U.S. patent application number 10/325730 was filed with the patent office on 2003-05-22 for method for the manufacture of steel products of a precipitation hardened martensitic steel, steel products obtained with such method and use of said steel products.
Invention is credited to Blaauw, Hubert Sjoerd, Nilsson, Jan-Olof, Olsson, Soren, Van der Grijn, Adriaan.
Application Number | 20030094218 10/325730 |
Document ID | / |
Family ID | 20416728 |
Filed Date | 2003-05-22 |
United States Patent
Application |
20030094218 |
Kind Code |
A1 |
Nilsson, Jan-Olof ; et
al. |
May 22, 2003 |
Method for the manufacture of steel products of a precipitation
hardened martensitic steel, steel products obtained with such
method and use of said steel products
Abstract
A method for the manufacture of steel products and products thus
produced, wherein steel is subjected to precipitation hardening in
a martensitic structure subsequent to soft annealing and thereafter
shaping. The method steps include shaping followed by solution
annealing between 1200.degree. C. and 1050.degree. C., quenching
from the solution annealing temperature with a quenching speed of
at least 5.degree. C. per second to a temperature below 500.degree.
C., subjecting said steel to an isothermal martensitic
transformation and subsequently hardening the steel at a
temperature between 450.degree. C. and 550.degree. C. to
precipitate particles out from solution into said martensitic
structure.
Inventors: |
Nilsson, Jan-Olof;
(Sandviken, SE) ; Olsson, Soren; (Sandviken,
SE) ; Blaauw, Hubert Sjoerd; (Drachten, NL) ;
Van der Grijn, Adriaan; (Drachten, NL) |
Correspondence
Address: |
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
20416728 |
Appl. No.: |
10/325730 |
Filed: |
December 23, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10325730 |
Dec 23, 2002 |
|
|
|
09644079 |
Aug 23, 2000 |
|
|
|
6531007 |
|
|
|
|
Current U.S.
Class: |
148/325 ;
148/320; 148/605 |
Current CPC
Class: |
C21D 6/02 20130101; C21D
2211/008 20130101; C21D 6/004 20130101; C21D 6/04 20130101 |
Class at
Publication: |
148/325 ;
148/320; 148/605 |
International
Class: |
C22C 038/24 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 1999 |
SE |
9902977-9 |
Claims
What is claimed is:
1. A method for the manufacture of a steel product comprising the
steps of: (a) subjecting the steel to precipitation hardening in a
martensitic structure subsequent to soft annealing; (b) shaping
said steel, followed by solution annealing between a temperature of
1200.degree. C. and 1050.degree. C. and for a time period of from 5
to 30 minutes; (c) quenching the steel from the solution annealing
temperature to a temperature below 500.degree. C. with a quenching
rate of at least 5.degree. C. per second, said quenched steel being
subjected to an isothermal martensitic transformation; and (d)
hardening at a temperature between 450.degree. C. and 550.degree.
C. for at least 3 minutes to cause particles to precipitate out
from solution into the martensitic structure.
2. The method according to claim 1, wherein the quenched steel is
subjected to an isothermal martensitic transformation at a
temperature between -30.degree. C. and -50.degree. C. for at least
one hour.
3. The method according to claim 1, wherein, between the solution
annealing and the quenching of the steel, the steel is subjected to
a sensitizing procedure between 950.degree. C. and 850.degree. C.
for at least 5 minutes thereby optimizing initiation of the
isothermal martensitic transformation.
4. The method according to claim 3, wherein the sensitizing
procedure homogeneously nucleates martensite during the isothermal
martensitic transformation.
5. The method according to claim 1, wherein the steel comprises
chromium (Cr) in a weight percentage between 10% and 14%.
6. A steel product manufactured by the method of claim 1, wherein
the steel product is a cap of an electric rotary shaver.
7. A steel product manufactured by the method of claim 1, wherein
the steel product is a cutter of an electric rotary shaver.
8. A steel product manufactured by the method of claim 1, wherein
the steel product is a cap of an electric translation vibration
shaver.
