EP2257652B1 - Method of manufacturing sheets of austenitic stainless steel with high mechanical properties - Google Patents

Description

The present invention relates to the manufacture of hot-rolled sheets of austenitic stainless steels having high mechanical characteristics, and in particular a combination of very advantageous mechanical strength and distributed elongation.

For the manufacture of structural elements in the automotive industry, various grades of coated carbon steel sheets with more or less complex microstructures are commonly used. The parts are made from sheets of thickness ranging from 1 to 3 mm. For some parts, however, we would like to simultaneously have a higher corrosion resistance combined with a high deformation capacity so as to produce parts with a complex stamping. It is also known that austenitic stainless steels are commonly used because of their excellent resistance to corrosion and their mechanical characteristics, in particular their high ductility. For example, known austenitic stainless steels designated according to the standards EN 10088-1 by the reference 1.4318, the composition of which contains (content expressed by weight): C≤0.030%, Si≤1.00%, Mn≤2.00% , P ≤ 0.045%, S ≤ 0.015%, Cr: 16.50 to 18.50%, Ni: 6.00 to 8.00%, N: 0.10 to 0.20%. These steels have high mechanical properties due to the formation of martensite during deformation at room temperature. The typical mechanical properties of these steels in the annealed state are the following: yield strength Rp _0.2 (yield strength corresponding to 0.2% elongation): 300-400 MPa, distributed elongation: A ≥45%, Rm (maximum resistance) ≥ 700 MPa. Product P = Rp _0.2 (MPa) x extended elongation = about 15750 MPa.%. It is possible to use these grades in cold rolled condition: C850, C1000 - Standard EN-10088-2, these designations respectively corresponding to a minimum strength of 850 and 1000MPa. The increase of elastic limit conferred by this operation (Rp _0.2 ≥ 600MPa) results in a simultaneous decrease in elongation (A = 30%). The product P then reaches about 18,000 MPa. These characteristics are satisfactory for some applications. They nevertheless remain insufficient in the case where high resistance in service is desired, for example for a gain in lightening, and a great aptitude for prior shaping operations.

An alternative method to cold rolling hardening is hot rolling work at a sufficiently low temperature. This method confers a better compromise elongation - resistance, but has the major disadvantage of leading to localizations of the deformation during shaping, resulting in vermiculures. To avoid these vermiculures on standard steel 1.4318 not recrystallized after hot rolling, it is necessary to anneal after hot rolling.

The documents US 4,975,131 and US 5,000,801 describe stainless steels belonging to the same type of steel grades as the steels according to the invention.

The object of the invention is therefore to have hot-rolled sheets of austenitic stainless steel with mechanical characteristics greater than or equivalent to those of the grades of the type 1.4318 presented above, which are inexpensive to manufacture, and which have no sensitivity to appearance of vermiculures.

Another object of the invention is to provide hot-rolled sheets of austenitic stainless steel having a product P greater than 21000 MPa.%, Which can be associated with a yield strength Rp _0.2 greater than 650 MPa, or else with a distributed elongation greater than 45%.

• Nb/8+0,1% ≤ N ≤ Nb/8+0,12%, à titre optionnel : 0.0005%≤ B ≤ 0,0025%, Mo ≤0,6%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration.

For this purpose, the subject of the invention is a hot-rolled sheet made of austenitic stainless steel whose product P (Rp _0.2 (MPa) x extended elongation (%)) is greater than 21000 MPa.% And whose chemical composition includes, the contents being expressed by weight: 0.015% ≤ C ≤ 0.030% 0.5% ≤ Mn ≤ 2% If ≤ 2%, 16.5% ≤ Cr ≤ 18%, 6% ≤ Ni ≤ 7%, S ≤ 0.015%, P ≤ 0.045%, Al ≤ 0.050%. 0.15% ≤ Nb ≤ 0.31%, 0.12% ≤ N ≤ 0.16%, the Nb and N contents being such that:

• Nb / 8 + 0.1% ≤ N ≤ Nb / 8 + 0.12%, optional: 0.0005% ≤ B ≤ 0.0025%, Mo ≤ 0.6%, the rest of the composition being iron and impurities inevitable resulting from the elaboration.

