EP1016732A1

EP1016732A1 - Ferritic stainless steel for the production of tubes by electric resistance welding (erw) process

Info

Publication number: EP1016732A1
Application number: EP99302787A
Authority: EP
Inventors: Tarcisio Reis De Oliveira; Ronaldo Claret Ribeiro Da Silva; Washington Maurilio Alves Fernandes; Helcio De Araujo Quintao
Original assignee: Acesita SA
Current assignee: Aperam South America
Priority date: 1998-12-30
Filing date: 1999-04-09
Publication date: 2000-07-05
Also published as: AR018329A1; UY25468A1; BR9805859A

Abstract

This patent refers to a ferritic stainless steel which presents good resistance to corrosion and oxidation at high temperatures, with good performance when used for producing tubes by a welding process called ERW - Electric Resistance Welding (resistance welding at high frequency)..

Such tubes are commonly employed in automobile exhaust systems and must present good expansion and bending ability so that they can be properly used for that specific object and present the following chemical composition: %C %Mn %Si %P %S %Cr %Ni %Ti %N %Nb Min. - - - - - 10.50 - 0.09 - 0.05 Max. 0.014 1.00 1.00 0.045 0.045 11.75 0.50 0.20 0.013 0.10 Fe = balance, except incidental impurities, with the provision that the (C + N) content is limited to a maximum of 220 ppm.

Description

This patent refers to a ferritic stainless steel which presents good resistance to corrosion and oxidation at high temperatures, with good performance when used for producing tubes by a welding process called ERW - Electric Resistance Welding (resistance welding at high frequency). Such tubes are commonly employed in automobile exhaust systems and must present good expansion and bending ability so that they can be properly used for that specific object.

Background of the Invention

The steel already known from the prior art for this purpose and which shows the best performance is specified by ASTM A240/A240m as ASTM 409, and its chemical composition is as follows: %C = 0,03 max.; %Mn = 1.00 max.; %Si = 1,00 max.; %P = 0.040 max; %N = 0.03 max; %S = 0.010 max.; %Cr = 10.50 - 11.70; %Ni = 0.50 max.; %Ti > 6 x (%N + %C) and < 0.50%; Fe = remaining, except incident impurities (weight %). This kind of steel presents the following disadvantage when used for producing tubes using the ERW process: only with very low content of carbon and nitrogen ((%C + %N) < 0,0150) it is possible to obtain tubes which will have bending and expansion capacity adapted to the most critical production requirements of exhaust systems without further thermal treatment.
In a paper published by KAWASAKI STEEL CORPORATION (KSC), "Weld Zone Toughness of ERW Ti-Stabilized 11% Cr Steel Pipe for Automobile Exhaust Systems", it is taught that when the 409-steel is welded using the ERW process it shoes a weak behavior on the welding zone (mainly on Heat Affected Zone - HAZ), due to the precipitation of chromium carbides.
The above effect may be explained by the fact that during the heating phase, the tubes regions (edges) that reach temperatures higher than 900 °C have the titanium carbide (TiC) dissolved, i.e., part of the carbon fixed as TiC during the base metal production goes to the solid solution. The titanium nitride (TiN), much more stable, remains practically unaltered, almost not occurring the presence of nitrogen in solid solution. Therefore, in a region that reach temperatures higher than 900 °C , the carbon that was in the solid solution precipitates as chromium carbides during the subsequent cooling step (which quickly occurs after pressing the welding point), since there is not enough time for the carbon to diffuse and react with the titanium that once had fixed it. These chromium carbides act in such a way that they damage the mechanical properties on that region. If they could precipitate as titanium carbides, the effect would be less harmful and the mechanical properties would be improved.
Considering that there is not much freedom to act directly on the ERW process (potency and speed parameters that considerably influence the above mentioned precipitation) since there will always be a region with temperatures higher than 900 °C KSC worked on the chemical composition of the steel, limiting the content of (C + N) up to a maximum of 150 ppm. Therefore, even if there is precipitation of chromium carbides, they will be present in small amounts and will not influence so much the mechanical properties.
However, due to the difficulty and high costs involved in the production of 409 stainless steel with ultra-low contents of carbon and nitrogen, it was still desirable to develop a new process which could overcome the above mentioned drawbacks but maintaining higher concentration of those elements.
It is also already known from the prior art to add Nb to the steel composition. For example, the use of elevated contents of niobium, that is to say, amounts superior to 0.1 %, conjugated with elevated values of (C + N) is already disclosed in Brazilian patent PI 8706954-7. The purpose of patent PI 8706954-7 is, however, to demonstrate that the superficial defects found on the surface of sheets of stabilized ferritic steels are related to the formation of titanium inclusions, and that the problem could be minimized by controlling the contents of N and partially substituting the Ti by Nb, by using concentrations higher than 0.1 % of the latter. Therefore the object of PI 8706954-7 is not related to the stabilization of stainless steel by avoiding the precipitation of carbides during manufacture of tubes by ERW process.

Summary of the Invention

It was now surprisingly found out that it is possible to overcome the disadvantages of this kind of stainless steel known from the prior art by adding a small amount of niobium to its composition. By doing this, it is possible to achieve expansion and bending properties during the manufacture of tubes through ERW process equivalent to the ones obtained with steels with ultra-low contents of carbon and nitrogen.

