U.S. patent application number 15/761373 was filed with the patent office on 2018-09-13 for method of producing a tool steel.
The applicant listed for this patent is Boehler Edelstahl GmbH & Co KG. Invention is credited to Siegfried GELDER, Harald LEITNER.
Application Number | 20180258504 15/761373 |
Document ID | / |
Family ID | 56618151 |
Filed Date | 2018-09-13 |
United States Patent
Application |
20180258504 |
Kind Code |
A1 |
LEITNER; Harald ; et
al. |
September 13, 2018 |
METHOD OF PRODUCING A TOOL STEEL
Abstract
In a method of producing a tool steel, in particular a hot-work
tool steel, a steel of the following analysis: C: 0.25-0.6
weight-%, Si: max 0.15 weight-%, Mn: max. 0.3 weight-%, Mo: 2-5
weight-%, Cr: 0-2 weight-%, W: 1-3 weight-%, V: 0-2 weight-%, Ni:
0-3 weight-%, and a residue of iron and unavoidable impurities due
to smelting is smelted and alloyed, wherein a workpiece of this
steel is heated up and austenitized at temperatures >Ac.sub.3
and then cooled down, wherein the cooling is effected down to a
temperature of 330.degree. C. to 360.degree. C. and the workpiece
is held isothermally at this temperature, until the workpiece is
completely bainitically transformed, followed by cooling down to
room temperature, together with a hot-work tool steel for this
purpose and its use.
Inventors: |
LEITNER; Harald; (St.
Marein, AT) ; GELDER; Siegfried; (St. Peter
Freienstein, AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Boehler Edelstahl GmbH & Co KG |
Kapfenberg |
|
AT |
|
|
Family ID: |
56618151 |
Appl. No.: |
15/761373 |
Filed: |
August 3, 2016 |
PCT Filed: |
August 3, 2016 |
PCT NO: |
PCT/EP2016/068495 |
371 Date: |
March 19, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/02 20130101;
Y02P 10/212 20151101; C21D 6/00 20130101; C21D 6/005 20130101; C21D
6/008 20130101; C22C 38/04 20130101; C22C 38/002 20130101; C22C
38/22 20130101; C21D 1/20 20130101; C22C 38/44 20130101; C22C 38/24
20130101; Y02P 10/20 20151101; C22C 38/50 20130101; C21D 2211/002
20130101; C22C 38/06 20130101; C22C 38/46 20130101; C21D 6/004
20130101 |
International
Class: |
C21D 6/00 20060101
C21D006/00; C21D 1/20 20060101 C21D001/20; C22C 38/50 20060101
C22C038/50; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 7, 2015 |
DE |
10 2015 113 058.0 |
Claims
1. Method of producing a tool steel, in particular a hot-work tool
steel, wherein a steel of the following analysis C: 0.25-0.6
weight-% Si: max 0.15 weight-% Mn: max. 0.3 weight-% Mo: 2-5
weight-% Cr: 0-2 weight-% W: 1-3 weight-% V: 0-2 weight-% Ni: 0-3
weight-% and a residue of iron and unavoidable impurities due to
smelting is smelted and alloyed, wherein a workpiece of this steel
is heated up and austenitized at temperatures >Ac.sub.3 and then
cooled down, wherein the cooling is effected down to a temperature
of 330.degree. C. to 360.degree. C. and the workpiece is held
isothermally at this temperature, until the workpiece is completely
bainitically transformed, followed by cooling down to room
temperature.
2. Method according to claim 1, wherein after complete through
heating for a period of 20 to 40 min., austenitizing takes
place.
3. Method according to claim 1, wherein isothermal holding takes
place for a period of 30 to 90 min.
4. Method according to claim 1, wherein cooling from the
austenitizing temperature down to the isothermal holding
temperature is conducted at cooling rates from .lamda.=0.4 to
.lamda.=2.
5. Method according to claim 1, wherein after cooling to room
temperature, the workpiece is tempered to set the hardness.
6. Method according to claim 1, wherein the tempering temperature
lies between 580.degree. C. and 660.degree. C., while the duration
of tempering lies between 1 and 2 hours.
