TWI463020B - Acier inoxydable duplex - Google Patents

Description

Double stainless steel

The present invention relates to double stainless steel, and more particularly to structural components for use in material manufacturing equipment (chemical, petrochemical, paper and offshore industries) or energy manufacturing equipment in a manner that does not impose any limitation thereto, and also relates to A method of making plates, strips, rods, sticks, wires or blocks from this steel.

In general, in many applications, such as in the aforementioned industries or in the agro-food industry, including self-forming sticks or wires (welded grids, etc.), blocks (stress applicators, etc.), shafts .. and other parts, this steel can be used as a substitute for 304L stainless steel. It can also make castings and forgings.

For this purpose, stainless steel grades of type 304 and 304L are known which, in the annealed state, have substantially a Wolster iron microstructure. In the cold worked state, they may further contain a variable proportion of 麻田散铁. However, these steels include a large amount of added nickel, which is usually very expensive. In addition, from a technical point of view, these grades cause problems in some applications because of the poor tensile properties (especially yield strength) and stress corrosion resistance in the annealed state.

It is also known that the Worthfield-fertilizer iron type stainless steel is mainly composed of a mixture of ferrite iron and Worthite iron, for example, steels 1.4362, 1.4655, 1.4477, 1.4462, 1.4507, 1.4410, 1.4501 and according to the EP10088 standard. 1.4424, which all contain more than 3.5% nickel. These steels are particularly resistant to corrosion and stress corrosion.

It is also known that the ferrite iron type or the fat grain-Mita loose iron stainless steel grade is used for a clear heat treatment range, and its microstructure consists of two components (fertilizer iron and 麻田散铁, preferably 50/50 ratio, It is composed of 1.4017 grade according to EN10088 standard. These grades (typically less than 20% chromium) have high tensile mechanical properties, but corrosion resistance is not satisfactory.

In addition, it would be desirable to simplify the method of making stainless steel sheets, strips, rods, sticks, wires or blocks.

The object of the present invention is to provide high mechanical properties (especially tensile strength in the state of annealed or dissolved treatment of more than 400 or even 450 MPa), high corrosion resistance (especially the same or better than 304L) 304L), stainless steel with good microstructure stability and good toughness in the weld zone, and without the addition of expensive additional elements, and providing a method that is easier to implement, since the steel is used to make plates, strips, rods, sticks, wires or blocks, To remedy the shortcomings of the prior art steel and manufacturing methods.

For this purpose, the first object of the invention is a double stainless steel whose composition, in weight %, consists of: C 0.05%, 21% Cr 25%, 1% Ni 2.95%, 0.16% N 0.28%, Mn 2.0%, Mo+W/2 0.50%, Mo 0.45%, W 0.15%, Si 1.4%, Al 0.05%, 0.11% Cu 0.50%, S 0.010%, P 0.040%, Co 0.5%, rare earth metal (REM) 0.1%, V 0.5%, Ti 0.1%, Nb 0.3%, Mg 0.1%, the remainder is iron and impurities derived from smelting, and its microstructure consists of Worthite iron and 35 to 65 vol% ferrite iron, which further satisfies the following relationship: 40 I _F 70 to 40 I _F 60 is preferably wherein I _F = 6x (% Cr + 1.32 x % Mo + 1.27 x % Si) - 10 x (% Ni + 24 x % C + 16.15 x % N + 0.5 x % Cu + 0.4 x % Mn) - 6.17 and I _LCR 30.5 to I _LCR 32 is preferred wherein I _LCR = % Cr + 3.3 x % Mo + 16 x % N + 2.6 x % Ni - 0.7 x % Mn.

The steel according to the invention may also comprise the following individual or combined optional features: - the ratio of ferrite iron is between 35 and 55 vol%; - a chromium content of between 22 and 24% by weight; - a manganese content of less than 1.5% by weight; - a calcium content of less than 0.03% by weight; and - a molybdenum content of more than 0.1% by weight.

A second subject of the invention consists of a method for producing a hot rolled sheet, strip or coil from a steel according to the invention, wherein: - an ingot or slab of steel providing the composition of the invention; and - the ingot or The slab is hot rolled at a temperature between 1150 and 1280 ° C to obtain a plate, strip or coil.

In a particular embodiment, the ingot or slab is hot rolled at a temperature between 1150 and 1280 ° C to obtain a so-called quarto plate; thereafter, between 900 and 1100 ° C The temperature is heat treated; and the plate is cooled by air quenching.

