EP0388283B1 - Stainless ferritic steel and process for manufacturing this steel - Google Patents

Abstract

Ferritic stainless steel with ductility, shock-resistance and corrosion resistance in neutral or weakly acid chloridized media; also the prodn. process for fabrication of this steel. The steel is characterised by the following chemical composn.: 28.5-35 wt.% chromium, 3.5-5.50 wt.% molybdenum, 0.5-2 wt.% copper, less than 0.50 wt.% nickel, less than 0.40 wt.% manganese, less than 0.40 wt.% silicon, less than 0.030 wt.% carbon, less than 0.030 wt.% nitrogen, at least 0.10 wt.% but less than 0.60 wt.% titanium and/or niobium, and up to 0.10 wt.% deoxidizing additives such as aluminium, magnesium, calcium, boron or rare earth metals. The remainder of the material is composed of iron and impurities. The steel is worked in strip form. The steel strip is hot-rolled with reheating to a temp. of 900-1200 deg.C, followed by being cold-rolled. After this there is an intermediate stage of heating to 900-1200 deg.C. before a second cold-rolling. Finally the steel is reheated a last time to 900-1200 deg.C.

Description

The present invention relates to a ferritic stainless steel very resistant to corrosion in a neutral or weakly acidic chlorinated medium and more particularly suitable for the manufacture of heat exchangers for industry, in particular those cooled by brackish water and water from sea.

The present invention also relates to a process for the preparation of such a steel.

FR-A-2,377,457 discloses a ferritic steel with chromium nickel molybdenum resistant to corrosion and containing in particular from 18 to 32% of chromium, from 0.1 to 6% of molybdenum, from 0.5 to 5% nickel and not more than 3% copper.

The examples of steel described in this document relate to steels containing 1.99 to 2.15% molybdenum. Furthermore, it is specified, on page 9 lines 27 to 32, that the steels having the best alloy compositions are those containing 28% chromium, 2% molybdenum and 4% nickel, as well as those containing 20% chromium , 5% molybdenum and 2% nickel, because they have sufficient structural stability and can be produced economically on an industrial scale.

Also known from FR-A-2,352,893 is a ferritic stainless steel containing from 0.01 to 0.025% by weight of carbon, from 0.005 to 0.025% by weight of nitrogen, from 20 to 30% by weight of chromium , 3 to 5% molybdenum, 3.2 to 4.8% nickel, 0.1 to 1% copper, 0.2 to 0.7% titanium and / or 0.2 to 1% niobium.

This document claims more particularly a high nickel content of between 3.2 to 4.8% associated with a limitation of the copper content of between 0.1 to 1% to obtain the temperature ambient high ductility values.

FR-A-2,473,069 also discloses an iron-based ferritic stainless steel containing up to 0.08% by weight of carbon, up to 0.060% by weight of nitrogen, from 25 to 35% in weight of chromium, from 3.60 to 5.60% by weight of molybdenum, up to 2% by weight of nickel, up to 2% by weight of titanium, niobium and zirconium according to the following equation:

$\text{\% Ti / 6 +\% Zr / 7 +\% cb / 8>\% C +\% N}$

The sum of said carbon and nitrogen being greater than 0.0275% by weight.

FR-A-2,473,068 discloses a ferritic stainless steel which has the same composition as the preceding steel, but whose nickel content by weight is between 2 and 5%.

However, it is known that nickel is an expensive element which accelerates the formation of embrittling intermetallic phases and reduces the resistance to cavernous corrosion in chlorinated medium.

The present invention therefore relates to a ferritic stainless steel in which the addition of copper is limited to a value between 0.5 to 2% by weight so as to reinforce the impact resistance of the alloy while reducing the speed for the formation of hard and embrittling intermetallic phases of the sigma and chi type which can form during heat treatments for manufacturing welding. This results in the possibility of developing an alloy stabilized with titanium and / or niobium with a very high chromium and molybdenum content essential for obtaining maximum corrosion resistance while minimizing manufacturing difficulties and risks. degradation of other final properties.

