore be envisaged. In a first variant the Si
content merely results from the processing conditions for the bath
of liquid metal, namely from the raw materials used and possible
partial oxidation of Si, and no deliberate major addition of Si is
made. In this case the Si content obtained typically ranges from
traces to 0.3%. In a second variant, Si is purposely added,
preferably to obtain a content of 0.8 to 1.5%.
[0055] Ni is contained between traces and 1%, preferably between
traces and 0.5%. It may solely be present through its addition via
the raw materials as residual element, or it can be added in a
small amount to contribute towards reducing Bs temperature. However
its content is limited to 1%, better still 0.5% for cost-related
reasons, this element being costly and its price is likely to
fluctuate heavily on the market.
[0056] Mo is contained between traces and 0.5%, preferably between
0.04 and 0.5%. The role of molybdenum on quenchabiity has been well
established: it allows the formation of ferrite and pearlite to be
avoided but does not however slow the formation of bainite. It can
therefore be added in variable quantity depending on the diameter
of the part. A second advantage of molybdenum is that it limits the
sensitivity to reversible temper brittleness (see Bhadeshia, Mater.
Sci. Forum, High Performance Bainitic Steels, vol. 500-501, 2005).
Finally, molybdenum strengthens austenite by changing to a solid
solution. Insofar as the mechanical strength of austenite is one of
the main factors governing the fineness of a bainitic structure
(Singh and Bhadeshia, Mater. Sci. Eng. A, 1998, Vol 245, p. 72),
the addition of Mo indirectly contributes to obtaining a finer
structure. The upper limit is mainly fixed for economic
reasons.
[0057] V ranges from traces to 0.3%, preferably between 0.05 and
0.3%. The addition of vanadium allows additional hardening; however
contrary to the case with ferrite-pearlite steels, this hardening
does not appear to occur via precipitation; it has effectively been
experimentally shown that after hot working (hot forging or
rolling) and natural cooling, only a very small fraction of
vanadium is in precipitated form. Just like molybdenum, vanadium
reinforces austenite by precipitation and solid solution, and it
can therefore indirectly contribute towards the fineness of the
bainitic structure, hence its hardening effect. The addition
thereof is limited to 0.3% for economic reasons.
[0058] The Cu content ranges from traces to 1%. It may optionally
be used to contribute towards hardening, but would lead to
implementation difficulties with contents higher than 1%.
[0059] The Al content ranges from traces to 0.1%, preferably
between 0.005 and 0.1%. Al is optionally added to ensure
deoxidation of the steel and to avoid the excessive growth of
austenitic grains when held at high temperature (e.g. for case
hardening) which may be performed subsequently on the part after
the implementation of the process of the invention.
[0060] B ranges from traces to 0.005%, preferably between 0.0005
and 0.005%. This optional element can be used for parts of large
diameter, in particular if the Mo content is low, to guarantee the
homogeneity of the structure (limit the presence of ferrite). In
this case, it is preferable to combine the addition of B with an
addition of Ti which will capture the nitrogen to form nitrides and
thereby avoid the formation of boron nitrides. In this manner, all
the boron will be available to play its role of homogenizer of the
structure. This should give traces.ltoreq.N.ltoreq.0.0080% and Ti
%.ltoreq.3.5N %.
[0061] Ti ranges from traces to 0.03%, preferably between 0.005 and
0.03%. As just mentioned, this optional element is chiefly to be
used for grades with boron, with the ratio of Ti % to N % just
indicated.
[0062] Nb is contained between traces and 0.06%. This optional
element can be used to refine the austenitic structure after hot
forging or rolling, resulting in a reduction in the sizes of
bainite packets and accelerated bainitic transformation (Bhadeshia,
Proc. Royal Soc., 2010, Vol 466, p 3).
[0063] S is contained between traces and 0.1%. As is well known
this element may optionally be left at a relatively high level,
even purposely added to improve the machinability of the steel. Its
content is then 0.005 to 0.1%. Preferably this significant presence
of S is accompanied by an addition of Ca of up to 0.006%, and/or Te
of up to 0.03%, and/or Se of up to 0.05%, and/or Bi of up to 0.05%
and/or Pb of up to 0.1%. This improved machinability can
particularly be sought for applications in which the part is
subjected to fatigue stress or for applications in which its
mechanical properties are improved at least locally by sufficient
pre-stressing to prevent the propagation of cracks (roller
burnishing of crankshafts, autofrettage of high pressure injection
rails).