A third object of the invention consists of a method for producing a hot rolled rod or stick from steel according to the invention, wherein: - a continuous ingot or block of steel providing a composition according to the invention; - the ingot or block Hot rolling at a temperature between 1150 and 1280 ° C to obtain a rod (which is cooled by air) or a coil of wire (which is cooled by water); and thereafter, selectively: - between 900 Heat treatment at a temperature between 1100 ° C; and - the rod or strand is quenched and cooled.

In a particular embodiment, after cooling, a cold drawing operation is performed on the rod or a mold stretching operation is performed on the rod.

The present invention also encompasses a method of making a steel block, wherein the cold forming operation is performed on a hot rolled bar obtained in accordance with the present invention; and also includes a method of making a steel forging, wherein the hot rolled bar obtained in accordance with the present invention The forging operation is performed when it is cut into metal pieces and then between 1100 ° C and 1280 ° C on the metal block.

The invention further comprises a plurality of products obtainable by the process according to the invention and uses thereof, such as: - hot rolled steel four-open size plates having a thickness between 5 and 100 mm, and strips and coils, which can be used for manufacturing Structural components for the manufacture of materials or energy-generating devices, in particular for materials and energy manufacturing devices operating between -100 and 300 ° C (for operation between -50 and 300 ° C) Structural components; - cold rolled steel bars, which can be obtained by cold rolling of hot rolled coils; - hot rolled bars of 18 mm to 250 mm in diameter and cold drawn bars of 4 mm to 60 mm in diameter, this product is available For the manufacture of mechanical parts (such as pumps, valve shafts, motors and engine shafts) and adapters operating in corrosive media; - hot rolled rods from 4 mm to 30 mm in diameter and moulds from 0.010 mm to 20 mm in diameter Stretching rods or threads, this product can be used to make cold formed kits, agro-food industry, oil and mineral extraction, or to fabricate woven or knitted metal fabrics for chemical, mineral or food filtration; - block; - a forging that can be used to make a bracket or adapter; And castings which can be obtained by casting steel according to the invention.

Other features and advantages of the present invention will be apparent from the following description, taken as example.

The double stainless steel according to the present invention contains the content defined below.

The grade has a carbon content of 0.05% by weight or less, preferably less than 0.03% by weight. This is because the excessive content of this element reduces the local corrosion resistance by increasing the risk of precipitation of chromium carbide in the heat affected zone of the weld.

This grade has a chromium content between 21 and 25% by weight, preferably between 22 and 24% by weight, for good corrosion resistance, which is at least comparable to that of the type 304 or 304L grade. .

The nickel content of this grade is between 1 and 2.95%, preferably equal to or lower than 2.7%, or even less than 2.5% by weight. This element forming the Vostian iron is added to obtain good crack corrosion resistance. When the content is more than 1% and more preferably more than 1.2% by weight, it has an advantageous effect against the initiation of pitting corrosion. However, the content was limited, and when it was more than 2.95 wt%, the pit-resistant property was observed to be lowered. The addition of nickel also makes it possible to obtain a good toughness/ductility compromise due to its advantage of having a toughness transition curve towards a low temperature transition, which is especially true when manufacturing thick four-size plates, where the toughness properties are important. advantageous.

Since the nickel content of the steel according to the invention is limited, it has been found that in order to obtain a suitable Worth iron content after heat treatment between 900 ° C and 1100 ° C, it is necessary to add an unusually large amount of other formations. The elements and limits of the iron form the content of the element of the ferrite.

The nitrogen content of this grade is between 0.16 and 0.28% by weight, which generally means that nitrogen is added to the steel during the smelting period. This element of the formation of the Worthite iron first enables it to obtain the ferrite-iron-Worstian iron two-phase double steel (containing a proper proportion of Worthite iron, which has good stress and corrosion resistance), and also has high mechanical strength. Metal. It also enables it to have good microstructure stability in the heat affected zone of the weld. The maximum content is limited because, above 0.28%, solubility problems are observed; during solidification of slabs, blocks, ingots, castings or welds, bubbles are formed.

The content of manganese, which is also an element which forms Worth iron at less than 1150 ° C, is maintained below 2.0% by weight, and preferably less than 1.5% by weight, so that the element is disadvantageous in many respects. Therefore, this grade will have problems during the smelting and refining period due to some of the refractory used to attack the ladle. Therefore, these expensive elements must be replaced more frequently, thus making the process more interrupted.

