CS214787B2 - Semiferitic non-corroding steel and method of thermal treating thereof - Google Patents

Description

(54) Semi-ferritic stainless steel and method of its heat treatment

The invention solves semi-ferritic stainless steel and a method of its heat treatment.

Known stainless semi-ferritic steels, containing essentially 16 to 18% by weight of chromium, have a total of one of the following compositions:

The first steel has a weight composition:

C traces up to 0.1 ° / o,

Mn 1,0%

Si traces up to 1.0%,

P traces up to 0.04%,

S 0,03%,

Ni trace up to 0,5 ° / o,

Cr 16-18%;

other steel has mass composition:

C traces up to 0.1%,

Mn 1,0%

Si traces up to 1.0%,

P trace up to 0,04 ° / o,

With traces up to 0.03%,

Ni traces up to 0.5%,

Cr 16 to 18%,

Mo 0.9 to 1.3%;

and the other steel has a weight composition;

C traces up to 0.12%,

Mn trace up to 1.5%,

Si traces up to 1.0%,

P trace up to 0,06 ° / o,

Not less than 0,15%,

Ni traces up to 0,5%,.

Cr 16-18%.

These alloys are characterized by a mixed structure of ferrite δ -j-austenite χ in the range of temperatures usually used for their hot forming (forging, rolling). In this temperature range between approximately 1300 ° C (temperature 5 5) and 850 ° C (temperature 1 1), the percentage of austenite passes through the maximum at a temperature near 1100 ° C and is normally 10 to 40%. By rapid cooling, this austenite is converted to martensite. Annealing to a temperature below A1 allows to decompose the martensite so formed and to obtain a homogeneous structure consisting of ferrite and carbides according to the following overview:

hot structure: ferrite δ + austenite χ structure after ferrite δ martensite M cooling:

Annealing structure: ferrite--j- ferrite + + carbides C.

The last structure corresponds to the classical conditions of use of these species in the form of bars, sheets or wires.

The rapid cooling of the semi-ferritic steel structure by heat, which will be the case, in particular in the case of the welded zone, causes both an increase in the ferrite grain α and a conversion of the austenite formed into martensite. The existence of this transformation is the cause of the brittleness of the welds, because it leaves a fragile martensltic phase at the grain contact points of the ferritic structure.

The only acceptable solution designed to improve the weldability of semi-ferritic steels is to use a fully ferritic structure. The most commonly used titanium is:

C w 0.10 ° / o,

Si 1%,

Mn w 1%,

Cr 17%,

Ti%> 70% - or niobium,

Cš0,10%,

Th 1.5%,

Mn w 1%,

Cr 17.5%,

Nb > 20 ° / o.

This method does not make it possible to eliminate the problems associated with grain growth of fully ferritic structures. For example, it is known that a fully ferritic structure makes. problems in hot rolling on a continuous mill, ingot reheating, and slab grinding for brittleness of structure.

Finally, semi-ferritic steels have the advantage that they can easily be formed into sheets or wires, that they resist stress corrosion in chlorinated solutions, are inexpensive, but have the disadvantage that they are bumpy after welding and are sometimes insufficiently corrosion resistant.

The addition of nickel, manganese, or a suitable combination of these two elements makes it possible to convert the structure of semi-ferritic steels containing 17% by weight of chromium into a completely austenitic structure at each temperature and in particular under conditions of use. This new group of steels corresponds to stainless steel austenitic steels, some of the classical weight compositions of which are given below. One such steel has a weight composition:

C Ši Mni Ni 0,05 0,5 10 2,0 0,05 0,5 0.5 8 0,05 0,5 0.5 6.5 Normal thermal: processing before use-

this is quenching in water at 1050 to 1150 ° C. These steels have the advantage of improved weldability as compared to semi-ferritic steels, however, they retain their sensitivity to grain growth, have better stress corrosion resistance compared to austenitic steels.

The disadvantage of these steels is that they can

C traces up to 0.12%,

Mn trace up to 2.0%,

Si traces up to 1.0%,

P trace up to 0,04 ° / o,

With traces up to 0.03 ^iO / o,

Ni 8-10%, Cr 17-19%;

other steel has mass composition:

C traces up to 0.7%,

Mn trace up to 2.0%,

Si traces up to 1.0%,

P traces up to 0.04%,

With traces up to 0.03%,

Ni 10-12 ° / o

Cr 16 to 18%,

Mo 2 to 2.5%;

and other steel has mass composition:

C traces up to 0.15%,

Mn 5.5 to 7.5%,

Si tracks up to 1.0 ° / o,

P trace up to 0,06 ° / o,

With traces up to 0.03%,

Ni 3.5 to 5.5%,

Cr 16-18%.

These steels are used either in the rolled state or in the mechanically hardened state.

They have the advantage of good weldability and good corrosion resistance, the disadvantage that the price of steel increases considerably due to the addition of nickel or a combination of nickel and molybdenum, being particularly sensitive to stress corrosion, limiting their use for certain applications (water heaters), and They have a low elastic limit.