7. Hot-work tool steel produced by a method according to claim 1
and with an analysis as follows: C: 0.25-0.6 weight-% Si: max 0.15
weight-% Mn: max. 0.3 weight-% Mo: 2-5 weight-% Cr: 0-2 weight-% W:
1-3 weight-% V: 0-2 weight-% Ni: 0-3 weight-% and a residue of iron
and unavoidable impurities due to smelting, wherein the hot-work
tool steel has a residual austenite content <1% with an
otherwise completely bainitic structure, wherein hardness lies
between 46 and 53 HRC and notch-bend impact energy 4.5 and 8.5
joules.
8. Use of a tool steel according to claim 7 for hot-work tools such
as injection moulds, hot-working tools, press hardening tools,
forging dies, hammer dies, hot-piercing punches, hot-extrusion
punches.
Description
[0001] The invention relates to a method of producing a tool steel
and in particular a hot-work tool steel.
[0002] The term tool steels describes steel materials used in
particular for the working or reshaping of a multiplicity of
materials. It includes amongst others cool-work tool steels,
plastic mould steels, metal-ceramic steels and hot-work tool
steels.
[0003] Hot-work tool steels generally describes tool steels which,
in use, accept a constant temperature lying in excess of
200.degree. C., while this constant temperature is overlaid by
temperature peaks arising from the work cycle. Besides the general
mechanical stresses which such steels undergo due to the relevant
forming process, they are therefore subject to a further thermal
stress. General requirements for hot-work tool steels therefore
also include a good so-called fire crack resistance, a type of wear
resulting from frequent temperature changes in the surface
areas.
[0004] In addition, hot-work tool steels must have resistance to
the occurrence of heat cracks, which is ensured by a core
requirement of hot-work tool steels, namely so-called thermal
toughness.
[0005] Furthermore, such hot-work tool steels must also of course
possess high strength, since they are generally subject to impact
compression or tension under heat. Not least of importance are of
course good wear properties under heat, in particular a low
tendency to adhere relative to the materials to be processed, good
resistance to erosion, also to high-temperature corrosion and
oxidation, together with good dimensional stability, also in the
hot state.
[0006] Since these hot-work tools must be mechanically processed,
so that they can give other materials the necessary shape under
heat, good machinability is likewise a requirement. Especially high
stresses occur in particular during high-temperature working of
steel materials, in particular if these steel materials are hot
when placed in a tool made of a hot-work tool steel and then cooled
down in this tool in order to generate hardness.
[0007] The properties of steel materials described are determined
on the one hand by the chemical analysis of the steel material, but
primarily the structure of the hot-work tool steel is critical, in
particular for properties such as toughness and strength. In this
connection, properties such as toughness and strength, but also
thermal conductivity and other important properties, may not be
individually enhanced without possible negative effects on other
desirable properties. To this extent, hot-work tool steels often
require compromises, on the one hand in respect of their chemical
analysis, but on the other hand also in terms of their structural
formation.
[0008] Hot-work tool steels are known for example from CH 165893,
in which a chemical analysis is disclosed, also on the other hand a
suitable heat treatment for setting certain properties.
[0009] Similarly known from AT 144892 is a steel alloy, in
particular for hot-work tools and tools or articles which have to a
high degree insensitivity to temperature fluctuation, dimensional
stability, hot-tensile strength and toughness. A subject of this
document are chromium-tungsten-nickel steels, in which the nickel
content may be partly or wholly replaced by cobalt, and in which
tungsten, cobalt, nickel and chromium may be contained but are
preferably chromium-free.
[0010] Known from WO 2008/017341 A1 is a method of setting the
thermal conductivity of a steel, tool steel, in particular hot-work
tool steel and a steel article made from the latter. The steel
material according to this document should have much greater
thermal conductivity than known tool steels but, with an
essentially known steel analysis, gives no indication as to how
this higher thermal conductivity may be obtained in practice.
[0011] Known from DE 1090245 is the use of a steel for hot-work
tools, involving on the one hand a chemical analysis for a hot-work
tool steel and on the other hand a heat treatment with cooling down
from a temperature in the range of 900.degree. C. to 1200.degree.