The addition of iron-manganese (usually such that this grade is added to the composition) also contains appreciable amounts of phosphorus and selenium. It is undesirable to introduce these elements into the steel and it is difficult to remove these elements when refining this grade. . Manganese also interferes with this refining by limiting the possibility of decarburization. It is also problematic further downstream of the process, which suffers from the corrosion resistance of grades due to the formation of manganese sulfide (MnS) and oxidized inclusions. This element is often added to grades that are expected to be rich in nitrogen to increase the solubility of the element in the grade. When the manganese content is insufficient, this nitrogen content cannot be reached in the steel. However, the inventors have found that it is possible to limit the addition of manganese to the steel according to the invention, and at the same time add a sufficient amount of nitrogen to achieve the desired effect and stability on the ferrite-Worthian iron balance of the base material. Heat affected area of the weld.

Molybdenum (an element forming ferrite iron) is maintained at a content of less than 0.45 wt%, and tungsten is maintained at a content of less than 0.15 wt%. However, the content of these two elements is such that the sum of Mo+W/2 is less than 0.50% by weight, preferably less than 0.4% by weight, more preferably less than 0.3% by weight. This is because the inventors have found that by maintaining the two elements and their contents and below the values shown, no precipitation of intercalated intermetallics is observed, whereby by heat treatment or The sheet or strip after hot working is cooled by air, and in particular, the method of manufacturing the steel sheet or strip can be alleviated. Furthermore, it has been observed that by controlling these elements within the stated limits, the solderability of the grades is improved. Preferably, however, the minimum molybdenum content is maintained at 0.1% to improve the hot forgeability of this grade. In addition, the smelting of grades with a molybdenum content of less than 0.1% imposes significant restrictions on the use of recycled waste for this grade, thus causing processing problems, in particular the use of a feed consisting of 100% pure iron alloy.

Copper (an element forming Vostian iron) is present in an amount between 0.11 and 0.50% by weight, and preferably between 0.15 and 0.40% by weight. This element improves the corrosion resistance in an acid medium having a reducing power. However, its content is limited to 0.50% by weight to prevent the formation of a ruthenium phase (it is desirable to prevent this phase because these cause hardening of the ferrite grain iron phase and double alloy embrittlement).

The oxygen content is preferably limited to 0.010% by weight to improve its forgeability.

Boron is an optional element which may be between 0.0005 wt% and 0.01 wt% (preferably between 0.0005 wt% and 0.005 wt%, more preferably between 0.0005 wt% and 0.003 wt%). Amount is added to the grade according to the invention to improve its thermal conversion. In yet another embodiment, however, the boron content is preferably limited to less than 0.0005 weight percent to limit the risk of cracking during welding and continuous casting.

The content of cerium (an element forming ferrite iron) is less than 1.4% by weight. Aluminum (an element forming ferrite iron) is present in an amount of less than 0.05% by weight, preferably between 0.005% by weight and 0.040% by weight, to obtain a low-melting calcium aluminate inclusion. The maximum aluminum content is also limited to prevent excessive formation of aluminum nitride. The role of these two elements (矽 and aluminum) is essentially to ensure deoxidation of the steel bath during smelting.

Cobalt, an element forming the Vostian iron, is maintained at a level of less than 0.5% by weight, preferably less than 0.3% by weight. This element is a residual element derived from the raw material. In particular, it may be limited due to processing problems (having processing problems after illuminating components of nuclear power generation equipment).

The rare earth metal (expressed as REM) may be added to the composition in an amount of 0.1% by weight (preferably less than 0.06% by weight). Mention may be made in particular of bismuth and strontium. The content of these two elements is limited because they tend to form unwanted intermetallics.

Vanadium (an element forming ferrite iron) may be added to this grade in an amount of 0.5% by weight (preferably less than 0.2% by weight) to improve the crack corrosion resistance of the steel.

Niobium (an element forming ferrite iron) may be added to this grade in an amount of 0.3% by weight (preferably less than 0.050% by weight). It helps to improve the tensile strength of this grade by virtue of the formation of tantalum nitride. Limit its content to limit the formation of coarse-grained tantalum nitride.

Titanium (an element forming ferrite iron) can be added to this grade in an amount of 0.1% by weight (preferably less than 0.02% by weight) to limit the formation of titanium nitride (especially titanium nitride in liquid steel) Formation).