In a preferred embodiment, the niobium and nitrogen contents of the steel, expressed by weight, are such that: 0.20% ≤ Nb ≤ 0.31%, 0.12% ≤ N ≤ 0.16%. The subject of the invention is also a hot-rolled sheet made of austenitic stainless steel according to any one of the above compositions, whose yield strength Rp _0.2 is greater than 650 MPa, characterized in that the average size The austenitic grain of the steel is less than 6 microns, the non-recrystallized surface fraction is between 30 and 70%, and the niobium is completely in the form of precipitates. The invention also relates to a hot-rolled sheet of austenitic stainless steel according to any one of the above characteristics, the distributed elongation of which is greater than 45%, characterized in that the niobium is not totally precipitated. .

The invention also relates to a method for manufacturing a hot-rolled sheet of austenitic stainless steel whose yield strength Rp _0.2 is greater than 650 MPa, according to which a semi-finished steel product is supplied. according to any one of the above compositions, then the semi-finished product is heated to a temperature of between 1250 ° C and 1320 ° C, then the semi-finished product is rolled out with a rolling end temperature below 990 ° C and a cumulative reduction rate ε on the last two finishing cages, greater than 30%.

According to one particular embodiment, a semi-finished steel product of the above composition containing 0.20% ≤ Nb ≤ 0.31%, 0.12% ≤ N ≤ 0.16% is supplied, then the half produced with an end-of-lamination temperature below 970 ° C.

The invention also relates to a method for manufacturing a hot-rolled sheet of austenitic stainless steel, the distributed elongation of which is greater than 45%, according to which a semi-finished product of steel is supplied according to one of any of the above compositions, then the semi-finished product is heated to a temperature between 1250 ° C and 1320 ° C, and then the semi-finished product is rolled out with a rolling end temperature above 1000 ° C.

The invention also relates to a method of manufacturing a rolled sheet. wherein the product P (Rp _0.2 (MPa) x extended elongation (%)) is greater than 21000 MPa.%, according to which a semi-finished product of steel is supplied according to one of the following: any of the above compositions, then the semi-finished product is heated to a temperature between 1250 ° C and 1320 ° C, and then the semi-finished product is hot rolled.

Another object of the invention is the use of a stainless steel hot rolled sheet according to any of the above features, or manufactured by any of the above methods, for the manufacture of elements in the automotive field.

Other features and advantages of the invention will become apparent from the description below, given by way of example.

After numerous tests, the inventors have shown that the various requirements reported above were satisfied by observing the following conditions:

As regards the chemical composition of the steel, the carbon content must be less than or equal to 0.030% in order to avoid the risk of sensitization to intergranular corrosion. In order to obtain a yield strength greater than 650 MPa, the carbon content must be greater than or equal to 0.015%.

Manganese, like silicon, is an element known for its deoxidative properties in the liquid state and to increase the hot ductility, especially by combining with sulfur. On the other hand, at room temperature, it promotes the stability of the austenitic phase and reduces the stacking fault energy. It also increases the solubility of nitrogen. These favorable effects are obtained economically when the manganese content is between 0.5 and 2%.

Like manganese, silicon is an element usually added for the purpose of deoxidizing liquid steel. Silicon also increases the yield strength and the resistance, by hardening in solid solution or by its action on the δ ferrite content. However, beyond 2%, the weldability and the hot ductility are reduced.

Chromium is a well-known element for increasing resistance to oxidation and corrosion in aqueous media. This effect is satisfactorily obtained when its content is between 16.5% and 18%.

Nickel is an indispensable element to ensure sufficient stability of the austenitic structure of steel at room temperature. The optimum content should be determined in relation to other elements of the alphagene composition such as chromium, or those with a gamma-like character such as carbon and nitrogen. Its effect on the stability of the structure is sufficient when its content is greater than or equal to 6%. Above 7%, the cost of production increases excessively because of the high cost of this element of addition.

Molybdenum increases the resistance to pitting. Optionally, molybdenum addition up to 0.6% can be carried out.

Boron improves the forgeability of steel. As an optional addition boron in an amount between 0.0005 and 0.0025% can be carried out. Addition in greater quantity would critically decrease the burn temperature.

Sulfur is an element that particularly degrades hot forgeability and corrosion resistance, its content must be maintained less than or equal to 0.015%.

Phosphorus likewise degrades hot ductility, its content must be less than 0.045% to obtain satisfactory results.

Aluminum is a powerful deoxidation agent for the liquid metal. In combination with the silicon and manganese contents mentioned above, an optimal effect is obtained when its content is less than or equal to 0.050%.

Niobium and nitrogen are important elements of the invention for the manufacture of austenitic stainless steels with high mechanical properties.

