EP0031800B1

EP0031800B1 - Austenitic, precipitation hardenable stainless steel

Info

Publication number: EP0031800B1
Application number: EP19800850186
Authority: EP
Inventors: Frans Gustaf Stefan Angel; Nils Gunnar Eugén Malmgren; Barry Solly
Original assignee: Fagersta AB
Current assignee: Fagersta Stainless AB
Priority date: 1979-12-28
Filing date: 1980-12-10
Publication date: 1983-12-14
Also published as: SE420623B; ES8206655A1; DE3065923D1; ES498616A0; JPS56105457A; SE7910719L; EP0031800A1

Description

This invention deals with an austenitic stainless, precipitation hardenable chromium-nickel-aluminium steel with good cold workability. By tempering the martensite obtained by cold working, quenching or in some cases cooling to subzero temperatures, precipitation hardening is obtained. The new alloy can be produced as hot rolled products such as wire rod, bar, strip and plate.
Precipitation hardening of stainless steel using aluminium is a well-known technique. These steels can either be given an annealing treatment designed to raise the M_s temperature so much that martensite is formed on quenching, or they can be annealed at a higher temperature giving an austenitic structure when quenched. In the latter case the steel can be subsequently cold worked, thus transforming the austenite into martensite. After one of these treatments the material can be precipitation hardened.
The disadvantage with these steels is that the rate of martensite formation when cold working is extremely high, thus rendering the material difficult to work. The risk of stress cracks, for example, during the wire drawing process is extremely high.
The main purpose of the present invention is thus to improve the workability in the cold condition of Cr-Ni-AI steels by lowering the M_s and the M_d30 temperatures sufficiently to ensure that good workability is obtained without at the same time lowering them so far that the austenite is not transformed to martensite when cold worked. Martensite is desirable in the final product since it raises the mechanical strength considerably.
An austenitic stainless steel according to the invention is characterized in that the chromium equivalent Cr' defined as Cr'=% Cr+% Mo lies between 16.5% and 18.2%, and the nickel equivalent Ni' defined as Ni'=% Ni+1/2 (% Mn-1%) lies between 8.25% and 9.75% said steel containing C up to 0.15%, Si up to 3.0%, Mn up to 8.0%, P up to 0.045%, S up to 0.040%, Mo up to 2.0%, N up to 0.15%, 0.5-2.5% AI and preferably one or more of the metals Ti, Zr and U up to 2%, with the balance of iron and impurities normally occurring in stainless steels.
Preferably the steel contains either Ti up to 0.5% or Zr up to 0.5% or U up to 1%.
Two production heats of the steel according to the invention with the following analyses have been made under production conditions.
Both heats were melted in a 10-ton high-frequency induction furnace, ingot teemed and rolled to billets. These were conditioned and rolled to wire rod, 0 6.0 mm for heat 3423-71 and 0 5.6 mm for heat 4029-71. The wire rod was subjected to a normal anneal at 1050°C, pickled and inspected before being drawn to wire.
As reference material a heat (3226-71) in steel of type 17-7 PH (AISI 631) has been produced by the same method. This is a precipitation hardenable Cr-Ni-Al-steel with the following heat analysis (Table 2).
Heats 3423-71 (Table 1) and 3226-71 have been drawn from wire rod, 0 6.0 mm, using single drafts. The tensile strength (R_m), yield strength (R₈), and reduction of area (Z) as measured by tensile testing are compared in Figure 1. It is apparent that the yield and tensile strengths increase more slowly for the alloy in the invention (continuous curve) thus causing the drawability, measured as maximum achievable area reduction, to increase from 75% to 92%. Moreover, the ductility measured as reduction of area is higher for the alloy in the invention over the whole range and particularly at large area reductions. It should be noted that stress cracks are completely absent in the material for area reductions as large as 92% compared with 17-7 PH where a definite risk of stress crack formation exists at area reductions as low as 45%.
The precipitation hardening effect of the alloy in the invention has been tested for two different area reductions and the results are shown below in Table 3.
Thus, the tempering treatment gives an increase of yield and tensile strengths of about 200 N/mm² for heat 3423-71 and about 400 N/m M2 for heat 4029-71. The large difference between these effects can be explained by the fact that wire rod from heat 3421-71 has not been worked sufficiently, i.e. the austenite has not been transformed to martensite in sufficient quantities prior to tempering.
The alloy in this invention has mainly been developed in order to improve the cold workability of precipitation hardenable stainless Cr-Ni-AI steels. Within the area abcd in Figure 2, the composition can be set so that annealing in the temperature range 600-900°C leads to a rise in the M_s temperature due to carbide precipitation. The material can subsequently be cooled to ambient or subzero temperatures in order to obtain a transformation to martensite which can then be tempered.
A particularly interesting application is cold heading where tensile strengths as low as 540-560 N/mm² have been measured on material annealed at 1050°C. This is true for both heats 3423-71 and 4029-71.
The technical result of the invention is that the drawability is considerably improved at the same time as the precipitation hardening effect is retained. It is reasonable to assume that the good weldability can be improved by additions of zirconium and uranium instead of titanium. Since the alloy has an extremely sluggish martensitic transformation in the annealed state the material can be stored and transported outdoors under cold weather conditions without risk of spontaneous martensite formation.
Chromium and nickel equivalents should lie within the area abcd as shown in Figure 2 where chromium can be partly replaced by molybdenum with up to 2%, and nickel can be partly replaced by manganese according to the formula % Ni=1/2(% Mn-1 %).
The alloy according to the invention is annealed within the temperature range 600―950°C, quenched and cooled to below room temperature. The alloy can then be precipitation hardened by tempering.
The alloy can also be quenched direct after hot rolling and subsequently cold worked and precipitation hardened by tempering.

Claims

1. Austenitic, precipitation hardenable, stainless Cr-Ni-AI steel with good cold workability characterized in that the chromium equivalent Cr' defined as Cr'=% Cr+% Mo lies between 16.5% and 18.2%, and the nickel equivalent Ni' defined as Ni'=% Ni+1/2(% Mn-1%) lies between 8.25% and 9.75%, said steel containing Ni at least 5%, C up to 0.15%, Si up to 3.0%, Mn up to 8.0%, P up to 0.045%, S up to 0.040%, Mo up to 2.0%, N up to 0.15%, 0.5-2.5% AI and preferably one or more of the metals Ti, Zr and U up to 2%, with the balance of iron and impurities normally occurring in stainless steels.

2. Steel according to claim 1, characterized in that it contains either Ti up to 0.5% or Zr up to 0.5% or U up to 1%.