9. A steel product manufactured by the method of claim 1, wherein
the steel product is a cutter of an electric translation vibration
shaver.
10. A steel product manufactured by the method of claim 1, wherein
the steel product is a blade shaver.
11. A steel product manufactured by the method of claim 1, wherein
the steel product is a cutter in a domestic appliance.
12. A steel product manufactured by the method of claim 1, wherein
the steel product is a knife in a domestic appliance.
13. A steel product manufactured by the method of claim 1, wherein
the steel product is a spring in a domestic appliance.
14. A steel product manufactured by the method of claim 1, wherein
the steel product is a medical instrument.
15. A steel product manufactured by the method of claim 1, wherein
the steel product is a dental instrument.
16. A steel product manufactured by the method of claim 1, wherein
the steel product is a diaphragm plate spring in a fluid valve.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for the
manufacture of a steel product wherein the steel is subjected to
isothermal martensite formation and precipitation hardening in a
martensitic structure subsequent to soft annealing and shaping. The
invention also relates to a steel product obtained with such method
and to the use of said steel product.
[0003] 2. State of the Art
[0004] In the discussion of the state of the art that follows,
reference is made to certain structures and/or methods. However,
the following references should not necessarily be construed as an
admission that these structures and/or methods constitute prior
art. Applicant expressly reserves the right to demonstrate that
such structures and/or methods do not qualify as prior art against
the present invention.
[0005] In published international patent application WO93/07303, a
method of manufacture of the above mentioned kind has been
described wherein the transformation into the martensitic structure
is accomplished by air cooling after annealing in the austenitic
region or by cold working. Air cooling after annealing normally
results in the so-called athermal kinetic mode of martensite
transformation. The formation of air cooling induced martensite is
suppressed by alloying elements like nickel, titanium and aluminum,
which are used for precipitation of hardenable steel. It may be
that at relatively high concentrations of such alloying elements
the austenite is stabilized such that the martensitic
transformation start temperature becomes impracticably low.
SUMMARY OF THE INVENTION
[0006] It is an object of the invention to offer a method for the
manufacture of steel products, steel products so manufactured and
the use of said steel products whereby a practical optimum is
achieved between ductility, strength, wear and corrosion
resistance, homogeneity of martensite distribution and a practical
level of the martensite transformation temperature.
[0007] One aspect of the present invention is a method for the
manufacture of a steel product comprising the steps of subjecting
the steel to precipitation hardening in a martensitic structure
subsequent to soft annealing. The steel is then shaped into the
desired form followed by solution annealing between a temperature
of 1200.degree. C. and 1050.degree. C. and for a time period of
from 5 to 30 minutes. From the annealing temperature, the steel is
quenched at a rate of at least 5.degree. C. per second to a
temperature below 500.degree. C. The quenched steel being subjected
to an isothermal martensitic transformation. Hardening of the steel
is then accomplished at a temperature of between 450.degree. C. and
550.degree. C. for at least 3 minutes to cause precipitation of
particles out of solution into the martensitic structure.
BRIEF DESCRIPTION OF THE DRAWING FIGURE
[0008] The objects and advantages of the invention will become
apparent from the following detailed description of preferred
embodiments thereof in connection with the accompanying
drawing.
[0009] FIG. 1 is a temperature profile in time of the heat
treatment and processing method of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0010] A method for the manufacture of a steel product according to
the invention is characterized by shaping of the steel followed by
solution annealing between 1200.degree. C. and 1050.degree. C. for
from 5 to 30 minutes, after which the steel is quenched from the
solution annealing temperature to a temperature below 500.degree.
C. with a quenching speed of at least 5.degree. C. per second. The
quenched steel is then subjected to an isothermal martensitic
transformation and is subsequently hardened by being held at a
temperature between 450.degree. C. and 550.degree. C. for at least
3 minutes to precipitate out particles from solution into the
martensitic structure.