The so-called austenitic-ferritic steels form a transition group between semi-ferritic and austenitic steels. Their composition is chosen to create a two-phase structure: austenite + ferrite. The basic difference between this group of steels and between semi-ferritic steels, which also contain a relatively large amount of austenite in the heat, is in the particular stability of this two-phase structure as opposed to semi-ferritic steels whose structure after decomposition into ferrite + -j- mentioned above. Some weight compositions of steels of this group are given in the following table:

Cr Ti Cu Mo 18 0.4 _ _ 20 May 1.5 2.5 26 0.2 - -

difficult to hot roll and because of their composition they have a significantly higher price.

The steels of the invention belong to a group of stainless semi-ferritic steels with improved weldability and corrosion resistance compared to other steels of this group. Said heat treatment in the steel of the invention increases the productivity of the plant for the production of sheets, rods and wires due to a considerable reduction in the overall utilization time of the processing furnaces.

Hereinafter it will be referred to as · chromium equivalent of steel (° / o Cr) -f- (% Si) + + (O / o Mo) + 4 (θ / o Ti +% Nb) and as nickel equivalent: (% Ni ) -j- 0.5 (% Mn) + + 0.5 (% Cu) + (% Co) + 20 (O / o C +% N2) and the steel will be represented as a point in the orthogonal coordinate system according to the attached illustration, where a chrome equivalent is applied to the line segment and a nickel equivalent is applied to the ordinate, the lacquer is as defined above.

These steels have the same hot structure as semi-ferritic steels (ferrite + austenite), but retain this two-phase structure on cooling, without forming a brittle martensitic phase. As a result, they are better weldable than conventional semi-ferritic steels.

Their corrosion resistance is also better than that of conventional semi-ferritic steels. In addition, expensive alloying metals are saved.

The essence of the semi-ferritic stainless steel according to the invention is characterized in that it contains traces of up to 0.1% carbon, 3 to 6% of manganese, traces of up to 1% of silicon, traces of up to 1% of nickel, 15-18% of chromium. 5 to 3% molybdenum, traces up to 1% copper, traces up to 0.1% nitrogen, the rest iron and unavoidable impurities.

The group derived therefrom in which part of the molybdenum is replaced by silicon has comparable properties. It also contains 1 to 2% Si and 0.5 to 2% Mo by weight concentration, the other is unchanged.

In order to improve the machinability of the steels according to the invention, sulfur and / or selenium and / or tellurium can be added in a known manner in a total amount not exceeding 0.4%.

The steels according to the invention, which are supplied in the form of sheets, rods or wires, can be subjected to the usual manufacturing operations as known semi-ferritic steels, i.e. block rolling, hot rolling, annealing and decapping, cold or drawing, end annealing. The annealing after the last hot cylinder puncture is normally extended annealing, at a temperature of the order of 800 ° C, which leads to maximum softening, favorable for further cold forming operations.

The principle of the heat treatment of steel according to the invention is that after the last rolling or hot drawing during forming into sheets, rods, wires, the heat treatment is carried out in two stages, the first converting austenite retained at ambient temperature to martensite and the second being tempering to a maximum of 850 ° C, whereby martensite is converted to ferrite and carbides.

The first stage depends on one embodiment of the invention in that it is annealed at a temperature between 700 and 900 ° C, followed by slow cooling to 650 ° C and then cooling in air.

The annealing is preferably carried out at a temperature of between 750 and 800 ° C, for example for 4 hours, followed by slow cooling, of the order of 25 ° C per hour up to 650 ° C, then cooling in air.

According to another embodiment, the first stage of the treatment is freezing, eg keeping at -80 ° C for 3 hours. According to another embodiment of the invention, the first stage is mechanical consolidation at ambient temperature.

According to yet another embodiment of the invention, the first step is slow cooling from the outlet temperature of the last hot rolling operation to 650 ° C and then cooling in air.

The second stage is discharge to a temperature below 850 ° C only after the disappearance of martensite. Its duration, which can be an hour or less, is considerably less than a single annealing, usually lasting 10 to 20 hours, which makes it possible to improve the profitability of the furnaces.

For example, 5 melts of steels according to the invention were carried out. Their analyzes were:

Melting mass concentration in%

C Si Mn Ni Cr Mo WITH P Nž (1) 0,058 0.4 4.7 0.5 20 May 0.02 0.005 0.019 0.051 (2) 0,063 0.4 9.2 0.2 23.1 0.01 0.005 0.019 0.052 (3) 0,044 0.3 4.4 0.1 17.4 1.98 0.016 0,023 0.024 (4) 0,045 1.3 7.6 0.8 17.1 0.98 0.014 0,023 0,023 (5) 0,064 1.9 5.2 0.1 16.8 0.96 0,025 0,025 0.050 They were determined their mechanical Properties / h to 650 ° C, then cooled to air, annealing in two stages according to way and then there was tempering at 750 ° C during

minutes, followed by air cooling. The results are in the following table:

diameter 10 mm, coarse forged, annealed at 785 ° C for 4 hours, then cooled at 25 ° C /

Melting R (MPa) E (MPa) A% ε% (L _o = 50mm)

(1) 560 330 30 56 (2) 580 340 29 58 (3) 590 350 26 58 (4) 630 390 25 58 (5) 680 440 31 77

The bending capability, without any heat treatment, of the welds of the steels of the invention is higher than that of conventional semi-ferritic steels. A comparison of the microhardness of the various components in the weld of conventional semi-ferritic steels and steels according to the invention gives the following results:

Microhardness inside ferrite grains customary semi-ferritic 220 steel steel according to the invention 260

Low toughness of melt grain Characteristic of stainless semi-ferritic steels.