• 28,5 à 35 % de chrome,
• 3,5 à 5,50 % de molybdène,
• 0,5 à 2 % de cuivre,
• moins de 0,40 % de manganèse,
• moins de 0,40 % de silicium,
• moins de 0,030 % de carbone,
• moins de 0,030 % d'azote,
• un pourcentage en titane et/ou en niobium au moins égal à 0,10 % et inférieur à 0,60 % , lesdits pourcentages en titane et/ou niobium satisfaisant aux équations suivantes :
  
  $\text{\%Ti > 0,10 + 4 x (\%C) + 3,4 x (\%N),}$
  
  
  $\text{\%Nb > 0,10 + 7,7 x (\%C) + 6,6 x (\%N),}$
  
  
  
• et pouvant contenir du nickel en addition involontaire et en élément résiduel en tout cas à raison de moins de 0,5% et contenant jusqu'à 0,10% d'éléments ajoutés pour la désoxydation tels que l'aluminium,du magnésium, du calcium, du bore, des matériaux de terres rares, le reste étant du fer et des impuretés résultant de la fusion des matières nécessaires à l'élaboration

This result is obtained by the invention thanks to a ferritic stainless steel having the following chemical composition by weight:

• 28.5 to 35% chromium,
• 3.5 to 5.50% molybdenum,
• 0.5 to 2% copper,
• less than 0.40% manganese,
• less than 0.40% silicon,
• less than 0.030% carbon,
• less than 0.030% nitrogen,
• a percentage of titanium and / or niobium at least equal to 0.10% and less than 0.60%, said percentages of titanium and / or niobium satisfying the following equations:
  
  $\text{\% Ti> 0.10 + 4 x (\% C) + 3.4 x (\% N),}$
  
  
  $\text{\% Nb> 0.10 + 7.7 x (\% C) + 6.6 x (\% N),}$
  
  
  
• and may contain nickel in involuntary addition and residual element in any case at a rate of less than 0.5% and containing up to 0.10% of elements added for deoxidation such as aluminum, magnesium, calcium, boron, rare earth materials, the rest being iron and impurities resulting from the fusion of the materials necessary for the preparation

According to a preferred characteristic of the invention, the steel contains less than 0.010% of carbon and less than 0.015% of nitrogen, the sum of the carbon and of the nitrogen being less than 0.025%.

The invention also relates to a process for the production of a ferritic stainless steel from which a steel strip is formed which is hot rolled, characterized in that the hot rolled steel strip is subjected annealing at a temperature between 900 and 1200 ° C., then the steel strip is subjected to a first cold rolling followed by an intermediate annealing at a temperature between 900 and 1200 ° C. and finally the steel strip is subjected to a second cold rolling followed by a final annealing at a temperature between 900 and 1200 ° C.

• le recuit intermédiaire et le recuit final sont effectués en continu pendant 20 secondes à 5 minutes,
• les recuits sont suivis d'un refroidissement rapide.

Les caractéristiques et avantages de l'invention ressortiront d'ailleurs des diagrammes annexés aux figuresAccording to other characteristics of the invention:

• intermediate annealing and final annealing are carried out continuously for 20 seconds to 5 minutes,
• annealing is followed by rapid cooling.

The characteristics and advantages of the invention will moreover emerge from the diagrams annexed to the figures.

The examples which led to the present invention were obtained from 30 kg ingots produced in the vacuum induction furnace. Slabs from these ingots were heated between 1100 and 1250 ° C for hot rolling to a thickness of 5 mm.

The hot-rolled strips are then annealed between 1000 and 1200 ° C followed by cold rolling to a thickness of 2 millimeters. After this cold rolling, annealing on the order of 20 s to 5 min is carried out continuously at a temperature between 900 and 1200 ° C.

Additional cold rolling makes it possible to obtain strips of a thickness of 0.8 millimeters which then undergo a final annealing of the order of 20 s to 5 min and at a temperature between 900 and 1200 ° C.

All heat treatments are followed by rapid cooling. The heat treatment conditions are adapted so that the grain size is substantially constant.