C. expediently 1000.degree. C. to 1100.degree. C. and tempering
treatment at a temperature in the range of 500.degree. C. to
750.degree. C., so that a hardness of 30 to 60 HRC is obtained. In
this way, a substantially martensitic structure should be set.
[0012] From Maschinenmarkt [Machine Market] 24/2010, pages 58 to 61
it is known that it may be advantageous to induce austempering in
steel structures, since the bainite phase combines properties which
appear contrary to one another, such as high levels of hardness and
toughness. In many applications, though, austempering is not
cost-effective owing to the lengthy treatment time. A continuous
process of measuring the degree of austempering is intended to
optimise this austempering time, while the austempering should in
particular avoid residual austenite from the softer structure
constituents. It is stated that, in comparison with martensitic
hardening, austempering has the outstanding benefit of making
possible the simultaneous achievement of very high hardness and
high toughness levels. It is true that, through austempering,
slightly lower hardness is obtained than that of martensitic
structures but toughness, which can be detected as impact energy
for example in the notched-bar impact test, is markedly increased.
Further benefits of bainitic hardness should be less distortion,
good dimensional stability, enhanced resistance to crack growth and
the scope for generating residual compressive stress in the layer.
It has however emerged that the critical disadvantage of bainitic
hardening is the comparatively long holding time in the
austempering bath. The bainite transformation is a time-intensive
process, with the process time depending on the material structure,
the composition of the alloy, the temperatures of austenitizing and
of the bainite transformation itself. In principle, austempering
here takes place in a three-stage process, in which firstly
austenitizing takes place with, as far as possible, complete
transformation of the ferrite into austenite. The part is then
cooled down so rapidly to the austempering temperature that no
ferrite or pearlite occurs. Finally, the austempering temperature
is held constant and the transformation from austenite to bainite
takes place gradually.
[0013] From Design of advanced bainitic steels by optimisation of
TTT diagrams and T.sub.0 curves, ISIJ International, Volume 46
(2006), No. 10, pp. 1479 to 1488 it is known, that conventional
bainitic steels have a reproducibly good combination of toughness
and hardness, but remain behind quench-hardened and tempered
martensitic steels. This is attributed to the fact that cementite
particles have a worsening effect in the microstructure of the
steel, since cementite can act as a defect or crack initiator in
fracture-proof steels. The occurrence of cementite during bainite
transformation can however be suppressed by a steel composition
containing around two weight-% silicon. Thermodynamic and kinetic
models are presented so that steels with an optimal bainitic
microstructure may be developed; these are comprised of a mixture
of bainitic ferrite, plastic-enriched residual austenite and some
martensite. Using these models, a set of seven carbides-free
bainitic steels with 0.3 weight-% carbon is proposed for
production.
[0014] Known from DE 600 300 867 T2 is an ingot steel for the
production of injection moulds for plastic material or for the
production of workpieces for metal processing, which should have a
martensitic or martensitic-bainitic structure with a relatively
high chromium content.
[0015] Known from EP 2 662 460 A1 is a method for the production of
steel, in particular as a casting mould or tool, with both a
bainitic and also a martensitic phase, wherein the steel should
undergo heat treatment including austenitizing followed by rapid
cooling, in order to inhibit the formation of stable phases with a
transformation temperature above that of the bainite, and to hold
the temperature high and long enough to prevent the transformation
from austenite to martensite, so that at least 60 percent of the
transformation takes place below the martensite start temperature
plus 200.degree. C. but above the martensite start temperature
minus 50.degree. C., so that at least 70 percent bainitic
microstructure with fine carbide-like constituents is obtained and
impact toughness in excess of 8 joules is reached, within at least
20 millimetres of the surface of the heat-treated steel. Here the
martensite start temperature should be .ltoreq.480.degree. C. In
particular the silicon content, at 1.3 percent, should be
relatively high, as with many steels which are to be
austempered.
[0016] Known from EP 1 956 100 A1 is a hot-work tool steel and a
method for its processing, in which a hot-work tool steel of a
given analysis is cooled down to room temperature after solution
annealing and then reheated to a temperature above Ac1 and
subsequently cooled down to room temperature once more, then
subjected to heat treatment, with austenitizing carried out during
the first heat treatment.