Calcium may also be added to the grade according to the invention such that the calcium content is less than 0.03% by weight, preferably more than 0.0002% by weight, more preferably more than 0.0005% by weight, to control the nature of the oxide inclusions and Improve machinability. The content of this element is limited because it is easy to form calcium sulfide with sulfur (which reduces corrosion resistance). In a preferred embodiment, the calcium content is limited to less than 0.0005 wt%, preferably less than 0.0002 wt%.

The sulfur content is maintained below 0.010% by weight and is preferably less than 0.003% by weight. As mentioned above, this element forms a sulfide with manganese or calcium, and the presence of sulfide will impair corrosion resistance. Sulfur is considered an impurity.

Magnesium may be added in an amount such that the final content is 0.1% to modify the nature of the sulfides and oxides.

Selenium is preferably maintained at less than 0.005% by weight because of its damage to corrosion resistance. This element is usually introduced into this grade in the form of impurities in fermented iron and manganese ingots.

Phosphorus is maintained at a level of less than 0.040% by weight and is considered an impurity.

The remainder of the composition consists of iron and impurities. In addition to the foregoing, these impurities may be zirconium, tin, arsenic, lead or antimony. Tin may be present in an amount of less than 0.100% by weight and preferably less than 0.030% by weight to prevent soldering problems. Arsenic may be present in an amount of less than 0.030% by weight and preferably less than 0.020% by weight. Lead may be present in an amount of less than 0.002% by weight and preferably less than 0.0010% by weight. The cerium may be present in an amount of less than 0.0002% by weight and preferably less than 0.00005% by weight. Zirconium can be present in an amount of 0.02%.

Further, the inventors have found that when the weight percentages of chromium, molybdenum, nitrogen, nickel and manganese satisfy the following relationship, the grade has good local corrosion resistance, that is, resistance to formation of pits or cracks:

In the annealed state, the microstructure of the steel according to the present invention is composed of Worthite iron and ferrite iron. Preferably, after treatment at 1000 ° C for 1 hour, the phase is 35 to 65 vol% (35 to 55). A particularly good volume % is present in the proportion of fat granules.

The inventors have also found that the following formula suitably describes the ferrite iron content at 1100 ° C: I _F = 6 x (% Cr + 1.32 x % Mo + 1.27 x % Si) - 10 x (% Ni + 24 x % C + 16.15 x % N + 0.5 x % Cu + 0 .4x%Mn) - 6.17.

Therefore, in order to make the ratio of ferrite iron at 1100 °C between 35 and 65%, the ferrite iron index I _F must be between 40 and 70.

In the annealed state, the microstructure does not contain other phases that can damage its mechanical properties, particularly, such as the sigma phase and other intermetallic phases. In the cold worked state, a portion of the Worthite iron can be converted to a granulated iron, depending on the actual temperature of the deformation and the amount of cold deformation applied.

In general, the steel according to the invention can be smelted and can be made into a hot rolled sheet form (so-called four-open size sheet) from a slab or an ingot, but can also be in the form of a hot rolled strip or a hot rolled strip. Cold rolled strip form. It can also be hot rolled in the form of rods or sticks or blocks or forgings. These products can then be converted by hot forging or cold to a bar or a stretched block or die drawn line. The steel according to the invention may also be treated by casting, with or without heat treatment after casting.

In order to obtain the best possible performance, it is preferred to use the method according to the invention, which first comprises providing a steel ingot, slab or block having the composition according to the invention.

The ingot, slab or block is usually obtained by smelting the raw material in an electric furnace and then performing vacuum melting of AOD or VOD type having decarburization. This grade can then be cast continuously in a bottomless mold, and this grade can be cast into ingot form, slab form or block form. It is also possible to cast this grade directly into a sheet form, in particular by continuous casting between relatively rotating drums.

After the ingot, slab or block is provided, this can be selectively reheated to reach a temperature between 1150 and 1280 ° C, but it can also be directly cast in a continuous casting mode while it is still hot. Processing on thick sheets.

In the case of a panel, the slab or ingot is then hot rolled to obtain a four-open slab having a thickness typically between 5 and 100 mm. The reduction in the ratio used for this stage is usually between 3 and 30%. Thereafter, the plate is heat treated by reheating to a temperature between 900 and 1100 ° C so that the precipitate formed at this stage is returned to the solution and then cooled.