Niobium retards recrystallization during hot rolling: for a given hot rolling end temperature, its addition leads to maintain a higher rate of work hardening (it is called hot rolling "hardening"), thus increasing the strength of the steel. It is generally used as Ti to combat the formation of chromium carbides (austenitic stainless steels stabilized with EN 1.4580 and EN 1.4550). Finally, it can lead to intermetallic phase formation conferring an improvement in creep resistance.

Nitrogen is a hardening element in interstitial solid solution, which particularly increases the yield strength in this respect. It is also known, in solid solution, as a powerful stabilizer of the austenitic phase and as a retarder of the precipitation of Cr ₂₃ C ₆ chromium carbides. The solubility of the nitrogen during the solidification knows a maximum. Too high a content leads to the formation of volume defects in the metal.

The joint addition of niobium and nitrogen for curing is unusual in austenitic stainless steels. In the context of the invention, it has been demonstrated that stainless steels, the composition of which is close to that of the 1.4318 steels mentioned above, advantageously benefit from a particular joint addition of niobium and nitrogen, optimized for to obtain certain mechanical properties under specific conditions that will be exposed:

In the first place, it has been demonstrated that a nitrogen content ranging from 0.12 to 016%, together with a niobium content ranging from 0.15 to 0.31%, the contents of niobium and nitrogen being such that: Nb / 8 + 0.1% ≤ N ≤ Nb / 8 + 0.12% (relation 1), make it possible to manufacture a hot sheet with high mechanical characteristics intended to be stamped, and this without the need of a annealed after rolling as in conventional steel 1.4318, the stamped part not being prone to the formation of vermiculures.

The precipitation of NbN nitrides which occurs at the end of hot rolling reduces the amount of nitrogen in solid solution. The preceding relation (1) ensures that as much solid solution nitrogen remains after complete precipitation of all available niobium as in 1.4318 (N≥0.1%). This makes it possible to obtain the same metastability of the austenite at room temperature. The possibility of decreasing the Ni content by increasing the N content is limited by the solubility limit of nitrogen in the steel during solidification. For the contents of Cr, Mn and Ni steels according to the invention, the nitrogen content must be less than or equal to 0.16%.

Sufficient niobium must be present to achieve a hardening effect and delay recrystallization. This amount must be adapted to obtain a NbN solvus higher than the end of rolling temperature to obtain precipitation at the end of hot rolling.

The contents of niobium and nitrogen according to the invention make it possible to obtain a high precipitation of NbN after hot rolling.

A joint addition of 0.15 to 0.31% of niobium (preferably 0.20 to 0.31% of niobium) and 0.12 to 0.16% of nitrogen, the contents of niobium and of nitrogen being such that: Nb / 8 + 0.1% ≤ N ≤ Nb / 8 + 0.12%, makes it possible to obtain an advantageous combination (elasticity-elongation limit) whose product P is greater than 21000 MPa.

In addition to iron, the rest of the composition consists of unavoidable impurities resulting from the preparation, such as for example Sn or Pb.

The implementation of the manufacturing method according to the invention is as follows: A steel is produced whose composition has been explained above. This development can be followed by casting in ingots or, in the most general case, continuously, for example in the form of slabs ranging from 150 to 250 mm thick. It is also possible to perform the casting in the form of thin slabs of a few tens of millimeters thick between contra-rotating steel rolls. These cast semifinished products are first brought to a temperature of between 1250 and 1320 ° C. The purpose of the 1250 ° C temperature is to dissolve any niobium-based precipitates (nitrides, carbonitrides). However, the temperature must be less than 1320 ° C or it may be too close to the solidus temperature that could be reached in any segregated areas and cause a local start to pass through a liquid state that would be harmful for hot shaping. In the case of a direct casting of thin slabs between counter-rotating rolls, the step of hot rolling of these semi-products starting at a temperature below 1250 ° C. can be done directly after casting so that a step intermediate heating is not necessary in this case.

Selon l'invention, on a mis en évidence que lorsque la température de fin de laminage est inférieure à 990°C et que le taux de réduction cumulé ε est supérieur à 30%, la limite d'élasticité Rp_0,2 du produit final obtenu était supérieure à 650 MPa, le Nb se trouvant alors totalement sous forme de précipités.The rolling is generally carried out on a continuous hot stream comprising in particular roughing cages and finishing cages. It has been demonstrated that a particularly high yield strength Rp _0.2 is obtained by controlling in particular the reduction ratio in the two last finishing cages: if _n-2 is designated as the thickness of the sheet at the entrance of the penultimate finishing cage, and by the thickness of the sheet at the exit of the last finishing cage, the cumulative reduction rate is defined on the two last finishing stands by: $\mathrm{\epsilon }=\frac{{\mathrm{e}}_{\mathrm{NOT}-\mathrm{2}}-{\mathrm{e}}_{\mathrm{NOT}}}{{\mathrm{e}}_{\mathrm{NOT}-\mathrm{2}}}\mathrm{.}$

According to the invention, it has been demonstrated that when the end-of-rolling temperature is less than 990 ° C. and the cumulative reduction ratio ε is greater than 30%, the yield strength Rp _0.2 of the final product obtained was greater than 650 MPa, the Nb then being totally in the form of precipitates.