The exact chemical analyzes, that is to say the weight percentages of the experimental alloys, are specified in the table below:

We know that the elements favorable to corrosion resistance, namely chromium, molybdenum, titanium, niobium, etc. have harmful effects on other properties, such as mechanical properties. . Depending on the desired application, it is therefore necessary to adapt the chemical composition of the alloy in order to achieve a compromise between corrosion resistance and mechanical characteristics. A poorly adjusted chemical composition can also lead to insurmountable difficulties in manufacturing the alloy, in particular as a result of the precipitation of embrittling phases during the annealing heat treatment before or after cold rolling, for example, or even precipitation. weakening phases during a welding operation.

Furthermore, it is known that in a neutral chlorinated medium, the resistance to pitting corrosion of ferritic stainless steels increases with the chromium content. Molybdenum is a much more efficient alloying element than chromium because a Mo / Cr equivalent coefficient equal to 3.3 is generally accepted to qualify the improvement in resistance to pitting corrosion due to the action of molybdenum.

By using samples taken from known industrial ferritic stainless steel sheets, it has been verified that in concentrated and hot chlorinated medium, the potential, above which pitting corrosion takes place, is all the higher as the sum% Cr + 3.3 x (% Mo) is high. Consequently, the resistance to pitting corrosion is higher the higher the% Cr + 3.3 x (% Mo) parameter.

It is for this reason that a chromium content greater than 28.5% and a molybdenum content greater than 3.5% have been determined for ferritic stainless steel according to the present invention.

Tests conducted from the experimental flows listed in the previous table show that molybdenum promotes the precipitation of embrittling phases of the sigma type as shown in the diagram in Figure 1. The curves shown in this diagram show the influence of the holding time at 900 ° C on the elongation A% at break at room temperature of an experimental alloy at 29Cr 4Mo 2Ni Nb and 29Cr 3Mo 2Ni Nb, that is to say alloys with molybdenum content respectively equal to 3 and 4%.

The increase in the chromium content also accelerates the precipitation of the embrittling phases as shown in the diagram in Figure 2. The curves shown in this diagram show the influence of the holding time at 900 ° C on the elongation A% to rupture at room temperature of an experimental alloy at 29Cr 4Mo 4Ni Ti and 25Cr 4Mo 4Ni Ti.

The same is true of the increase in the nickel content as shown in the diagram in FIG. 3. The curves represented on this diagram show the effect of an addition of 2 to 4% of Ni on the elongation A % at failure at ordinary temperature of an experimental alloy at 29Cr 4Mo Ti after increasing holding times at 900 ° C.

Thus, when the contents of chromium, nickel and molybdenum increase, shorter and shorter holding times at 900 ° C. cause the precipitation of intermetallic phases detrimental to the ductility of the alloy, which leads to a very high increase. sensitive, even unacceptable, difficulties in the industrial manufacture of these ferritic stainless steels.

• du type 25 %Cr 4 %Mo 4 %Ni stabilisés au titane et au niobium, la plus faible teneur en chrome permettant d'adopter des teneurs élevées en molybdène et en nickel mais au détriment de la résistance à la corrosion par piqûres,
• du type 28 %Cr 2 %Mo 4 %Ni stabilisés au titane ou au niobium, les fortes teneurs en chrome et en nickel nécessitant une diminution de la teneur en molybdène pour réduire la vitesse de précipitation des phases fragilisantes.

It is therefore understandable that the industrial alloys currently available are:

• 25% Cr 4% Mo 4% Ni stabilized with titanium and niobium, the lower chromium content making it possible to adopt high molybdenum and nickel contents but at the expense of resistance to pitting corrosion,
• 28% Cr 2% Mo 4% Ni stabilized with titanium or niobium, the high chromium and nickel contents necessitating a reduction in the molybdenum content to reduce the rate of precipitation of the embrittling phases.

In patent FR-A-2,377,457 the addition of nickel up to 5% is justified as an improvement in the cold toughness, that is to say the impact resistance, and the resistance corrosion.