Air quenching is provided in accordance with the method of the present invention, which is easier to perform than conventional water cooling methods for grades of this type, which cool more rapidly. However, it can still be water cooled when needed.

This slow air cooling benefits in particular from the limited nickel and molybdenum content of the composition according to the invention, which does not occur in the case of precipitation of the intermetallic phase which would detract from its use properties. This cooling can be carried out in particular at a rate in the range from 0.1 to 2.7 ° C / sec.

After hot rolling, if it is intended to be transported in this state, the four-open size plate can be planarized, cut and pickled.

It is also possible to roll the strip in a strip rolling machine to a thickness between 3 and 10 mm.

In the case of the manufacture of long products from ingots or blocks, it can be hot pressed between the grooved rolls at a temperature between 1150 and 1280 ° C in a single hot rolling mill in a single hot rolling mill. To get the rod or coil of the strand, or to roll. The ratio of the cross-sectional area between the primary block and the final product is preferably greater than 3 to ensure the internal integrity of the rolled product.

When the rod is made, it is cooled by a simple air spray after leaving the rolling mill.

When a rolled bar having a diameter of more than 13 mm is produced, it can be cooled by quenching it in a water tank in the form of a coil when leaving the rolling mill.

When a rod having a diameter of 13 mm or less is produced, it can be spread on the conveyor in the form of a crucible, after the conveyor is passed through a dissolution treatment furnace (temperature between 850 ° C and 1100 ° C) for 2 to 5 minutes, Water quenched and cooled.

If it is desired to complete the recrystallization of the structure and to slightly reduce the tensile properties, a subsequent furnace heat treatment between 900 ° C and 1100 ° C can be selectively applied to the rod or coil (which has been subjected to hot rolling treatment).

After the rod or rod coil is cooled, various thermoforming or cold forming treatments can be performed depending on the end use of the product. After being cooled, the strip can be subjected to a cold drawing operation or a rod can be subjected to a mold stretching operation.

The hot rolled rod may also be cold formed, or the part may be produced after the rod is cut into small pieces of metal and then forged.

To illustrate the invention, a heated object is made and these are described with particular reference to Figures 1 to 5.

Instance

The pure iron alloy raw material is melted by vacuum induction, and then nitrogen is introduced by nitriding under a nitrogen partial pressure to add nitrogen, and a metal molded product is cast at an external nitrogen pressure of 0.8 bar to prepare a 25 kg laboratory ingot. . Of these heats, only the heat-receiving materials 14441 and 14604 are in accordance with the present invention.

According to the industrial heating of the present invention, 150 tons of reference material 8768 were obtained. This grade is smelted by melting in an electric furnace, followed by vacuum refining to decarburize to achieve the desired carbon content. It was then continuously cast into slabs (having a cross-sectional area of 220 x 1700 mm), and then, after heating to 1200 ° C, hot rolled into four-thickness plates having thicknesses of 7, 12 and 20 mm. Thereafter, the thus obtained sheet is heat-treated at about 1000 ° C so that many precipitates present at this stage are returned to the solution. After the heat treatment, the plate was cooled with water, followed by planarization, cutting and pickling.

The composition of the weight percentages of various grades refined on a laboratory scale or an industrial scale is shown in Table 1; the composition of various industrial products or semi-finished products refined in an electric furnace and then refined by AOD and cast into ingots or continuously cast is also shown in Table 1. For comparison.

Fertilizer iron content

1.1 Fertilizer iron content of the product just processed from these laboratory heats (in the state of just casting) or from the industrial product (in the state of just casting) cut samples of 1 to 8 cubic centimeters, these samples are in the salt In the bath, heat treatment was carried out for 30 minutes at various temperatures, followed by water quenching after treatment to determine the ratio of ferrite to iron at high temperatures. Since the ferrite is magnetic (this is different from the Worth iron, possibly carbides and nitrides), an analytical method for determining the saturation magnetization is used. The ferrite iron content thus measured is shown in Table 2 and plotted in Figure 1.

Figure 1 shows a good correlation between the index I _F and the ferrite iron content (measured as the base metal after treatment at 1100 ° C).

It has also been shown that the ferrite iron content of the heat-receiving material 14441 according to the present invention at less than 1300 ° C is suitable for thermal conversion into a dual structure. Further, after heat treatment in the range of 950 ° C to 1100 ° C, the ferrite iron content is suitable for stress corrosion resistance.