This toughness is greatly increased in the steels according to the invention, as shown in the table:

Steel Charpy toughness in J / cm ^{* 2} semi-ferritic <0.5 ferritic 0.5 to 1.5 austenitic-ferritic 1.5 to 3 austenitic according to the invention> 4

In terms of corrosion, there are two main types of adverse effects on stainless steels - hole corrosion in the presence of chlorine ions and total (area) corrosion in a dilute, non-chlorinated acid environment.

The following experiments were carried out on 10 mm thick sheets. The hot-rolled sheets of the steels of the invention were subjected to annealing in two stages according to the invention.

1. Corrosion in the presence of chlorine ions in a non-oxygenated but aerated solution

This corrosion refers to atmospheric corrosion, since chlorine ions are always present far from the sea, salt solutions (food products, etc.).

Corrosion resistance in this region was evaluated by the potential of the wells in sodium chloride solutions using an anodic polarization curve. The cathodic oxygen reduction reaction was shown to be insensitive to the well type, so that the determination of the well potential (anode characteristic) is a good measure of classification.

The following table shows the results of the comparative experiments for different steels.

Vickers / 50 g load on ferrite grain contact

530 (martensite phase)

320 (austenitic phase) well potential (in mV) in the environment

NaCl 0.02 M steel semi-ferritic steel ferritic steel austenitic-ferritic steel

530 to 580 540 to 590 650 to 730 austenitic manganese610 to 660 steels austenitic 640 to 700 nickel steels steel of the invention 620 to 680

2. Surface corrosion in a dilute, non-chlorinated environment

In the vast majority of cases, stainless steel will be in a passive state to resist corrosion in an acidic environment, otherwise it will corrode in active states.

In a certain acidic solution, the given stainless steel will be passive when the cathodic reaction of reducing the oxldant in solution - dissolved oxygen, Fe ³⁺ , Cu ²⁺ ions, etc. - brings the potential of the metal to the passivity area. This is done when the oxldant reduction rate, expressed in A / cm ² , is higher than the critical intensity of the paslation of ic per cm ² , measured on the anode polarization curve of the metal emitted without the oxidant. Conversely, for a given oxidant reduction rate, passive steels having ic less than this rate will be active, and steels having i _c greater will be active. Experience has shown that at least in a very large scale is the critical parameter _c: the steel is the "better" to _C will be smaller.

critical passivation current (10 ^A.3 A. cm ^-2 ) steel in H2SO4 environment 2M semi-ferritic 11 to 15 ferritic 10 to 15 austenitic-ferritic 0.5 to 4 manganese austenitic 5 to 9 nickel austenitic 0.7 to 1.5 steel according to the invention 2 to 6

214

The foregoing results make it possible to place the steel according to the invention at a level between nickel austenitic steels and classical semi-ferritic steels.

The use of the steels according to the invention in the form of bars, sheets and wires is very diverse. In addition to the usual use of semi-ferritic, austenitic manganese or low nickel steels, such as boiler construction, construction, sumps, wheel guards, bumpers, car trim rings, the steels of the present invention have succeeded in other 87 branches for their lower cost than the price of austenitic and austenitic-ferritic steels, and for its better quality than that of semi-ferritic steels. They are particularly suitable for the production of hot water tanks, automobile radiators (for their corrosion resistance under voltage, for their weldability and cost), paper pots, welded pipes, kitchen racks (for their weldability), roof slate hooks (for their corrosion resistance).

Claims (6)

pSedmEt 1. Semi-ferritic stainless steel, characterized in that it contains, by weight, traces up to 0.1% carbon, 3 to 6% manganese, traces up to 1% silicon, traces up to 1% nickel, Ί5 to 18% chromium, 1.5 up to 3% molybdenum, traces up to 1% copper, traces up to 0.1% nitrogen, the rest iron and unavoidable impurities. 2. The method of heat treatment of steel according to claim 1, characterized in that after the last rolling or hot drawing during forming into sheets, rods, wires, the heat treatment is carried out in two stages, the first of which converts the austenite retained at ambient temperature to martensite. and the second is tempering to a maximum of 850 ° C, whereby martensite is converted to ferrite and carbides. ynAlezu 3. The method of claim 2, wherein the first step is annealing at a temperature of between 700 and 900 ° C, followed by slow cooling to 650 ° C and then cooling in air. 4. The method of claim 2, wherein the first step is a frost treatment. 5. The method of claim 2 wherein the first step is mechanical consolidation at ambient temperature. 6. The method of claim 2, wherein the first step is slow cooling from the outlet temperature of the last hot rolling operation to 650 [deg.] C. and then cooling in air.