[0011] A combination of an isothermal martensitic transformation
and precipitation hardening is known (See Scripta Metallurgica et
Materialia, 1995, Vol. 33, No. 9, pp. 1367-1373). However, a method
of manufacture of the above-mentioned kind which allows a steel
product to be formed of a relatively complicated shape by
deformation whilst achieving an optimum between ductility,
strength, wear and corrosion resistance and homogeneity of
martensite distribution is not disclosed.
[0012] It is a further object of the invention to provide a very
efficient method for the manufacture of steel products with a
homogeneous distribution of martensite and precipitates.
[0013] Accordingly, a method for the manufacture of steel products
according to the invention is further characterized by subjecting
the quenched steel to an isothermal martensitic transformation by
holding the steel at a temperature between -30.degree. C. and
-50.degree. C. for at least one hour.
[0014] A method for the manufacture of steel products according to
the invention is still further characterized by a sensitizing
procedure in which the steel is held at a temperature between
950.degree. C. and 850.degree. C. for at least 5 minutes so as to
allow initiation of the martensitic transformation to become
optimal. The sensitizing procedure occurs between solution
annealing and quenching the steel. A steel subjected to a
sensitizing procedure avoids thermo-mechanical stresses which would
otherwise build up internally in the steel product. The absence of
internal thermo-mechanical stresses enables the manufacture of a
steel product with a very accurate size and which is stable in
use.
[0015] A further object of the invention is to provide a method of
manufacture of a steel product exhibiting a combination of superior
strength, corrosion resistance and ductility. Such a method is
further characterized in that the steel comprises chromium (Cr) in
a weight percentage between 10% and 14%. Generally, martensitic
steels with a low weight percentage of carbon, so-called maraging
steels, may be with or without chromium. Corrosion resistant
maraging steels comprise a weight percentage of chromium between
10.5 and 18%. A particular type of maraging steel, which may be
obtained by the method according to the invention, contains in
weight percentage 10-14% Cr, 7-10% Ni, 5-6% Mo, 0-9% Co, 0.5-4% Cu,
0.05-0.5% Al, 0.4-1.4% Ti and less than 0.03% C and N.
[0016] The invention will be elucidated further by the use of
practical examples:
EXAMPLE 1
[0017] A steel material suitable for use with the present invention
and containing a weight percentage of Cr of 10-14% was produced as
a strip material from a full scale seven ton melt in a high
frequency furnace and then subjected to rolling. The solidification
process after melting 1 is shown in FIG. 1 in which the temperature
profile over time is indicated by a solid line. Solidification of
the melt leads to crystallization of Ti (C, N), thereby binding the
free carbon and free nitrogen. The binding of free nitrogen is
important because the free nitrogen would otherwise prohibit the
isothermal martensitic transformation.
[0018] Before rolling, the steel is reheated to a temperature of
1150-1250.degree. C. and soaked at this temperature for at least 1
hour in order to give the material an austenitic structure and
sufficient ductility to be hot rolled. Reheating to a temperature
of 1150-1250.degree. C. 2 is followed by hot rolling 3. Hot rolling
3 produces a material in a strip shape with a suitable grain size
and evenly distributed intermetallic inclusions of carbon.
Additionally, hot rolling into a strip is accomplished without
resulting in a strain induced martensitic transformation.
[0019] Scale (oxide layers) formed during soaking and hot rolling
has to be removed by etching and/or grinding before the material
can be cold rolled to final dimensions. Cold rolling 4 gives the
strip steel the final thickness without formation of oxide layers.
Cold rolling 4, however, leads to strain induced martensitic
transformations and, to ensure sufficient ductility to form a
complicated product, the material has to be brought back into the
austenitic condition by annealing 5. This annealing 5 is carried
out in a continuous furnace at a temperature around 1050.degree.