1.2 The iron content of the final product is also determined by the grid method (according to ASTM E 562 standard) for the forged rod (which is heat treated at 1030 ° C) and the welded beads (which are stabilized by the coated electrode), derived from 700 The ferrite iron content of the heat affected zone deposited by °C at a cooling rate of 20 ° C / sec. The results (fertilizer iron content of the base metal and the heat affected zone) are shown in Table 3. This shows that the ferrite iron content in the base metal and in the heat affected zone in accordance with the present invention contributes to local stress corrosion resistance and also contributes to toughness (compared with reference to Table 5).

2. Castability

The ingot 14439 has air bubbles and cannot be used. In order to prevent this phenomenon during the casting at atmospheric pressure in the air, it was confirmed that the nitrogen content of the heat-receiving material according to the present invention must be limited to less than 0.28% by weight.

3. Thermal conversion capability

The thermal transition capability was evaluated using a thermal tensile test conducted on the sample. The corrected portion of the sample (8 mm in diameter and 5 mm in length) was heated by Joule heating at 1280 ° C for 80 seconds and then at 2 ° C / The rate of seconds is cooled to the test temperature (which varies between 900 and 1280 °C). Upon reaching this temperature, a rapid tensile test was performed at a rate of 73 mm/sec; after the rupture, the neck diameter of the rupture was measured.

The relative diameter variation (Table 4) defined below reflects the thermal deformability: Δψ = 100 x (1-(final diameter / initial diameter)).

In Table 4 and Figure 2, which represent the data in the form of a curve, it can be seen that the heat-reducible material 14441 according to the present invention has a thermal deformability similar to that of the comparative reference heat-receiving material 14382.

4. Mechanical properties

The tensile properties R _e0.2 and R _m are determined according to the NF EN 10002-1 standard. Toughness K _v is determined at a number of temperatures according to the NF EN 10045 standard.

The results of laboratory heat products 14441 and 14604 and industrial heat products 8768 (all of which are in accordance with the present invention) indicate that the yield strength can be greater than 450 MPa, i.e., twice the yield of the Worthfield iron profile steel of the AISI 304L type.

The laboratory heat samples 14441 and 14604 and the industrial heat products 8768 (all of which are according to the invention) have a toughness value greater than 200 joules at 20 ° C. It is satisfactory after considering the yield strength of these grades. . As for the heat-receiving product 14383 not according to the present invention, it has a low nitrogen content and a high fat iron content, and in the annealed state, the toughness value at 20 ° C is less than 100 joules. This confirms that a sufficient amount of nitrogen must be added to obtain a satisfactory toughness.

5. Corrosion resistance

Corrosion resistance tests were carried out on forged bars derived from laboratory heat and on sample embryos removed from hot rolled sheets derived from industrial heat.

5.1 Local Corrosion Resistance The pit corrosion resistance was evaluated by plotting the current-potential curve and measuring the cavity potential of i = 100 microamperes per square centimeter. In a neutral medium (pH = 6.4) with a high chloride concentration ([Cl ^- ] = 30 g / liter) at 50 ° C (E ₁ ) (this simulates the case of seawater encountered in seawater desalination equipment) This parameter was determined in a slightly acidic medium (pH = 5.5) at a low chloride concentration ([Cl ^- ] = 250 ppm) at room temperature (E ₂ ) (to simulate drinking water). The critical cavity temperature in the ferric chloride medium (6% FeCl ₃ ) is determined according to Method C of the ASTM G48-00 standard.

In another series of tests, the pit corrosion resistance was measured at 35 ° C in a degassed neutral medium containing 0.86 mol/liter NaCl (corresponding to 5% by weight NaCl). A 900 second floating potential measurement was performed. Thereafter, the electromotive force curve is plotted at a floating potential at a rate of 100 millivolts per minute until the cavity potential is reached. The cavity potential (E ₃ ) was measured at i = 100 μA / cm ^ 2 . Under these conditions, the sample according to the present invention and the reference sample 304L grade and the type 1.4362 of the Worthite iron-fertilizer iron grade and others were tested.

The crack corrosion resistance was investigated by measuring the critical crack temperature in a neutral medium (pH = 6.4) having a high chloride concentration ([Cl ^- ] = 30 g / liter). Arrangements that contribute to floating crack corrosion are based on the recommendations listed in the ASTM G78-99 standard. The critical crack temperature is the lowest temperature at which the crack depth is observed to be greater than 25 microns.