Tests have shown that the improvement in impact resistance which the addition of 4% nickel can provide to a ferritic stainless steel of the 25% Cr 4% Mo 0.5% Ti type was no longer observed when the content chromium is greater than 28% as shown in the diagram in Figure 4. The diagram in Figure 4 shows the evolution of impact resistance as a function of temperature and nickel content. This diagram does not show any beneficial effects of nickel when the impact rupture test of a notched test piece takes place above 0 ° C in the case of a ferritic stainless steel containing about 29% of chromium , 4% molybdenum and 0.5% titanium.

Contrary to popular opinion, the effect of nickel appears harmful because the energy necessary to break the test piece is, in this case, significantly lower than that of ferritic stainless steel containing no nickel. The beneficial influence of nickel only appears for the lower chromium contents.

Thus, the alloy with approximately 25% chromium, 4% molybdenum, 4% nickel and 0.5% titanium does not exhibit brittleness when cold between 0 and -50 ° C. unlike the alloy containing approximately 29 % of chromium, 4% of molybdenum, 4% of nickel and 0.5% of titanium as it appears on the diagram of figure 5 which shows the evolution of the resistance to the impact rupture according to the temperature and the chromium content.

This same diagram also reveals that, in the ductile state, the fracture energy of steel at around 25% chromium, 4% molybdenum, 4% nickel and 0.5% titanium is much higher to that of steel containing a higher chromium content and substantially similar contents of molybdenum, nickel and titanium.

Furthermore, in a chlorinated medium, resistance to cavernous corrosion, that is to say in confined spaces under deposits or construction gaps, is a primary use criterion. In fact, in a cave, it is known that progressive acidification occurs by the formation of hydrochloric acid originating from the hydrolysis of corrosion products.

Contrary to the teachings of FR-A-2,377,457, the addition of 4% of nickel to a ferritic stainless steel stabilized with titanium or niobium results in a marked reduction in the resistance to cavernous corrosion. Indeed, examinations carried out on samples after ASTM G48 test show that steel samples containing 4% of nickel undergo a severe attack.

Given the accelerating effect of nickel on the hot precipitation of intermetallic phases which weaken the alloy and reduce its resistance to corrosion, the alloy according to the present invention contains no voluntary addition of nickel which is considered to be an element residual. This absence of a significant amount of nickel allows the adoption of high chromium contents greater than 28.5% and molybdenum greater than 3.5% necessary for obtaining optimal resistance to cavernous and pitting corrosion. for ferritic stainless steel containing titanium and niobium. In ferritic steel according to FR-A-2,377,457, up to 3% copper and preferably 0.5 to 2% copper are added to the steel, which according to this patent increases the resistance to corrosion in non-oxidizing acids, and in particular in hot sulfuric acid solutions. However, according to research carried out within the framework of the present invention and presented in the diagram of FIG. 6, the results reveal that copper does not cause any improvement in the resistance to corrosion in chlorinated media. weakly acid analogous to corrosive media that form in caves.

This diagram shows the corrosion rates (mm / year) deducted from the weight losses observed after 24 hours of immersion in NaCl 2M-0.2M HCl medium deaerated by nitrogen bubbling, at the temperature of 30 ° C respectively for the alloys 6 and 7 of Table 1 above.

Consequently, in the absence of nickel, the addition of copper of between 0.5 and 2% does not degrade and does not improve the resistance to cavernous and pitting corrosion in a chlorinated medium.

According to the present invention, 0.5 to 2% of copper is added to ferritic stainless steel with a high chromium and molybdenum content and containing titanium or niobium.

The diagram in FIG. 7, the curves of which show the influence of 1% of copper on impact resistance, indicates that the addition of approximately 1% of copper to an alloy containing approximately 29% of chromium, 4% of molybdenum and 0.5% of titanium results in a decrease of the order of 20 ° C in the transition temperature between the brittle state characterized by very low breaking energies and the ductile state corresponding to high breaking energies . This results in a very significant improvement in the impact resistance of the alloy due to the addition of copper.

The demonstration of the beneficial effect of copper on cold brittleness constitutes an essential characteristic of the present invention. Indeed, the addition of copper is generally recommended to improve the corrosion resistance in hot sulfuric acid solutions as specified in FR-A-2,377,457, and not to improve the impact resistance at room temperature .

