Austenitic Stainless Steel for Cold Working Suitable For Later Machining

Abstract

Austenitic stainless steel for cold working suitable for later machining, characterized by the following composition by weight: carbon<0.030% 0.3%<silicon<1% 7%<manganese<9% 4.55%<nickel<7% molybdenum<0.8% 2%<copper<4% 0.02%<nitrogen<0.060% sulfur<0.01% phosphorus<0.030%

Description

This is a Continuation-In-Part of application Ser. No. 10/201,968 filed Jul. 25, 2002. The entire disclosure of the prior applications is hereby incorporated by reference.

The invention concerns an austenitic stainless steel for cold working such as cold heading. Steels containing a minimum of 11% chrome are called stainless and exhibit good corrosion resistance. Austenitic stainless steels also contain nickel, which provides valuable properties for processing, particularly cold working

BACKGROUND OF THE INVENTION

One of the most commonly used austenitic steels has the following composition: C<0.08, Si<1%, Mn<2%, S<03%, P<0.045%, 18<Cr 20%, 8<Ni<12%. This steel is well suited for cold working, but has a limited forming range due to the relatively high hardening coefficient which leads to significant work hardening via the formation of hardening martensite.

Another austenitic stainless steel is known with the following composition: C<0.12%; Si<1%; Mn<2%; S<0.03%; P<0.045%; 17<Cr<19%; 10<Ni<13%, providing a solution to the hardening issue through a relatively high nickel content, which limits the formation of hardening martensite. However, the presence of nickel in such concentrations also limits the elongation properties of the steel; furthermore, this steel is costly due to the high nickel content.

An interesting alternative has been partially identified by adding copper to the composition. Indeed, copper acts in the same way as nickel, limiting the formation of hardening martensite. There is, however, an upper limit for copper concentration, typically 3.5%, which should not be exceeded in order to prevent burning while hot rolling the steel. Nevertheless, such high copper content provides good forming capabilities, enabling the overall nickel content to be diminished to around 8%. Such a steel structure is particularly employed in the cold heading of long stainless steel products.

A widespread approach that improves to some extent the cold processing of austenitic stainless steels is to add manganese to the composition, which acts in the same way as nickel or copper. Several patents propose solutions of this nature. For instance,

• • French patent 2,229,776 claims a steel with the following composition: 3<Ni<15%; 6%<Mn<16%; 10%<Cr<25%; Si>2%.
    This steel has valuable abrasion resistance properties.
  • U.S. Pat. No. 3,910,788 describes a steel with 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 working and die-stamping in the flat product industry and is resilient to delayed failure. The nickel content in the composition is relatively high; the steel also contains sensible amounts of silicon to increase the hardening coefficient.

• • Japanese patents J63060051 and J63060050 both propose the following steel composition: C<0.15%; Si<1.5%; 0.5%<Mn<6%; 17%<Cr<23%; 10%<Ni<15%; 0.1%<Cu<3%; 0.02%<N<0.35%; with stabilization using chemical elements Ti+Nb+V. The high carbon and nitrogen contents result in high hardness being rapidly attained during cold working, which disqualifies the steel for cold heading applications.
  • U.S. Pat. No. 3,753,693 presents a steel composition such as: 17<Cr<19%; 7%<Ni<10%; 11%<Mn<13%; 0.01%<N<0.07%; C<0.06%; Si<1%; Mo<2%; Cu<1.5%.

This composition is particularly well-suited for cold heading applications where it provides a low hardening coefficient due to the very high nickel and manganese contents.

While being well-suited for cold heading, this composition has no economical interest because of the high nickel and chrome contents.

• • JP 55,031,173 presents a grade with the following composition: 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%.

The document provides a relatively high nickel content, and a very high nitrogen content. This steel is used to manufacture stainless steel rivets and screws.

• • U.S. Pat. No. 4,911,883 concerns a stainless steel for cold working, with the following composition: C<0.04%; Si<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 similar to that previously cited, but has a rather high nickel content.

