CA2395628C - Austenitic stainless steel for cold forming possibly followed by machining - Google Patents

Abstract

Austenitic stainless steel for cold deformation which can be followed by machining, characterized by the following weight composition: carbon <0.030% 0.3% <silicon <1% 5% <manganese <9% 4.55% <Nickel <7 % molybdenum <0.8% 2% <copper <4% 0.02% <nitrogen <0.060% sulfur <0.01% phosphorus <0.030%

Description

Austenitic stainless steel for cold deformation which can be followed by machining The present invention relates to austenitic stainless steel for cold type cold stamping. Steels that contain a minimum of 11% of Chrome are said to be rustproof and have a good resistance to corrosion. The steels Austenitic stainless steels also contain nickel, which gives them of the interesting properties of implementation, especially in transformation to cold.
One of the most commonly used austenitic steels is a steel whose composition is as follows: C <0.08, Si <1%, Mn <2%, S <03%, P <0.045%, 18 <Cr20%, 8 <Ni <12%, which is suitable for cold processing, but is limit in the field of deformation due to its coefficient of hardening relatively high, defining a significant consolidation due to the formation of martensite hardening.
Another austenitic stainless steel of the following composition is known:
C <0.12%; If <1%; Mn <2%; S <0.03%; P <0.045%; 17 <Cr <19%; 10 <Ni <
13%
which provides a solution to the problem of hardenability due to a nickel relatively high, which decreases the formation of martensite hardening.
However, the presence of nickel at these levels also limits the properties lengthening of steel, moreover, this steel is expensive given the content high in nickel.
An interesting alternative has been partially found by adding copper in the composition. Copper indeed acts like nickel by limiting the training of martensite hardening. However, there is an upper limit for the use of copper, typically 3.5%, which can not be exceeded, so avoid a burning phenomenon during hot rolling of steel. This content higher in copper, however, makes it possible to obtain a good aptitude for deformation, even 3o reduce the nickel content globally to around 8%. Such a composition steel is mainly used in cold-stamping long steel products stainless.
A common idea to improve implementation a bit more cold austenitic stainless steels is to introduce into the composition of - - - -------- - - - -----

2 manganese, which acts in the same direction as nickel or copper. Many documents offer solutions of this type. For example, the patent FR2229776 claims a steel of the following composition: 3 <Ni <
15%; 6% <Mn <16%; 10% <Cr <25%;If> 2%.
The interest of this steel concerns the resistance to abrasion.
the US Pat. No. 3,910,788 discloses a steel of the following composition: 1% <Si <
2.5%, 1.5% <Mn <5%, 1% <Cu <4%, 6% <Ni <9%; 15% <Cr <19%; N <
0.03%; C + N <0.04%.
This steel is suitable for cold processing and stamping in the The field of flat products and resists breaking apart. The contents in nickel the composition are relatively high, and the steel contains silicon in a not negligible amount, which increases the coefficient of hardening.
Japanese documents J 63060051 and J63060050 both propose the composition of a steel which is the following: C <0.15%; If <1.5%; 0.5% <Mn <
6%; 17% <Cr <23%; 10% <Ni <15%; 0.1% <Cu <3%; 0.02% <N <0.35%;
with stabilization with elements Ti + Nb + V. High levels in carbon and the nitrogen content presented means that high hardnesses are rapidly reached by cold processing, which is crippling for the field of cold.
US Pat. No. 3,753,693 discloses a steel composition such that:
Cr 19%; 7% <Ni <10%; 11% <Mn <13%; 0.01% <N <0.07%; C <0.06%; If <
1%; Mo <2%; Cu <1.5%.
This composition is recommended especially for applications strikes to cold, for which it offers a low coefficient of work hardening thanks to of the very high levels of nickel and manganese.
No doubt powerful in striking, this composition is not interesting because of the respective high levels of nickel and chromium.
the document JP 55 031 173 has a composition grade of C <
0.02%; 0.04% <N <0.1%; 2.5% <Cu <4%; 6% <Ni <8%; 17% <Cr <19%
and 3% <Mn <4%; S <0.003%.
In this document, the nickel content remains relatively high, and that in very high nitrogen. This steel is used for the manufacture of rivets and screws in steel stainless.