[0064] The other elements contained in the steel of the invention
are iron and impurities resulting from processing, present in their
usual contents.
[0065] The preferred ranges cited for the various elements are
independent of each other. A steel that comes under only one or
some of these preferred ranges and not under others would therefore
be considered as being included in the invention.
[0066] Industrially, the part can be produced by hot working a
blank or bar having the above-described composition, such as hot
forging or hot rolling, or by machining a ready-to-use bar.
[0067] In the first case the industrial process entails a hot
working step conducted in austenitic phase (typically
1100-1250.degree. C.) followed by natural cooling. One of the
important points of the invention is the possible obtaining of high
mechanical characteristics without the use of heat treatments after
forging or rolling, or any particular highly restricting control
procedure over the rate of cooling which can take place naturally
in still air. Nevertheless, if the installations so allow, adapted
cooling may be used in some cases either on account of the diameter
of the parts (with parts of large size cooling that is too slow may
lead to the onset of ferrite and/or pearlite in an excessive
amount), or to obtain higher mechanical characteristics than those
which would be obtained with natural cooling. Cooling with forced
air may be sufficient to reach this objective. Attention must be
paid however so that cooling is not rapid to the point of causing a
massive onset of martensite as would happen with quenching.
[0068] In addition, tempering heat treatment at low temperature
(200 to 350.degree. C. for periods of 30 minutes to 4 hours), on
the grades of the invention, allows a highly significant increase
to be obtained in yield strength without increasing hardness and
without reducing impact strength.
[0069] Depending on the parts concerned, several machining
operations may be performed after forging or rolling and before or
after tempering to obtain the precise dimensions and surface
characteristics of the end part.
[0070] The mechanical characteristics being obtained by natural
cooling, they are also able to be obtained from a hot rolled bar
ready for use if it already has the desired metallurgical structure
(essentially bainitic) which is described below. The composition of
the steels used in the invention is such that the probability of
obtaining the targeted structure naturally after mere cooling in
air of the hot rolled bar under usual conditions is not negligible,
if the dimensions of the bar lead to an adequate cooling rate.
[0071] The results are given below obtained with the steel
compositions conforming to the requirements of the invention and
those of reference compositions. These results were obtained on
laboratory castings forged into 40 mm rounds or on industrial
castings forged into rounds of equivalent diameter. To allow a
significant comparison between results, the mechanical
characteristics are evaluated after austenization at 1000.degree.
C. followed by natural cooling in still air or cooling under forced
air. In addition, for reference, two bainitic grades were added
which impart high mechanical characteristics to hot forgings and
are already used on crankshafts, rails and other forged parts
having high mechanical strength: specimen A (corresponding to
EP-B-0 787 812) and B (corresponding to EP-A-1 426 453). The
compositions of these samples are given in Table 1 together with
their bainitic transformation onset temperature Bs calculated as
previously on the basis of C, Mn and Cr contents.
TABLE-US-00001 TABLE 1 Compositions and Bs of the tested specimens.
Bs Specimen C % Mn % Cr % Si % Ni % Mo % V % Others (.degree. C.) A
(Ref) 0.25 1.52 0.84 0.80 0.17 0.10 0.19 Ti, B 567 B (Ref) 0.29
1.13 1.15 0.80 0.20 0.12 0.18 Nb, Ti, B 570 C (Inventn) 0.18 1.62
1.51 0.18 0.17 0.19 0.09 530 D (Inventn) 0.16 1.68 1.54 0.94 0.06
0.20 0.09 Ti, B 528 E (Inventn) 0.21 1.78 1.38 1.06 0.10 0.19 0.09
Ti, B 517 F (Inventn) 0.18 1.60 1.54 0.86 0.96 0.19 0.20 Ti, B 530
G (Inventn) 0.18 1.53 1.45 0.97 0.10 0.17 0.09 Ti, B 542
[0072] The contents of Ti, Nb and B are typically 0.030%, 0.025%
and 0.003% respectively when these elements are present.
