Niobium-stabilized 14% chromium ferritic steel, and use of...

Details Niobium-stabilized 14% chromium ferritic steel, and use of... Niobium-stabilized 14% chromium ferritic steel, and use of...

2000-09-08

2002-07-23

King, Roy (Department: 1742)

Metal treatment

Stock

Ferrous

C148S610000

Reexamination Certificate

active

06423159

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a niobium-stabilized 14% chromium ferritic steel, and to use of same in the automobile sector.
DISCUSSION OF THE BACKGROUND
The steels used for parts situated upstream from an exhaust system of a motor vehicle, the hot part of the system, must have both good resistance to oxidation and good creep resistance. Good formability is also necessary for manufacture of the manifold. The steels used for these hot parts are often either austenitic steels, which are relatively expensive and have poor oxidation resistance, although their formability is good, or bistabilized ferritic steels. The bistabilized ferritic steels have good oxidation resistance but are relatively difficult to form.
OBJECT OF THE INVENTION
One goal of the invention is to provide an economical ferritic steel which exhibits very good resistance to both creep and oxidation at temperatures up to 1000° C. as well as improved hardness for forming purposes.
DETAILED DESCRIPTION OF THE INVENTION
The object of the invention is realized with a process for producing a sheet-metal strip of niobium-stabilized 14% chromium ferritic steel, wherein steel with the following composition by weight based on total weight:
carbon≦0.02%,
0.002%≦nitrogen≦0.02%,
0.05%≦silicon≦1%,
0%<manganese≦1%,
0.2%≦niobium≦0.6%,
13.5%≦chromium≦16.5%,
0.02%≦molybdenum≦1.5%,
0%<copper≦1.5%,
0%<nickel≦0.2%,
0%<phosphorus≦0.020%,
0%<sulfur≦0.003%,
0.005%<tin≦0.04%,
impurities inherent to smelting and iron
wherein the content of niobium, carbon and nitrogen satisfy the relationship:
9.5≦Nb/(C+N),
is subjected to:
reheating before hot rolling at a temperature of between 1150° C. and 1250° C. and preferably at about (±15° C.) 1175° C.,
coiling at a temperature of between 600° C. and 800° C. and preferably of about (±15° C.) 600° C.,
cold rolling of the coil with or without preliminary annealing,
final annealing of the sheet-metal strip at a temperature of between 800° C. and 1100° C. for a duration of between 1 minute and 5 minutes and preferably at a temperature of about (±15° C.) 1050° C. for a time of about (±15 sec) 2 minutes.
The other optional characteristics of the invention are:
after final annealing or before use, the sheet metal is subjected to heat treatment at a temperature of between 800° C. and 1000° C. for a time of between 1 minute and 100 hours and preferably at a temperature of about (±15° C.) 850° C. for a time equal to or less than 30 minutes.
The invention also relates to a niobium-stabilized 14% chromium ferritic steel comprising, consisting of, and consisting essentially of, iron and the following by weight based on total weight:
carbon≦0.02%,
0.002%≦nitrogen≦0.02%,
0.05%≦silicon≦1%,
0%<manganese≦1%,
0.2%≦niobium≦0.6%,
3.5%≦chromium≦16.5%,
0.02%≦molybdenum≦1.5%, 0%<copper≦1.5%, 0%<nickel≦0.2%, 0%<phosphorus≦0.020%, 0%<sulfur≦0.003%, 0.005%<tin≦0.04%,
impurities inherent to smelting,
wherein the content of niobium, carbon and nitrogen satisfy the relationship:
9.5≦Nb/(C+N).
Other optional characteristics of the invention are:
the Nb content satisfies the relationship 0.1≦&Dgr;Nb≦0.5, where &Dgr;Nb=Nb−7(C+N) and preferably 0.2≦&Dgr;Nb≦0.3,
the contents of niobium, silicon and molybdenum satisfy the relationship: &Dgr;Nb/(Si+Mo)≦0.9 ,
the contents by weight of niobium and tin satisfy the relationship:
&Dgr;Nb/Sn≦50,
the contents of manganese and silicon satisfy the relationship:
Si/Mn>1.
The contents of niobium, titanium, zirconium and aluminum satisfy the relationship:
Nb/(Ti+Zr+Al)>0.16,
after heat treatment, the steel contains an intermetallic phase of Fe2Nb3 type with tetragonal structure at the grain boundaries.
The invention also relates to use of the ferritic steel sheet metal in the automobile sector, particularly for production of exhaust system manifolds.


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Chemical Abstracts, vol. 130, No. 20, May 17, 1999; Columbus, Ohio, U.S.; Abstract No. 270203; Abe, Masayuki et al: “Ferritic stainless steels having excellent surface properties and their preparation” XP002139529 & JP 11092872, Apr. 6, 1999.

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