[0017] Known from EP 2 006 398 A1 is a method for the production of
a steel material in which a steel material is completely
austenitised and then cooled down to the temperature of the
pearlite nose of the corresponding steel alloy, where it is held
until complete pearlite transformation.
[0018] Known from EP 1 887 096 A1 is a hot-work tool steel which is
intended to have a considerably higher thermal conductivity than
known hot-work tool steels, for which purpose it has a special
analysis, which is however practical for any hot-work tool
steel
[0019] The problem of the invention is to create a method for the
production of a hot-work tool steel which ensures bainite
transformation in an economically feasible short length of
time.
[0020] The problem is solved by a method with the features of claim
1.
[0021] A further problem is to create a hot-work tool steel with
which the method may be implemented.
[0022] The problem is solved by a method with the features of claim
7.
[0023] The austempering according to the invention of the steel
material is brought about by providing that, after austenitizing
treatment of the steel, it is cooled down to s holding temperature
and is held at this holding temperature until austempering is
completed.
[0024] Here the steel material according to the invention makes it
possible for the austempering holding time to lie in roughly the
same time range as the austenitizing holding time, which means that
austempering is made possible in a time which is absolutely
economical. In this connection the holding time depends in
particular on material strength and material quantity respectively,
i.e. in particular the time within which the necessary holding
temperatures occur.
[0025] According to the invention this austempering may be measured
in a dilatometer and, for the respective steel material, the
relevant period of time depending on material strength may be
determined. The reduction in temperature following austenitizing is
also accompanied by shrinkage due to reduced thermal expansion
wherein, immediately after the holding temperature has been reached
a relative change in length occurs due to the formation of the
bainite. Once the relative change in length is concluded, the
possible bainite transformation is completed wherein, as explained
above, a complete bainite transformation may be achieved with the
steel material according to the invention in contrast to customary
hot-work tool steels. Cooling down to room temperature then takes
place, leading to further shrinkage, wherein however as compared
with the material before bainite transformation, the material after
bainite transformation does not return to the value before
transformation, but remains somewhat above it.
[0026] A steel with the following composition has proved to be
advantageous: [0027] C: 0.25-0.6 weight-% [0028] Si: max 0.15
weight-% [0029] Mn: max. 0.3 weight-% [0030] Mo: 2-5 weight-%
[0031] Cr: 0-2 weight-% [0032] W: 1-3 weight-% [0033] V: 0-2
weight-% [0034] Ni: 0-3 weight-%
Residue of Iron and Impurities Due to Smelting
[0035] Within this analysis it is possible, with an isothermal
holding step between 330.degree. C. and 360.degree. C., to bring
about a suitable complete bainite transformation.
[0036] Temperatures below this temperature window effect martensite
formation; higher temperatures lead to formation of the upper
intermediate stage, which has poorer mechanical properties.