• • WO 00/26428 describes the following composition: 0.025%<C<0.15%; 4%<Mn<12%; Si<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%; with the conditions Cr %+Mo %<17.75% and Cr %+3.3Mo %+13N %>20.5%. This composition has a low nickel content.
    In the same category, JP 2001011579A presents the following composition: 0.05%<C<0.5%; 6%<Mn<15%; Si<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%, with a relatively high nitrogen+carbon content, as in the composition proposed by the previous document.

In conclusion, solutions proposed to obtain compositions of austenitic stainless steel for cold working containing manganese may be classified in the following manner:

• • highly alloyed grades, for very specific applications;
  • nitrogen and/or carbon grades with high mechanical characteristics;
  • less alloyed grades, with relatively high levels of nickel, and therefore relatively low levels of manganese, providing little improvement over classical grades, both in terms of workability and cost reduction.

SUMMARY OF THE INVENTION

The goal of the invention is to propose an austenitic stainless steel for cold forming suitable for later machining exhibiting improved mechanical properties over standard austenitic stainless steels; these properties are very valuable for cold working applications, particularly cold heading.

The goal of the invention is an austenitic stainless steel for cold working suitable for later machining, characterized by the following composition by weight:

• • carbon<0.030%
  • 0.3%<silicon<1%
  • 7%<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 characteristics of the invention are:
  • the composition also comprises boron in a concentration below 50·10⁻⁴%;
  • the Md30 index, defined by the formula: Md30=551−462(C %+N %)−9.2Si %−20Mn %−13.7Cr %−29Ni %−29Cu %−18.5Mo %, is below −60.
  • the ferrite rate after reheating of the raw solidification structure to 1,240° C. is below 10%, as per the following formula:
    % 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 Ni_eq=Ni %+20.04 C %+21.31 N %+0.46 Cu %+0.08 Mn %.

DESCRIPTION OF THE INVENTION

The following description, provided without limitation, will facilitate the understanding of the invention. The invention concerns an austenitic stainless steel for cold working suitable for later machining, characterized by the following composition by weight:

• • carbon<0.030%
  • 0.3%<silicon<1%
  • 7%<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, the carbon content is controlled to be below 0.030% in order to limit the formation and hardening of hardening martensite. In the same manner, and for the same purposes, the combined carbon and nitrogen content must be controlled to be below 0.07%.

The nitrogen content is controlled to a level between 0.02%<N<0.060%, preferably 0.02%<N<0.04%, in order to stabilize the austenite and guarantee a hot ferrite content <15%. Indeed, to facilitate hot working, the ferrite rate must be maintained below 10% after reheating of the raw solidification structure to 1,240° C., as per the following formula: % 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 Ni_eq=Ni %+20.04 C %+21.31 N %+0.46 Cu %+0.08 Mn %.

In the composition as per the invention, the nickel content has been lowered to between 4.55% and 7%, preferably to around 5%.

The lower nickel content is compensated by a manganese content between 7% and 9%, preferably around 8%. More specifically, with the substitution of nickel by manganese in austenitic stainless steels, manganese levels around 6% should be avoided as deleterious ferrite may be produced during industrial processing or subsequent heat treatment such as solution annealing.

As an example, stainless steels containing 5% Ni with various amounts of manganese have been processed as follow : (1) hot deformation of cast products and (2) reheating for 15 min at 1080° C. followed by fast cooling to room temperature. Ferrite measurement was performed by metallographic examination of polished samples.

	Steel	A	B	C	D
	% C	0.023	0.026	0.023	0.027
	% Mn	4.0	5.0	6.0	7.0
	% Si	0.30	0.30	0.30	0.31
	% S	0.004	0.004	0.004	0.004
	% P	0.008	0.008	0.008	0.008
	% Ni	4.95	4.99	4.99	4.97
	% Cr	17.0	17.1	17.1	17.1
	% Mo	<0.005	<0.005	<0.005	<0.005
	% Cu	3.0	3.1	3.1	3.1
	% N	0.027	0.027	0.028	0.028
	% S	0.004	0.004	0.004	0.004
	% P	0.008	0.008	0.008	0.008
	Ferrite	1.5	2.7	4.7	2.9
	Content
	Calc. Md30	−18	−45	−64	−86

In the above, the highest ferrite content heat exhibits the lowest cold heading ability.

