EP0493218A1 - Process for manufacturing stovenless steel with two phase ferrite-martensite structure and steel thereby obtained - Google Patents

Abstract

Process for the manufacture of a stainless steel with a high elastic limit and a high breaking load, with a ferrite martensite two-phase structure exhibiting a good malleability and a good abrasion resistance, in which the steel of the following composition by weight:   - carbon lower than 0.10 %   - chromium between 16 and 20 %   - nickel between 0.2 and 2 %   - manganese lower than 2 %   - copper lower than 2% the remainder being iron and impurities inherent in the method of manufacture, is subjected to quenching after being raised to a temperature of between 800 and 1200 DEG C and to at least one cold rolling at a ratio of more than 15 %. <??>The invention also relates to a stainless steel obtained by this process.

Description

The present invention relates to a process for the production of a stainless steel with a high elastic limit and a high breaking load, with a two-phase ferrite martensite structure, having good malleability and high abrasion resistance.

The present invention also relates to a stainless steel with two-phase ferrite martensite structure, obtained according to this process and a conveyor chain made of such a stainless steel.

• carbone inférieur à 0,1%
• chrome compris entre 10 et 20%
• azote inférieur à 0,12%
• carbone + azote compris entre 0,01 et 0,2%
• silicium inférieur à 2%
• manganèse inférieur à 4%
• nickel inférieur à 4%
• cuivre inférieur à 4%
• Ni+(Mn+Cu)/3 compris entre et 0,5 et 5%,
  le reste étant du fer.

L'acier est soumis à un traitement de recuit au défilé, dans un four, pour obtenir une structure ferrite-austénite. Cet acier est ensuite trempé pour obtenir de la martensite à partir de l'austénite.We know in EP-A-0.273.278 and EP-A-0.273.279, a stainless steel with two-phase ferrite martensite structure whose weight composition is as follows:

• carbon less than 0.1%
• chromium between 10 and 20%
• nitrogen less than 0.12%
• carbon + nitrogen between 0.01 and 0.2%
• silicon less than 2%
• manganese less than 4%
• nickel less than 4%
• copper less than 4%
• Ni + (Mn + Cu) / 3 between and 0.5 and 5%,
  the rest being iron.

The steel is subjected to a process annealing treatment, in an oven, to obtain a ferrite-austenite structure. This steel is then quenched to obtain martensite from austenite.

Such a method makes it possible to obtain high breaking loads, but does not meet the need for a high elastic limit. For example, for given breaking loads of 800 to 950 MPa, the elastic limit varies from 415 to 635 MPa, or in a ratio of 50 to 70% of the breaking load.

• carbone compris entre 0,03 et 0,06%
• silicium inférieur à 1%
• manganèse inférieur à 1%
• chrome compris entre 16 et 17,5%
• nickel compris entre 0,8 et 1%,
  le reste étant du fer.

It is also known from DE-A-2923.532, a ferritic stainless steel for conveyor chains, obtained from the following weight composition:

• carbon between 0.03 and 0.06%
• silicon less than 1%
• manganese less than 1%
• chromium between 16 and 17.5%
• nickel between 0.8 and 1%,
  the rest being iron.

• charge à la rupture comprise entre 750 et 800 MPa
• limite élastique supérieure à 600 MPa
• allongement supérieur à 10%

After annealing, the stainless steel is rolled at a rate of between 18 and 25%, in order to obtain the following mechanical characteristics:

• breaking load between 750 and 800 MPa
• elastic limit higher than 600 MPa
• elongation greater than 10%

Such a steel has an elastic limit corresponding to approximately 75% of the breaking load, but the mechanical characteristics obtained are too weak for an envisaged application such as, for example, the manufacture of conveyor chains.

The object of the present invention is to obtain a stainless steel with a high elastic limit and a high breaking load and which also has good characteristics in slitting, cutting, malleability and resistance to corrosion and abrasion.