[0037] The invention is explained below by way of example with the
aid of a drawing showing in:
[0038] FIG. 1 the measurement of the change in length over the
temperature programme in a dilatometer test of a steel according to
the invention;
[0039] FIG. 2 a representation of the change in length relative to
temperature during heating up and cooling down including the
isothermal holding time;
[0040] FIG. 3 the course of the change in length relative to
temperature during heating up and cooling down after the isothermal
transformation;
[0041] FIG. 4 the micro-section of the completely austempered but
not tempered steel material at an isothermal holding temperature of
330.degree. C.;
[0042] FIG. 5 the micro-section of a steel material according to
the invention, which shows complete transformation, austempered at
360.degree. C. and not tempered;
[0043] FIG. 6 the course of the relative change in length and
temperature during a dilatometer test with a conventional hot-work
tool steel;
[0044] FIG. 7 the course of the relative change in length depending
on the temperature during heating up and cooling down including an
isothermal holding step, for a conventional hot-work tool
steel;
[0045] FIG. 8 the course of the relative change in length and
temperature during the dilatometer test at a holding temperature of
340.degree. C., for a conventional hot-work tool steel;
[0046] FIG. 9 the course of the relative change in length depending
on the temperature during heating up and cooling down including an
isothermal holding at 340.degree. C., for a conventional hot-work
tool steel;
[0047] FIG. 10 the bainite content depending on holding temperature
and holding time for a conventional hot-work tool steel, showing
that a complete bainite transformation is not possible;
[0048] FIG. 11 hardness and notch-bend impact energy for a steel
according to the invention which has undergone bainite
transformation according to the invention, dependent on tempering
temperature;
[0049] FIG. 12 temperature conductivity and thermal conductivity of
a steel which has been heat-treated according to the invention and
a steel which has not been austempered according to the
invention;
[0050] FIG. 13 the residual austenite content depending on the
cooling rate for a steel according to the invention which is not
heat-treated according to the invention;
[0051] FIG. 14 the micro-section, the relative change in length and
the hardness of a steel according to the invention quenched at
.lamda.=0.08, without complete bainite transformation;
[0052] FIG. 15 structure, relative change in length and hardness
after quenching at .lamda.=1.1;
[0053] FIG. 16 structure, relative change in length and hardness
after quenching at .lamda.=3;
[0054] FIG. 17 time and temperature pattern for cooling at
.lamda.=1.1 and .lamda.=3;
[0055] FIG. 18 an example of a heat treatment curve.
[0056] The steel according to the invention for implementing the
method according to the invention with the result of a complete
bainitic structure (if not otherwise stated, all percentage details
are weight-%) has the following composition: [0057] C: 0.25-0.6
weight-% [0058] Si: max 0.15 weight-% [0059] Mn: max. 0.3 weight-%
[0060] Mo: 2-5 weight-% [0061] Cr: 0-2 weight-% [0062] W: 1-3
weight-% [0063] V: 0-2 weight-% [0064] Ni: 0-3 weight-%
[0065] The remainder is iron and impurities due to smelting.
[0066] Such a steel is smelted and alloyed in the usual manner for
hot-work tool steels.
[0067] In the heat treatment according to the invention (FIG. 18)
such a steel is firstly austenitized at a temperature which lies at
least above the austenitizing temperature (Ac.sub.3) and ensures a
complete full austenitizing of the material or workpiece. For this
purpose a certain holding time may be necessary, depending on the
workpiece and its dimensions. After austenitizing, which is
conducted at heating rates of 300.degree. C./h to 400.degree. C./h,
rapid cooling down is effected at cooling rates of .lamda.=0.4 to
.lamda.=2, in particular to temperatures between 330.degree. C. and
360.degree. C. This temperature is held, with the holding time
similarly depending on the workpiece geometry.
[0068] In particular the alloying position determines the cooling
rate. Irrespective of the cooling rate, the cooling must be carried
out with sufficient speed that no bainite transformation takes
place during cooling.
[0069] After the bainite transformation is concluded in the holding
phase, the workpiece may be cooled down to room temperature,
wherein the cooling rate here lies between .lamda.=0.3 and
.lamda.=1. As a result, a complete bainitic structure is then
available, with residual austenite of less than 1 weight-%.
[0070] An example of heat treatment is revealed by FIG. 18, wherein
the specimen has dimensions of 370 mm.times.150 mm.times.60 mm.
[0071] Here the broken line indicates the required furnace value or
furnace temperature and the solid line shows the temperature
development of the test specimen material. It can be seen that,
during a first heating-up to 650.degree. C., the material follows
and with a holding time of four hours, the furnace required
temperature is also reached by the charge actual value. There then
follows a further heating stage, which includes a rise of approx.
200.degree. C./h and lasts for around two hours. After around one
and a half hours, the material here also reaches the required
temperature value and is then heated at a heating rate of approx.
260.degree. C./h to the austenitizing temperature of over
1100.degree. C. This temperature is reached relatively quickly by
the material.
[0072] From this heat treatment, the cooling down and holding at a
predetermined temperature is then effected, as is evident for
example from FIG. 1.
[0073] Within the specified steel analysis it is possible to
obtain, with the heat treatment process according to the invention,
a complete bainitic structure within reasonable heat treatment
times, including heating up, quenching and holding time.