Besides the particularly favorable effect of copper on impact resistance, another essential feature of the present application also resides in the demonstration of an inhibition of the precipitation of brittle intermetallic phases. by the addition of copper as the diagram in FIG. 8 proves, the curves of which represent the influence of the addition of copper on the kinetics of precipitation of the embrittling intermetallic phases in a ferritic stainless steel at 29Cr 4Mo Ti. The addition of copper therefore very clearly delays the appearance of embrittling phases in the temperature range 750 to 950 ° C.

On the other hand, to avoid intergranular corrosion due to the precipitation of carbide and chromium nitride resulting in a depletion of chromium in the immediate vicinity of the grain boundaries, additions of titanium or niobium are commonly carried out on ferritic stainless steels to fix carbon and nitrogen to the state of carbide and nitride of titanium or niobium.

However, these additions of titanium or niobium have two harmful effects known qualitatively, but not quantified so far. They accelerate the precipitation of embrittling intermetallic phases and reduce impact resistance.

By reducing the carbon and nitrogen content, which makes it possible to reduce the amount of titanium or niobium necessary to fix carbon and nitrogen, it has been found in the context of the present invention that very much improved clearly the impact resistance of a ferritic stainless steel with a high chromium and molybdenum content and that the speed of formation of the embrittling intermetallic phases was simultaneously delayed.

Thus, a decrease in the transition temperature from the brittle to the ductile state of the order of 20 ° C can be observed in the case of a sheet 2 mm thick as shown in the diagram in Figure 9 whose curves show the difference in impact resistance of a super stainless steel -ferritic at 29Cr 4Mo 0.21Ti (C + N = 0.013%) and a super-ferritic stainless steel at 29Cr 4Mo 0.56Ti (C + N = 0.045%).

The area of appearance of the embrittling faces is, moreover, strongly shifted to the right, on the side of the longer isothermal holding times as indicated by the curves of the diagram in FIG. 10 which compare the kinetics of precipitation of the embrittling phases for a 29Cr 4MB 0.56Ti super-ferritic stainless steel (C + N = 0.045) and for 29Cr 4MB 0.21Ti super-ferritic stainless steel (C + N = 0.013).

After holding for 1 hour at 900 ° C, an alloy of 0.018% carbon, 0.027% nitrogen, 28.90% chromium, 3.75% molybdenum, 0.035% nickel and 0.56% titanium , only has an elongation at break of 6% at room temperature while an alloy of 0.03% carbon, 0.010% nitrogen, 28.90% chromium, 3.97% molybdenum, 0.041% nickel and 0.21% titanium has an elongation at break of 26%.

The reduction in the carbon and nitrogen contents associated with an addition of copper also makes it possible to obtain a transition temperature from the brittle to the ductile state clearly below 0 ° C. for a sheet 2 mm thick.

Furthermore, the present invention voluntarily excludes the addition of nickel, which is an expensive element and which accelerates the formation of embrittling intermetallic phases and reduces the resistance to cavernous corrosion in chlorinated medium.

Given the accelerating effect of titanium and niobium on the formation of embrittling intermetallic phases and their harmful influence on impact resistance when combined with carbon and nitrogen, ferritic stainless steels according to the present invention are all the more resistant to shocks and have structural stability in the range between 650 and 1000 ° C, the higher the lower the contents of C, N, Ti and Nb. To optimize resistance to intergranular corrosion, the contents of titanium and / or niobium to be added must be equal to the minimum necessary to fix carbon and nitrogen and take into account the fact that titanium and / or niobium solid solution in ferrite do not participate in the sequestration of carbon and nitrogen.

et en particulier à l'équation :
$\text{\%Ti > 0,15 + 4 x (\%C) + 3,4 x(\% N)}$

pour que la résistance à la corrosion intergranulaire soit optimale.Thus, the titanium content must satisfy the following equation:
$\text{\% Ti> 0.10 + 4x (\% C) + 3.4 x (\% N)}$

and in particular to the equation:
$\text{\% Ti> 0.15 + 4 x (\% C) + 3.4 x (\% N)}$

so that the resistance to intergranular corrosion is optimal.