[0073] Table 2 gives the mechanical characteristics measured on the
products obtained from these specimens. It is to be pointed out
here that the results obtained, in absolute value, are only be
analysed in the precise context to which they refer. Differences
are frequently observed in the mechanical properties obtained on
forged or rolled parts of same composition but of different size,
generally with an increase in mechanical characteristics for an
equivalent diameter. The hierarchy between the examined grades will
nevertheless remain identical for specimens all having the same
dimensions which may be different from those of the examples cited
herein. The indication "FA" after the specimen reference means that
cooling in this case was forced air cooling.
TABLE-US-00002 TABLE 2 Mechanical characteristics of the specimens
after austenization and cooling. Re Rm KCU Specimen Structure (MPa)
(MPa) Re/Rm A (%) Z (%) (J cm.sup.-2) A bainite 666 1114 0.60 19 39
39 B bainite 739 1226 0.60 18 41 27 A (FA) bainite 694 1119 0.62 14
30 32 C bainite 738 1185 0.62 15 53 50 D bainite 709 1173 0.60 14
44 44 D (FA) bainite 759 1203 0.63 15 57 69 E bainite 796 1303 0.61
15 39 47 E (FA) bainite + 10% 989 1344 0.74 12 46 58 martensite F
bainite 745 1213 0.61 17 49 44 F (FA) bainite 774 1238 0.63 16 50
50 G bainite 769 1212 0.63 17 51
[0074] The mechanical characteristics of the steel specimens of the
invention, C to G, therefore show a significant increase in
mechanical strength compared with the medium carbon bainitic grades
A and B whose carbon content comes under the medium-high carbon
category. The yield strengths are 60 to 130 MPa higher and the
mechanical strengths 70 to 190 MPa higher, other things being
equal. They also allow an increase in impact strength of up to
about 100% compared with medium-high carbon grades (C: 50
J/cm.sup.2 compared with 39 J/cm.sup.2 for A, 32 J/cm.sup.2 for A
(FA) and 27 J/cm.sup.2 for B) again with other things being
equal.
[0075] As indicated in Table 3, the structure is bainitic in all
cases with the exception of the E (FA) casting cooled under forced
air. This is supported by the Re/Rm ratio which works out at a
value of about 0.6 typical of a bainitic structure, except in the
case of E (FA) in which martensite is present and in which Re/Rm
assumes a higher value.
[0076] The presence of martensite is not prohibitive in itself,
insofar as the mechanical characteristics remain very high (in
particular impact strength remains higher than 40 J/cm.sup.2). On
the other hand, since the fraction of formed martensite is highly
sensitive to the exact conditions of cooling, major dispersion of
the mechanical characteristics can be expected in parts produced
under industrial conditions for which the controlled cooling of the
part cannot always be optimal. The objective must therefore be set
of limiting the total presence of martensite, pro-eutectoid ferrite
and pearlite to no more than 20%.
[0077] However, the major role must be stressed of the size of the
parts when analysing mechanical characteristics: for example, while
grade E cooled under forced air exhibits martensite on a round 40
mm in diameter, it was ascertained that conversely it allows a
homogeneous bainitic structure to be guaranteed on diameters of 50
to 300 mm.
[0078] If a particularly high value of Re is sought, it is possible
to apply tempering to the part at low temperature, before or after
final machining. As shown in Table 3, such tempering allows a yield
strength to be obtained that is up to 200 MPa higher than that
obtained after normalization, whilst maintaining even increasing
impact strength (up to +25%). Machining thereof will not be
affected. It is also found that the results obtained vary little
over a temperature range of 250-350.degree. C. for tempering.
Industrial treatment can therefore easily be carried out without
the need for very precise control over tempering conditions.
TABLE-US-00003 TABLE 3 Mechanical characteristics obtained after
tempering Re Rm KCU Specimn Treatment (MPa) (MPa) RE/Rm A (%) Z (%)
(J cm.sup.-2) F no temper 745 1213 0.61 17 49 44 temper 940 1184
0.79 12 59 59 300.degree. C., 2 h temper 937 1165 0.80 15 60 57
350.degree. C., 2 h C no temper 738 1185 0.62 15 53 50 temper 897
1156 0.78 15 58 250.degree. C., 2 h temper 920 1144 0.80 14 60
300.degree. C., 2 h temper 912 1138 0.80 14 56 350.degree. C., 2
h
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