[0074] Here it is noteworthy that the holding times for
austempering lie substantially in the range of holding times for
austenitizing, which was previously unachievable in any
circumstances.
[0075] Surprisingly a temperature range was found in which the
special material according to the invention may be transformed
completely into bainite in a technically reasonable time and if
necessary adjusted in terms of strength by a tempering process.
[0076] In contrast to conventional materials, in the case of the
material according to the invention, no residual austenite which
transforms into martensite remains after the heat treatment
according to the invention.
[0077] The invention is explained further with the aid of
examples.
EXAMPLE 1 (ACCORDING TO THE INVENTION)
[0078] A material according to the invention with the following
analysis:
TABLE-US-00001 C 0.31 weight-% Si 0.10 weight-% Mn 0.24 weight-% P
<0.005 weight-% S 0.0004 weight-% Cr 0.05 weight-% Mo 3.22
weight-% Ni 1.95 weight-% V <0.005 weight-% W 1.74 weight-% Ti
<0.005 weight-% Al 0.01 weight-%
is smelted and alloyed in the customary manner. The material is
brought into the form of a dilatometer specimen, as a cylinder with
a diameter of 5 mm and a length of 10 mm. This is used for the
dilatometer tests.
[0079] A further test specimen in the form of a notched-bar
specimen measuring 55 mm.times.10 mm.times.10 mm is heated to
austenitizing temperature, with the austenitizing set at
1090.degree. C. The test specimen is held at this temperature for
15 min. and then cooled down to 330.degree. C. Here the rate of
cooling is around 40.degree. C. per second.
[0080] The test specimens are held isothermally for 17 min. and
then cooled down to room temperature.
[0081] The resultant change in length of the dilatometer test
specimen over the temperature range is shown in FIG. 1. There the
temperature rise on the one hand and the relative change in length
on the other hand are shown as percentages, wherein it is evident
that, after the rapid cooling down from the austenitizing
temperature to 330.degree. C., a relative extension takes place,
which approaches a maximum which is held. With the subsequent
cooling down there is an irreversible lengthening even at room
temperature as compared with the very small extension after
reaching the austempering temperature. In FIG. 2, the relative
change in length is plotted as a percentage against temperature,
wherein it may be seen that, on completion of cooling down from the
austenitizing temperature to the austempering temperature with
isothermal holding, a change in length results, so that a
hysteresis loop occurs, in particular between heating up and
cooling down.
[0082] After the isothermal holding time the relative change in
length runs in percentage terms proportionally to the cooling down
(FIG. 3).
[0083] FIG. 4 shows the micro-section of the dilatometer test
specimen, with the Vickers hardness amounting to 494, while the
Rockwell hardness (HRC) comes to 50.5. A complete transformation of
the material into bainite may be seen from the micro-section. The
residual austenite content is <1% and is therefore insignificant
for the material properties.
EXAMPLE 2 (ACCORDING TO THE INVENTION)
[0084] The material of example 1 is smelted and alloyed in a
comparable manner and then subjected to comparable heat treatment,
wherein however the cooling down from the austenitizing temperature
of 1090.degree. C. is to 360.degree. C., with the cooling rate
coinciding with that of example 1.
[0085] The micro-section, made after cooling down to room
temperature, is shown in FIG. 5. Here too the residual austenite
content is >1%, while Vickers hardness is 494 and Rockwell
hardness (HRC) comes to 47. Here too a complete transformation is
obtained.
EXAMPLE 3 (NOT ACCORDING TO THE INVENTION)
[0086] A conventional hot-work tool steel with a chemical
composition of [0087] C=0.38 weight-% [0088] Si=0.10 weight-%
[0089] Mn=0.40 weight-% [0090] Cr=5.00 weight-% [0091] Mo=1.30
weight-% [0092] V=0.40 weight-% conforming to DIN EN as material
1,2343 or X 38 Cr Mo V5-1 is, after smelting and alloying, heated
to an austenitizing temperature of 1030.degree. C. and held at that
temperature until austenitizing is completed. The material is then
cooled down quickly to a holding temperature of 320.degree. C.
where it is held until change in length is constant and then cooled
down to room temperature.