Brief Description of the Drawings

Figures 1 and 2 show microhardness profiles on the welding regions.
Figures 3 and 4 show the comparative metallography data of welding regions for tubes prepared with an steel in accordance with present invention and with 409 steel stabilized only with Ti and carbon plus nitrogen lower than 150 ppm.
Figure 5 shows the results of destructive tests in tubes fabricated with steel from the current invention.

Detailed Description of the Invention

According to the present invention, the ferritic stainless steel used in the production of tubes using ERW process is characterized by a small addition of niobium. The content of (C + N) is limited to 220 ppm maximum, i.e., a value much higher than the value suggested by the materials known from the prior art. Moreover, the stabilization process now developed by ACESITA for 409-steels is different from the prior art knowledge since it also adds niobium to the material in a range from 0.05 to 0.1 %, by weight. In spite of its small amount and of being less efficient on the stabilization than titanium (its atomic weight is approximately twice as much higher than Ti, which decreases the number of atoms really available for stabilization), it can be noticed that only the carbon is stabilized by Nb and by the Ti that remained from the reaction with nitrogen, since all nitrogen becomes stable and fixed as TiN.
During the ERW process which imparts very high heating and cooling rates to the part to be welded, the niobium carbides show higher stability to dissolution when compared to titanium carbides and this stability characteristic prevents the carbon from being free in solution and from further re-precipitating as chromium carbide. The small addition of niobium thus allows the use of higher contents of carbon and nitrogen on type 409 steel without bringing about the disadvantages previously found in the prior art steels.

The steel according to the present invention comprises the following chemical composition:

	%C%	%Mn	%Si	%P	%S	%Cr	%Ni	%Ti	%N	%Nb
Min.	-	-	-	-	-	10.50	-	0.09	-	0.05
Max.	0.014	1.00	1.00	0.045	0.045	11.75	0.50	0.20	0.013	0.10
Fe = remainder, except incident impurities. The content (C + N) is limited to a maximum of 220 ppm. The above composition presents ferritic microstructure and mechanical properties similar to conventional type 409 stabilized with titanium.

In comparative studies and considering the same conditions for produces tubes using ERW process, the steel of the current invention showed very similar results regarding mechanical properties, metallography of the welding zone and behavior in mechanical tests of expansion and bending properties to those steels prepared according to the prior art, that is to say, steels which should be prepared with carbon and nitrogen contents lower than 150 ppm. Two different types of tubes were used: one with diameter = 38.10 mm and1.20 mm thickness, and the second with diameter = 50.80 mm with 1.50 mm thickness.
Conventional steel coils were used with average carbon plus nitrogen content of 125 ppm (409 Ti). In ACESITA's production conditions, the steel of the current invention was tested for contents of carbon plus nitrogen of 170 ppm (409 TiNb) as well as for contents of 210 ppm (409 TiNb2), being this value close to the superior limit of the content established in the invention.

TABLE 1 shows the results of tension tests with the steel of the current invention (409 TiNb1 and 409 TiNb2), with different contents of carbon and nitrogen, and the conventional 409 Ti, with carbon plus nitrogen concentration lower than 150 ppm. It should be observed that all properties of the steels of the invention are very similar to those of convention Ti 409 steel.

	38.10 x 1.20 mm	50.80 x 1.50 mm
	409 Ti < 150 ppm	409 TiNb1 170 ppm	409 TiNb2 210 ppm	409 Ti < 150 ppm	409 TiNb1 170 ppm	409 TiNb2 210 ppm
Yield Point (MPa)	375.0	364.7	379.3	360.0	377.7	389.0
Tensile Strength (MPa)	408.0	410.3	427.7	404.5	403.3	411.0
Elongation (%)	56.0	56.9	56.6	62.0	60.0	62.3

Figures 1 and 2 show that microhardness profiles on the welding region are also very close. In this case it can be distinguished that, in tubes conventionally manufactured with type 409 stabilized only with Ti and with (C + N) content above 150 ppm, the hardness on the welding regions is situated above 200 HV, i.e., within much superior values than those shown on Figures 1 and 2.
Figures 3 and 4 show metallograph of the welding region for 38.10 x 1.20 mm tubes produced with the current invention steel and with 409 stabilized only with Ti and carbon plus nitrogen content lower than 150 ppm.
Figure 5 shows the results of destructive tests in tubes manufactured with steel with a composition in accordance with the present invention. The tubes were approved on the expansion, flaking and flattering. In the straight section expansion test, considered a very severe test, the tubes were approved for expansion of 1.2 (20 %) and 1.3 (30 %), which shows the good performance of the tubes fabricated with the current invention.
It can also be distinguished that the steel according to the current invention has good performance in other types of welding processes (TIG, MIG, coated electrode, etc.), as well as an excellent drawability, presenting an average normal anisotropy of 1.40.

Claims

Ferritic stainless steel for the production of tubes by Electric Resistance Welding (ERW) process, characterized by the following chemical composition, all percentages being by weight:

from 0 to 0.014 % of C;

from 0 to 1.00 % of Mn;

from 0 to 1.00 % of Si;

from 0 to 0.045 % of P;

from 0 to 0.045% of S;

from 10.50 to 11.75% of Cr;

from 0 to 0.50 % of Ni;

from 0.09 to 0.20% of Ti;

from 0 to 0.013 % of N;

from 0.05 to 0.10 % of Nb;

the balance being Fe except for the incidental impurities, provided that the (C + N) is up to a maximum of 220 ppm.