C., to prevent the material from transforming to martensite before
shaping of the product. The product is cold formed in the
austenitic condition 6 leading to a partial transformation to
strain induced martensite. To ensure a homogeneous martensite
transformation throughout the product and sufficient hardenability
of the formed martensite by precipitation hardening, the material
has to be solution annealed 7 for 5 to 30 minutes at a temperature
between 1050.degree. C. and 1200.degree. C. Solution annealing 7
also causes Al, Mo, Cu, Ti, C and N to go into substitutional
and/or interstitial solution in the austenitic structure and
reversion of strain induced martensite to austenite. The elements
Al, Cu, Mo and Ti in solution are used for precipitation hardening
of the isothermal martensite in a later stage of the
manufacture.
[0020] In order to achieve an optimal isothermal martensitic
transformation 10, the martensitic transformation 10 should be
carried out at a temperature between -30.degree. C. and -50.degree.
C. for at least one hour. More preferably, the isothermal
martensitic transformation 10 is preceded by a sensitizing process
8. The sensitizing process 8 is positioned between a solution
annealing step 7 and a quenching step 9. The sensitizing process 8
occurs when the steel is held at a temperature between 850.degree.
C. and 950.degree. C. for at least five minutes. The sensitizing
process 8 causes destabilization of the austenitic structure of the
steel material and so facilitates the later isothermal martensitic
transformation 10. It has been determined that during the
sensitizing process 8, Mo and Ti are removed from the solution and
it is believed that Mo concentrates along crystal boundaries. The
behavior of Ti is not yet clear. Sensitization further ensures
homogeneous nucleation of martensite during the isothermal
martensitic transformation 10. Quenching 9 to room temperature or
even lower prevents premature precipitation of essential
intermetallic compounds in the austenite.
[0021] After quenching 9, the steel material is subjected to an
isothermal martensitic transformation 10. This transformation is
accomplished by holding the steel at a temperature of -30.degree.
C. to -50.degree. C. for at least one hour. The result is a
homogeneous martensitic structure with regularly distributed
retained austenite in a fine grain size. The isothermal martensitic
transformation 10 is followed by a hardening procedure 11 during
which intermetallic compounds like .eta.-Ni.sub.3(Al, Mo, Ti) and
.beta.-NiAl precipitate out from substitutional and/or interstitial
solution into the martensitic structure. The steel product so
treated will have a homogeneous hardness of more than 500 HV.
[0022] A steel product which is obtained by the present method
exhibits excellent properties with respect to wear and corrosion
resistance, homogeneous hardness and ductility during the
austenitic phase of the manufacture. This makes the strip steel
product very attractive for shaver caps of electric rotary shavers,
which are subjected to deep drawing during manufacture in order to
obtain the necessary bowl shape. The same applies to the heavily
deformed cutters of shavers, the strongly shaped knives of blenders
and the strongly folded return springs for thermostats in
irons.
[0023] The chemical composition in weight percentages of a steel
material, which is very well suited to be subjected to the
treatment method according to the present invention is as follows
(so-called Sandvik 1RK91 steel):
1 C + N <0.05 Cr 12.00 Mn 0.30 Fe balance Ni 9.00 Mo 4.00 Ti
0.90 Al 0.30 Si 0.15 Cu 2.00
EXAMPLE 2
[0024] A steel material or product with the same chemical
composition as in Example 1 may be produced as a diaphragm plate
spring functioning as a return spring in a fluid valve. Depending
on the required accuracy of the diaphragm plate spring dimensions,
it may be allowed to have so-called rest austenite in the product
after quenching 9. In the event that maximum accuracy is required,
it is preferred to anneal the solution 7 followed by sensitizing
the solution 8 which causes destabilization of the austenite so
that the later isothermal martensitic transformation 10 is
facilitated. Diaphragm plate springs for many applications use
complicated shapes which require strong deformations during
forming. Such deformations cause strain induced martensite which
has to be reversed into austenite by solution annealing 7. The
method of the present invention is well suited to preparing the
steel stock for this application.
[0025] Although the present invention has been described in
connection with preferred embodiments thereof, it will be
appreciated by those skilled in the art that additions, deletions,
modifications, and substitutions not specifically described may be
made without department from the spirit and scope of the invention
as defined in the appended claims.
* * * * *