The coefficients 4 and 3,4 logically follow the approximate values of the atomic masses of titanium (48), carbon (12) and nitrogen (14) as well as the formulas of titanium carbide and titanium nitride, respectively TiC and TiN.

If ferritic stainless steel is stabilized with niobium, the equation becomes:

$\text{\% Nb> 0.10 + 7.7 x (\% C) + 6.6 x (\% N).}$

The atomic mass of niobium being taken equal to 93 grams.

In the particular case corresponding to optimal resistance to intergranular corrosion, the equation becomes:

$\text{\% Nb> 0.20 + 7.7 x (\% C) + 6.6 x (\% N).}$

Taking into account the cost of titanium and niobium and the possible harmful effects of an excess of these elements, it is desirable to approach as much as possible the excess of the quantity theoretically necessary to fix carbon and nitrogen.

According to the present application, the addition of copper is limited to less than 2%, the precipitation of copper-rich particles resulting in a significant degradation of hot forgeability when the copper content is greater than 2%.

An addition of aluminum to the ferritic stainless steel according to the present application can be added during the preparation for deoxidation purposes.

Consequently, the addition of copper between 0.5 and 2% strengthens the impact resistance of the alloy while reducing the speed of formation of hard and embrittling intermetallic phases of the sigma and chi type which can form during thermal treatments. manufacturing or welding. This results in the possibility of developing an alloy stabilized with titanium or niobium with very high chromium content between 28.5 to 35% and molybdenum between 3.5 and 5.5%, essential for obtaining a maximum corrosion resistance while minimizing difficulties of manufacturing and the risks of degradation of the other final properties.

Due to its properties, the ferritic alloy according to the present invention is particularly suitable for the use in the form of sheets and strips whose thickness may be greater than that generally used in practice (less than one mm) for a steel. ferritic stainless with the same chromium and molybdenum content containing titanium or niobium.

The stainless steel described by the present invention is particularly intended for the manufacture of welded tubes for heat exchangers conveying chlorinated water. It can, for example, be produced by the steel, electrical, AOD and / or vacuum refining, continuous casting and hot rolling on strip train industry.

Claims (5)

1. Stainless ferritic steel resistant to corrosion in neutral or slightly acidic chlorinated media, ductile and shock-resistant, of which the chemical composition by weight is as follows:

   - 28.5% to 35% chromium

   - 3.5% to 5.50% molybdenum

   - 0.5% to 2% copper

   - less than 0.40% manganese

   - less than 0.40% silicon

   - less than 0.030% carbon

   - less than 0.030% nitrogen

   - a percentage of titanium and/or niobium at least equal to 0.10% and less than 0.60%, the said titanium and/or niobium percentages satisfying the following equations:
   
   $\text{\%Ti > 0.10 + 4 x (\%C) + 3.4 x (\%N)}$

   

   
   $\text{\% Nb > 0.10 + 7.7 x (\%C) + 6.6 x (\%N)}$

   

   
   

   - and able to contain nickel as an involuntary addition and as a residual element, in any case in a proportion of less than 0.5%, and containing up to 0.10% elements added for deoxidisation such as aluminium, magnesium, calcium, boron, rare earth materials, the remainder being iron and impurities resulting from the fusion of the substances necessary for preparation.
2. Stainless ferritic steel according to claim 1, characterised in that it contains less than 0.010% carbon and less than 0.015% nitrogen the total amount of carbon and nitrogen being less than 0.025%.
3. A process for the preparation of a stainless ferritic steel according to either one of claims 1 and 2, by means of which process a steel strip is formed which is hot-rolled, characterised in that the hot-rolled steel strip is annealed at a temperature between 900°C and 1200°C, then the steel strip undergoes a first cold-rolling process followed by intermediate annealing at a temperature between 900°C and 1200°C, and finally the steel strip undergoes a second cold-rolling process followed by final annealing at a temperature between 900°C and 1200°C.
4. A proccess according to claim 3, characterised in that the intermediate and final annealing processes are carried out continuously for 20 seconds to 5 minutes.
5. A process according to claim 3, characterised in that the annealing processes are followed by rapid cooling.

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