3 US 4,911,883, relates to a stainless steel for transformation to cold, of the following composition: C <0.04%; If <0.6%, 6% <Ni <8%; 2.2% <
Mn <3.8%; 17% <Cr <19%; 2.5% <Cu <4%; S <0.002%; N <0.010%.
This composition is close to the previous mentioned, with in particular a nickel content which remains quite high.
the document WO 00/26428 deals with a composition comprising: 0.025% <C <
0.15%; 4% <Mn <12%; If <1%; P <0.2%; S <0.1%; 15.5% <Cr <17.5%;
1% <Ni <4%; 0.25% <Mo <1.5%; 1.5% <Cu <4%; W <1%; Co <1%;
0.05% <N <0.3%; respecting the relations Cr% + Mo% <17.75% and Cr% + 3.3Mo% + 13N%> 20.5%. This composition comprises Ni levels bass.
In the same field, it is known document JP 2001011579A presenting the following composition: 0.05% <C <0.5%; 6% <Mn <15%; If <0.5%; S <
0.03%;
10% <Cr <20%; 0.04% <Ni <0.3%; Mo <3%; Cu <3%; Al <0.1%; 0.05% <N <
0.3%, whose proportion of nitrogen plus carbon is relatively high as in the composition of the previous document.
In summary, the proposed solutions for obtaining steel compositions austenitic stainless steel for cold processing containing manganese can be classified as follows:
- very allied shades, for very specific applications, - Nitrogen and / or carbon grades with high mechanical characteristics, - shades with fewer alleys, with nickel levels remaining high, and therefore, with relatively low levels of manganese, which does not very marked evolution compared to classical shades, both in term of implementation properties, only in terms of cost reduction.
The object of the invention is to propose an austenitic stainless steel for cold deformation that can be followed by machining with properties improved mechanical properties compared to standard stainless steel steels austenitics, very interesting properties for transformation to cold, and particularly in cold stamping, The subject of the invention is an austenitic stainless steel for deformation cold which can be followed by machining, characterized in weight composition next :
carbon <0.030%
0.3% <silicon <1%

4

5% <manganese <9%
4.55% <nickel <7%
15% <chromium <18%
molybdenum <0.8%
2% <copper <4%
0.02% <nitrogen <0.060%
sulfur <0.01%
phosphorus <0.030%
The other features of the invention are:
the composition further comprises boron at a content of less than 50.10 "4%
the index Md30, defined by the relation: Md30 =. 551 - 462 (C% + N%) - 9.2Si% -Mn% - 13.7 Cr% - 29 Ni% - 29 Cu% - 18.5 Mo% is less than - 60.
- the ferrite content after reheating of the solidification solidification structure at 1240 C is less than 10%, satisfying the following relation:
% Ferrite = 0.034 x2 + 0.284x - 0.347, in which, x = 6.903 [Cr eq - Ni eq 11,029

- 6.998], with Cr eq = Cr% and Ni eq = Ni% + 20.04 C% + 21.31 N% + 0.46 Cu% +
0.08 Mn%.
The following description, presented in a non-limiting way, will do well understand the invention. The invention deals with austenitic stainless steel for cold deformation which may be followed by machining, characterized in that composition following weight:
carbon <0.030%
0.3% <silicon <1%
5% <manganese <9%
4.55% <nickel <7%
15% <chromium <18%
molybdenum <0.8%
2% <copper <4%
0.02% <nitrogen <0.060%
sulfur <0.01%
phosphorus <0.030%
In the proposed composition, carbon is controlled at a lower level at 0.030% to limit the formation and hardening of martensite hardening. In the same way, and for the same reason, it is necessary of control the sum of the carbon and nitrogen contents to a value less than 0.07%.
The nitrogen content is controlled at a content of between 0.02% <N <
0.060% and preferably 0.02% <N <0.04% to stabilize the austenite and to guarantee a ferrite content when hot <15%. Indeed, to facilitate the transformation to hot, it will be ensured that the ferrite content is less than 10% after reheating of the solidification solidification structure at 1240 C, with the following relation:
% Ferrite =
0.034 x2 + 0.284x - 0.347, where x = 6.903 [Cr eq - Ni eq / 1.029 -6.998]
with Cr = Cr% and Ni eq = Ni% + 20.04 C% + 21.31 N% + 0.46 Cu% + 0.08 % Mn.
In the composition according to the invention, the nickel content has been lowered enter 4.55% and 7% and preferably about 5%. Lowering the Ni content is compensated by a content of. manganese between 5% and 9% and preferably about 8%. The chromium content guarantees a good resistance to corrosion, she is chosen between 15% and 18%.
Copper is present in the composition to stabilize the austenite. It is limited, as specified in the prior art to a content of less than 4%. We prefer a content greater than 2% for a better stabilization of the austenite.
Sulfur is limited to a content of less than 0.01% and preferentially to less than 0.002% in order to limit the formation of sulphides manganese, which is detrimental to hot processing and keeping corrosion.
Other elements such as silicon with a content below % and preferably 0.3%. The composition also contains molybdenum at a less than 0.8% and phosphorus less than 0.030%.
The possible addition of boron at a content of less than 10-4% may facilitate the hot and cold transformation.
In the end, the steel will have to reach a value of Md30 as low as possible, and preferably less than -60, the value Md30 being related to the relation next :
Md30 = 551 - 462 (C% + N%) - 9.2Si% - 20 Mn% - 13.7 Cr% - 29 Ni% - 29 Cu% -18.5% Mo.
In an exemplary embodiment, it has been compared a composition of a steel according to the invention with a reference steel, austenitic, standard whose compositions are presented in the following Table 1:

Table 1.
% C% Si% Mn% Ni% Cr% Mo% Cu% N% S Md3% a 1 0.02 0.36 8.1 5.1 15.0 0.30 3.3 0.03 0.00 -3 5 1 71.6 Ref 0.02 0.33 1.8 8.1 17.2 0.29 3.2 0.02 0.00 -9 7 2 59.3 In the field of cold stamping, the steel according to the invention has a cold working ability, better than reference steel standard. The Crossed typing tests interrupted show that:
Steel 1 according to the proposed invention does not exhibit cracking.
- The reference steel has ductile failure primers at the corners of the cross.
In the field of corrosion, the steel according to the invention has a good corrosion resistance, comparable to that of a standard 304 type steel.
In the fields of magnetism, welding and machining, steel according to the invention has a non-magnetic character in the annealed state, and weakly magnetic in the hardened state, in weldability, weldability equivalent to that of a standard type 304 steel and machinability, similar machinability to that of a standard 304 type steel.
The steel of the invention is commercially attractive at a cost using of the cheaper alloy elements than nickel, and retains a simplicity of implementation at all stages of its development, in particular through a at hot and mechanical properties compatible with a development in current production of standard austenitic stainless steels.
The steel of the invention lends itself well to the production of parts comprising a first cold stamping operation followed by finishing machining.
The field of its use is potentially, the fixation as per example, screws, bolts, nuts, threaded rods, and all parts Special implemented by cold stamping as for example, coins chandlery, automotive parts such as fasteners, air-bags, support and body of - -------- ----

7 probes or else, parts for the connection of large series as for example, pieces of electrical screws.
We can also mention applications involving the transformation to cold importantly by looking for a good cost performance ratio in the s field of wire drawn and fine wire, for example, for filters or strainers, or in the field of springs.

Claims (3)

1. Austenitic stainless steel for cold forming which can be followed by machining, characterized in the following composition by weight:

carbon <0.030%
0.3% <silicon <1%
5% <manganese <9%
4.55% <nickel <7%
15% <chromium <18%
molybdenum <0.8% 2% <copper <4%
0.02% <nitrogen <0.060%
sulfur <0.01%
phosphorus <0.030%

and the sum of the carbon and nitrogen contents has a value less than 0,07% and that the index Md30, defined by the relation:
Md30 = 551 - 462 (C% + N%) - 9.2Si% - 20 Mn% - 13.7 Cr% -29 Ni% - 29 Cu% - 18.5 Mo% is less than - 60.

2. Steel according to claim 1, characterized in that the composition further comprises boron at a content of less than 50 × 10 -4%. 3. Steel according to claim 1, characterized in that the ferrite content after reheating of the solidification solid structure at 1240 ° C is lower than 10%, satisfying the following relation:% Ferrite = 0.034 x2 + 0.284x -0.347, where x = 6.903 [Cr eq - Ni eq / 1.029 - 6.998], with Cr eq = Cr% and Neither eq =
Ni% + 20.04 C% + 21.31 N% + 0.46 Cu% + 0.08 Mn%.