• carbone inférieur à 0,10%
• chrome compris entre 16 et 20%
• nickel compris entre 0,2 et 2%
• manganèse inférieur à 2%
• cuivre inférieur à 2%
  le reste étant du fer, et des impuretés inhérentes au mode d'élaboration,
  et dont les différentes teneurs répondent à la relation : $$\text{20 C\% + 1,1 Ni\% +}\frac{\text{1}}{\text{3}}\text{(Mn + Cu)\% = 1,5 à 2,5}$$
  
  est soumis successivement à :
• une trempe après élévation à une température comprise entre 800 à 1200°C,
• et au moins un écrouissage à un taux supérieur à 15%.

Selon d'autres caractéristiques :

• la teneur en carbone est inférieure à 0,05%,
• l'acier inoxydable comprend, en outre, éventuellement dans sa composition pondérale moins de 2,5% de molybdène,
• l'acier est soumis à une trempe après une élévation de température comprise entre 900 et 1100°C.

The present invention therefore relates to a process for the preparation of a stainless steel with a two-phase ferrite martensite structure, characterized in that the steel of the following composition by weight:

• carbon less than 0.10%
• chromium between 16 and 20%
• nickel between 0.2 and 2%
• manganese less than 2%
• copper less than 2%
  the rest being iron, and impurities inherent in the method of production,
  and whose different contents correspond to the relationship: $$\text{20 C\% + 1.1 Ni\% +}\frac{\text{1}}{\text{3}}\text{(Mn + Cu)\% = 1.5 to 2.5}$$
  
  is successively subject to:
• quenching after raising to a temperature between 800 to 1200 ° C,
• and at least one work hardening at a rate higher than 15%.

According to other characteristics:

• the carbon content is less than 0.05%,
• stainless steel also comprises, optionally in its weight composition less than 2.5% of molybdenum,
• the steel is subjected to quenching after a temperature rise of between 900 and 1100 ° C.

The present invention also relates to a stainless steel with two-phase ferrite martensite structure, in which the ferritic or martensitic phases are in a proportion of between 40 and 60% and preferably, in a proportion of approximately 50%.

Such a steel has a breaking load greater than 950 MPa and an elastic limit greater than 900 MPa, the elastic limit being greater than or equal to 90% of the breaking load.

The present invention particularly relates to a conveyor chain made of such a stainless steel.

The characteristics and advantages will become apparent during the description which follows, given solely by way of example, made with reference to the appended drawings which represent the load-breaking curves (Rm), of elastic limit (Rp 0.2 ) and elongation (A%), depending on the rate of work hardening.

Acier A :

C = 0,02% ; Cr = 16,5% ; Ni = 1,4%
Mn= 0,40% ; Cu = 0,05% ;

Acier B :

C = 0,04% ; Cr = 16,5% ; Ni = 1,4% ;
Mn= 0,35% ; Cu = 0,05% ;

le reste étant du fer et des impuretés inhérentes au mode d'élaboration.In an exemplary embodiment according to the invention, two stainless steels known as 17% chromium were produced, the compositions of which are as follows:

Steel A:

C = 0.02%; Cr = 16.5%; Ni = 1.4%
Mn = 0.40%; Cu = 0.05%;

Steel B:

C = 0.04%; Cr = 16.5%; Ni = 1.4%;
Mn = 0.35%; Cu = 0.05%;

the rest being iron and impurities inherent in the method of production.

la somme algébrique des compositions étant égale à 2,1 pour l'acier A et 2,5 pour l'acier B.The defined contents correspond to the relationship: $$\text{20C\% + 1.1 Ni\% +}\frac{\text{1}}{\text{3}}\text{(Mn + Cu)\% = 1.5 to 2.5,}$$

the algebraic sum of the compositions being equal to 2.1 for steel A and 2.5 for steel B.