[0093] FIG. 6 shows the pattern of the relative change in length
and temperature during the dilatometer test.
[0094] The pattern of the relative change in length depending on
temperature during heating and including the isothermal holding
time as described is evident from FIG. 7, wherein it is clear that
there is no closed hysteresis.
EXAMPLE 4 (NOT ACCORDING TO THE INVENTION)
[0095] The material according to example 3 is austenitized in the
same manner at 1030.degree. C., but cooled down to an isothermal
holding temperature of 340.degree. C. The pattern of the relative
change in length depending on temperature during heating and
cooling down including the isothermal holding time is here shown in
FIGS. 8 and 9, while here too it is evident that, although a
certain lengthening due to bainite formation does take place, the
lengthening then reduces and in particular a closed hysteresis
curve is not obtained.
[0096] The bainite contents of example 3 (holding
temperature=320.degree. C.) and example 4 (holding
temperature=340.degree. C.) with the respective holding times in
hours are shown in FIG. 10.
[0097] From this it is evident that, in examples 3 and 4, maximum
austempering is achieved only after 5 hours, which is many times
the maximum achievable austempering of the material according to
the invention, obtainable within a maximum of 20 min. Moreover,
with a holding temperature of 340.degree. C. a maximum austempering
of around 55% is obtained, whereas with a holding temperature of
320.degree. C. a maximum austempering of 80% is achieved. The
remaining structure is here similarly in the form of residual
austenite or martensite.
[0098] In the case of tempering after heat treatment, for example 1
a pattern of notch-bend impact energy and hardness respectively
corresponding to FIG. 11 may be observed. Depending on the
tempering temperature for two hours tempering time in each case,
the Rockwell hardness may be varied between 47 and 52, while in
each case notch-bend impact energy lies between 8 joules at room
temperature (i.e. in the untempered state) and 5 to 6 joules,
indicating very even hardness and toughness.
EXAMPLE 5 (NOT ACCORDING TO THE INVENTION)
[0099] The material according to the invention is subjected to a
conventional heat treatment on the dilatometer, with the test
specimen dimensions corresponding to those of the previous
examples.
[0100] Here, differing .lamda.-values are used, with .lamda. in
this case being the cooling parameter, which is usual in the heat
treatment of tool steels. It indicates in hectoseconds the time
needed for cooling down a steel from 800.degree. C. to 500.degree.
C.
[0101] Hardness and residual austenite patterns for different
.lamda.-values are evident from FIG. 13 wherein, with slow cooling,
hardness as expected falls from around 550 Vickers hardness to 325
Vickers hardness, whereas residual austenite content increases with
rising .lamda.-values.
[0102] With .lamda.=0.08 and a conventional heat treatment, the
material according to the invention has a martensitic structure,
with the structure pattern shown in FIG. 14, similarly the relative
change in length and temperature during cooling.
[0103] With .lamda.=1.1 the structure is characterised by
martensite and an intermediate stage in which the change in length
adopts a different pattern.
[0104] In FIG. 16 the structure is specified for .lamda.=3, with
martensite and an intermediate stage and a basically different
change in length to temperature relationship.
[0105] FIG. 17 shows, for cubes with edge lengths 75 mm and 180 mm,
how temperature and time behave, wherein cooling of the smaller
cube is effected with .lamda.=1.1, while cooling of the larger cube
is effected with .lamda.=3.
[0106] The tests show impressively the success in finding a
combination of a chemical composition of the steel on the one hand
and a heat treatment on the other hand, which makes possible a
completely bainitic structure of a hot-work tool.
[0107] The material according to the invention discloses this
potential only with the heat treatment according to the invention;
a conventional heat treatment does not lead to the desired
result.
[0108] Conversely, the heat treatment according to the invention
similarly does not produce the result with materials which are not
according to the invention.
[0109] In the case of the invention it is advantageous that, in a
reproducible and reliable way, it is possible to produce a tool
steel which has a bainitic structure and therefore outstanding
tough-hard properties, which may be controlled by suitable
tempering and in accordance with specific hardness properties, with
altogether high notch-bend impact energy.
* * * * *