Moreover, another drawback with high ferrite content is the potential of corrosion healing due to carbide precipitation at the ferrite/austenite boundaries. The balance in austenite forming and ferrite forming agents imply that when Ni is set to as low as 5%, the content in manganese should be increased to 7% or above.

The presence of chrome guarantees good corrosion resistance, with the content chosen between 15% and 18%.

Copper is present in the composition to stabilize the austenite. As described in prior art, it is limited to below 4%. A content above 2% is preferred to improve stabilization of the austenite.

Sulfur is limited to a content below 0.01% and preferably below 0.002% in order to limit the formation of manganese sulfides which are detrimental to hot workability and corrosion resistance.

Other elements are also controlled, e.g. silicon content should be below 1%, preferably below 0.3%. The composition also contains molybdenum in a concentration below 0.8% and phosphorus in a concentration below 0.030%.

Boron may be added in a concentration below 50.10⁻⁴% to facilitate cold and hot working.

Eventually, the Md30 index of the steel must be as low as possible, preferably below −60; the value of Md30 is given by the following formula: Md30=551−462(C %+N %)−9.2Si %−20Mn %−13.7Cr %−29Ni %−29Cu %−18.5Mo %.

In an example of implementation, the composition of a steel 1 as per the invention was compared to that of a reference sample, a standard austenitic steel with the composition presented in table 1 below:

	TABLE 1
	% C	% Si	% Mn	% Ni	% Cr	% Mo	% Cu	% N	% S	Md30	%*
Steel 1	0.023	0.36	8.1	5.1	15.0	0.30	3.3	0.035	0.001	−71.6
Ref. steel	0.029	0.33	1.8	8.1	17.2	0.29	3.2	0.027	0.002	−59.3

For cold heading applications, the steel as per the invention exhibits improved cold workability compared to the standard reference steel. Cruciform impact testing shows that:

• • Steel 1 as per the proposed invention does not exhibit any cracking;
  • The reference steel exhibits incipient ductile failure at the angles of the cross.

For corrosion, the steel as per the invention exhibits good corrosion resistance, comparable to that of a standard 304 steel.

For magnetism, welding and machining, the steel as per the invention is non-magnetic in annealed temper, and lightly magnetic in hard temper; has a weldability equivalent to that of a standard 304 steel; and a machinability close to that of a standard 304 steel.

The steel as per the invention is commercially attractive seeing as it employs alloy elements which are less costly than nickel, and facilitates working at each stage of processing, particularly due to a hot structure and mechanical properties which are compatible with existing processing techniques for standard austenitic stainless steels.

The steel as per the invention is well-suited for parts involving an initial cold heading operation followed by a finishing machining operation.

Its field of application may include fitting products such as fasteners, bolting, nuts, threaded rods, and any particular parts manufactured by cold heading, e.g. fixtures, automobile parts such as mountings, airbags, probe support and body elements, or mass-produced connection parts such as electrical connection equipment.

Other applications can also be cited that significantly involve cold working in order to provide a good performance/cost ratio, e.g. in the fields of drawn wires and fine wires, filters and screens, or spring manufacturing.

Claims

1. Austenitic stainless steel for cold working suitable for later machining, characterized by the following composition by weight:

carbon<0.030%

0.3%<silicon<1%

7%<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%

2. Steel as per claim 1, characterized in that the composition also comprises boron in a concentration below 50.10−4%.

3. Steel as per claim 1, characterized in that the Md30 index, defined by the formula: Md30=551−462(C %+N %)−9.2Si %−20Mn %−13.7Cr %−29Ni %−29Cu %−18.5Mo %, is below −60.

4. Steel as per claim 1, characterized in that the ferrite rate after reheating of the raw solidification structure to 1,240° C. is below 10%, as per the following formula: % Ferrite =0.034 x2+0.284x−0.347, where x=6.903 [Creq−Nieq/1.029−6.998], with Creq=Cr % and Nieq=Ni %+20.04 C %+21.31 N %+0.46 Cu %+0.08 Mn %.