According to the invention, a ferritic-martensitic structure has been obtained by subjecting the steels of compositions defined above to quenching after a rise in temperature of said steels between 900 and 1100 ° C for a few minutes.

Traditionally, steels rich in martensite are little or not used because of their brittleness and the risks of breakage which they involve in rolling operations and during the shaping of parts.

Unexpectedly, no incident appeared during the work hardening of these two-phase ferrito-martensitic steels by rolling.

The steels after production and thermal and mechanical treatments, have, as shown in Figures 1 and 2, on the one hand a tensile strength of 1070 MPa, an elastic limit substantially equal to 1050 MPa, an elongation of 6% for a work hardening of 40% of steel A and, on the other hand a breaking load of 1180 MPa, an elastic limit equal to approximately 1140 MPa, an elongation of 3.5% for a work hardening of 40% of steel B.

In addition, the improvement in abrasion resistance ensured by an increase in the mechanical characteristics by work hardening: load at break, elastic limit, etc., is reinforced by the presence of a martensitic phase in a ferritic structure forming l two-phase steel.

After obtaining the two-phase and work-hardened steel sheet, shaping operations made it possible to produce a conveyor chain element, the shaping comprising slitting, cutting and rolling operations.

It has been found that the steels A and B according to the invention, produced in the form of sheet metal, despite a percentage of 50% and 55% respectively of martensite and despite a work hardening of more than 40% have excellent aptitude for the production of parts , such as for example conveyor chain links.

The tensile strength of the conveyor chain element, compared to existing conveyor chains of hardened type 430 steel, is 40% higher. It is also found that the corrosion resistance is improved with the steel according to the invention.

Too high a carbon content causes sensitization to intergranular corrosion. The carbon contents chosen in the composition of the steels and the method according to the invention greatly reduce the precipitation of chromium carbides, which causes stainless steels to sensitize to intergranular corrosion, but also to corrosion in an aqueous medium. chloride.

The introduction into the steel composition of less than 2.5% molybdenum increases the resistance to corrosion and more particularly the resistance to corrosion in a chlorinated aqueous medium.

The process according to the invention can be used with products of various shapes, sheets, bars, tubes, wires, etc.

Claims (10)

Process for the production of stainless steel with high elastic limit and high breaking load, with a two-phase ferrite martensite structure, having good malleability and high abrasion resistance, characterized in that the steel of composition by weight next : - carbon less than 0.10% - chromium between 16 and 20% - nickel between 0.2 and 2% - manganese less than 2% - copper less than 2%
the rest being iron and impurities inherent in the method of production,
and whose different contents correspond to the relation $$\text{20 C\% + 1.1 Ni\% +}\frac{\text{1}}{\text{3}}\text{(Mn + Cu)\% = 1.5 to 2.5}$$

is successively subject to: - quenching after raising to a temperature between 800 to 1200 ° C, - and at least one work hardening at a rate higher than 15%. Process according to claim 1, characterized in that the carbon content is less than 0.05%. A method according to claim 1, characterized in that the steel further comprises, in its composition by weight, less than 2.5% of molybdenum. Method according to claim 1, characterized in that the steel is subjected to quenching after a temperature rise of between 900 and 1100 ° C. Stainless steel with two-phase martensite ferrite structure, characterized in that it is obtained by the method according to any one of claims 1 to 4. Steel according to claim 5, characterized in that the ferritic or martensitic phases are in a proportion of between 40 and 60%. Steel according to claims 5 and 6, characterized in that the ferritic or martensitic phases are in a proportion of about 50%. Steel according to any one of Claims 5 to 7, characterized in that it has a breaking load greater than 900 MPa. Steel according to any one of Claims 5 to 8, characterized in that it has an elastic limit greater than 800 MPa, the elastic limit being greater than or equal to 90% of the breaking load. Conveyor chain, characterized in that it is produced from a two-phase ferrite martensite steel, according to any one of claims 5 to 9.

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