The values obtained are listed in Table 6. Comparing the results of the panels made of UNS S32304 and the rods obtained from the heat-receiving material 14382 (the chemical compositions of the two are similar), it is pointed out that the corrosion resistance of the rods is lower than that of the hot-rolled sheets having the same composition.

The inventors have found that the local corrosion index (i.e., resistance to formation of corrosion pits or cracks) is abbreviated as I _LCR and is defined as: I _LCR = Cr + 3.3 x Mo + 16 x N + 2.6 x Ni - 0.7 x Mn (Cr, Mo, N, Ni, and Mn) Content is expressed in % by weight)

Note that the nickel content of all components is considered to be less than 6% when considering local corrosion resistance (please refer to Figures 3, 4 and 5).

The heat-receiving materials 14383 and 14660 (having an index of I _LCR equal to 28.7 and 29.8) according to the present invention have less corrosion behavior than the steel of the AISI 304L type. The heat-receiving materials 14604 and 14441 (having an I _LCR index of 30.9 and 33) according to the present invention behave at least as similar as the 304L steel. It has been found that to obtain corrosion resistance at least equal to the AISI 304L grade, preferably, the steel according to the present invention must have an I _LCR greater than 30.5 and preferably greater than 32.

5.2 Uniform Corrosion Resistance The characteristics of uniform corrosion were determined by evaluating the corrosion rate after soaking in a 2% dilute sulfuric acid solution heated to 40 ° C for 72 hours.

Comparing the experimental heat containing 2.5% Ni and 0.2% N (14441 according to the invention and 14660 not according to the invention) clearly shows the negative effect of the high Mn content on the uniform corrosion resistance in the sulfuric acid medium.

^* : According to the invention ¹ , the oxidation potential of the solvent, no pit condition E ₁ was observed: in a neutral medium (pH = 6.4) having a high chloride concentration ([Cl ^- ] = 30 g / liter), Cavity potential E ₂ at 50 ° C: in a slightly acidic environment (pH = 5.5) with a low chloride concentration ([Cl ^- ] = 250 ppm), a cavity potential E ₃ at 25 ° C: in a neutral chloride medium (5% NaCl), the cavity potential at 35 ° C T _CP : critical pit temperature T _CC in ferric chloride medium: in the presence of high chloride concentration ([Cl ^- ] = 30 g / liter) Critical crack temperature in a medium (pH = 6.4) V: uniform corrosion rate at 40 ° C in 2% sulfuric acid medium

5.3 Repassivation potential The steel sample was polished to 1200 in water using SiC paper and then aged in air for 24 hours.

The floating potential was measured for 15 minutes, and then the self-floating potential was dynamically polarized at a current of 300 mA/cm 2 at a current of 100 mA/min, and then returned to a potential of 0, thereby performing in a chloride medium. Cyclic polarization test.

Thereby, the pit potential (P- _cavity ) and the re-passivation potential of the previously formed pit (V _{re-passivation} ) were measured. The results obtained are shown in Table 7.

From the repassivation potential test in NaCl medium, the higher the nickel content, the greater the difference between the pit potential and the repassivation potential. This shows that nickel is detrimental to the repassivation of grades according to the invention which have previously been corroded.

Figure 1: Relationship between % ferrite iron after treatment at 1100 °C and product I _F processed in this way; Figure 2: plot of relative diameter change Δψ versus deformation temperature; Figure 3: pit potential E1 and E2 (measured on a forged rod) versus index I _LCR ; Figure 4: graph of uniform corrosion rate V (measured on a forged rod) versus index I _LCR ; and Figure 5: critical temperature T Diagram of _CC and T _CP (measured on forged rods) versus index I _LCR .

Claims (26)

A double stainless steel whose composition, in weight %, consists of the following: C 0.05%, 21% Cr 25%, 1% Ni<2.5%, 0.16% N 0.28%, Mn 2.0%, Mo+W/2 0.50%, Mo 0.45%, W 0.15%, Si 1.4%, Al 0.05%, 0.11% Cu 0.50%, S 0.010%, P 0.040%, Co 0.5%, rare earth metal (REM) 0.1%, V 0.5%, Ti 0.1%, Nb 0.3%, Mg 0.1%, the remainder is iron and impurities derived from smelting, and its microstructure consists of Vostian iron and 35 to 65 vol% ferrite, which further meets the following relationship: 40 I _F 70 wherein I _F = 6x (% Cr + 1.32 x % Mo + 1.27 x % Si) - 10 x (% Ni + 24 x % C + 16.15 x % N + 0.5 x % Cu + 0.4 x % Mn) - 6.17 and I _LCR 30.5 wherein I _LCR =%Cr+3.3x%Mo+16x%N+2.6x%Ni-0.7x%Mn; and the steel is characterized by a yield strength greater than 450 MPa and a toughness value greater than 200 joules at 20 °C . According to the steel of the first application of the patent scope, it is further characterized by: I _LCR 32. The steel according to item 1 or 2 of the patent application is further characterized in that the ratio of the ferrite iron is between 35 and 55 volume%. According to the steel of claim 1 or 2, it is further characterized by: 40 I _F 60. A steel according to item 1 or 2 of the patent application is further characterized in that the chromium content is between 22 and 24% by weight. The steel according to item 1 or 2 of the patent application is further characterized in that the manganese content is less than 1.5% by weight. A steel according to item 1 or 2 of the patent application is further characterized in that the calcium content is less than 0.03% by weight. The steel according to item 1 or 2 of the patent application is further characterized in that the molybdenum content is more than 0.1% by weight. A method of producing a hot rolled sheet, strip or coil from the steel according to any one of claims 1 to 8 wherein: the ingot of the steel according to any one of claims 1 to 8 is provided or The slab; and the ingot or slab are hot rolled at a temperature between 1150 and 1280 ° C to obtain a plate, strip or coil. The method of claim 9, wherein the ingot or slab is hot rolled at a temperature between 1150 and 1280 ° C to obtain a so-called quarto plate; between 900 and 1100 The temperature between ° C is heat treated; and the plate is cooled by air quenching. A hot-rolled steel four-size slab made according to the method of claim 10, and having a thickness of between 5 and 100 mm. A use of a hot-rolled coil produced according to the method of claim 11 or the method of claim 9, which is used for the manufacture of structural components for the manufacture of materials or energy-manufacturing devices. The use according to claim 12, wherein the temperature operation of the material and the energy producing device is between -100 and 300 ° C, preferably between -50 and 300 ° C. A cold rolled steel strip obtained by cold rolling a hot rolled coil obtained by the method of claim 9 of the patent application. a steel according to any one of items 1 to 8 of the patent application scope A method of producing a hot rolled rod or rod, wherein: a continuous ingot or block of steel according to any one of claims 1 to 8; the ingot or block is between 1150 and 1280 ° C Hot rolling to obtain a rod (which is air cooled) or a coil of wire (which is cooled by water); and thereafter, optionally: heat treated at a temperature between 900 and 1100 ° C; The coil of the rod or strand is quenched and cooled. A hot rolled rod or hot rolled rod obtained by the method of claim 15 and having a hot rolled rod diameter of 18 mm to 250 mm and a hot rolled rod having a diameter of 4 mm to 30 mm. The method of claim 15, wherein after cooling, a cold drawing operation is performed on the rod or a mold stretching operation is performed on the rod. A cold drawn rod or mold stretching rod or wire obtained by the method according to claim 17 and having a cold drawing rod diameter of 4 mm to 60 mm, a mold stretching rod or a wire diameter It is from 0.010 mm to 20 mm. A use according to the bar of claim 16 or 18 for the manufacture of mechanical parts such as pumps, valve shafts, motors and engine shafts, and adapters for operation in corrosive media. A use of a rod or wire according to claim 16 or 18 for the manufacture of cold formed kits, for the agricultural food industry, for oil and ore extraction, or for the manufacture of chemicals, minerals Or woven or knitted metal fabric for food filtration. A method of manufacturing a steel block, wherein a cold forming operation is performed on a hot rolled bar produced according to the method of claim 15 of the patent application. A steel block produced by the method of claim 21 of the patent application. A method of manufacturing a steel forging by cutting a hot rolled bar obtained according to the method of claim 15 into a metal block, and then performing a forging operation between 1100 ° C and 1280 ° C on the metal block. A steel forging made by the method of claim 23 of the scope of the patent application. A use of a forging according to item 24 of the patent application for the manufacture of a bracket or adapter. A casting obtained by casting a steel according to any one of claims 1 to 8.