For a content of Nb between 0.20 and 0.31% and a nitrogen content of between 0.12 and 0.16%, this minimum value of 650 MPa is obtained when the end-of-rolling temperature is less than 970 ° C. C and ε greater than 30%.

According to the invention, it has also been demonstrated that a hot-rolled sheet with a distributed elongation greater than 45% is obtained when the end-of-rolling temperature is greater than 1000 ° C. The Nb is in this case partially precipitated.

After hot rolling, a sheet is obtained which has no sensitivity to the appearance of vermiculides and does not require intermediate annealing.

By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Example:

Casting semi-finished products of steels, the composition of which is presented in the table below (percentage by weight): Table 1: Composition of steels (percentage by weight) Underlined values: not in accordance with the invention Steel VS mn Yes Cr Or MB S P al Nb NOT I1 (according to the invention) 0,023 1,100 0.48 17.45 6.67 0.25 0.005 0,020 0,002 0,152 0.13 I2 (according to the invention) 0,024 1.19 0.55 17.36 6.66 0.25 0.005 0,020 0,002 0.302 0.15 R (reference) 0,026 1,030 0.6 17.5 6.6 0.25 0.0008 0,026 0,002 0002 0.13

The semi-finished steel products were reheated at 1280 ° C for 30 minutes. A hot rolling was then carried out by varying the end of rolling temperature between 900 and 1100 ° C and the cumulative reduction rate ε, to reach a final thickness of 3 mm. The sheets 11-1, I1-2, 11-3 ... designate sheets from the same half-product I1, laminated under different conditions. The microstructure of the steel obtained was characterized by measuring in particular the recrystallized austenitic phase surface fraction, the fraction of niobium precipitated relative to the total niobium, and the average grain size. In the case of a structure not completely recrystallized, the latter measurement is performed on the recrystallized part of the structure. The mechanical tensile characteristics, in particular the yield strength Rp _0.2 and the distributed elongation, were also determined. The presence of a localization of the deformation during the tensile test has also been noted. It is known that the presence of such a location is associated with the appearance of vermiculides during shaping operations.

The results are shown in Table 2 below: Table 2: Manufacturing conditions and microstructural and mechanical characteristics of hot-rolled sheet No. of test TFL (° C) ε> 30% Average grain size less than 6 microns Non-recrystallized fraction between 30 and 70% Niobium totally precipitated RP _0.2 (MPa) AT (%) Rp _0.2 x A (MPa%) Location of the deformation I1-1 905 Yes Yes Yes Yes 689 40 27628 No I1-2 935 Yes Yes Yes Yes 651 40 25520 No I1-3 1040 Yes No No (<30%) No 432 49 21340 No I1-4 1050 Yes No No (<30%) No 467 46 21715 No I2-1 930 Yes Yes Yes Yes 677 38 25997 No I2-2 965 Yes Yes Yes Yes 681 39 26559 No I2-3 980 No No Yes Yes 631 41 26186 No I2-4 1000 No Yes No (<30%) No 627 46 28277 No I2-5 1100 Yes No No (<30%) No 547 53 29100 No R-1 900 Yes - Yes - 702 29 20428 Yes R-2 925 Yes - Yes - 638 29 18566 Yes R-3 950 Yes - Yes - 632 30 19150 Yes R-4 1020 Yes - No (<30%) - 482 31 14749 No TFL: End of rolling temperature
Rp _0.2:: Conventional yield strength at 0.2% deformation
A: Distributed extension
ε: cumulative reduction rate of the last two rolling passes

It is thus apparent that the steels I1 and 12 according to the invention have a combination Rp _0.2 x A (MPa.%) Greater than 21000 MPa.% Particularly advantageous while the reference steel R does not have such a combination. , whatever the rolling conditions.

It is also shown that when the non-recrystallized fraction is between 30 and 70% and when the average grain size is less than 6 microns, the yield strength Rp _0.2 is greater than 650 MPa (tests I1-1 , I1-2, I2-1, I2-2 Furthermore, when the non-recrystallized fraction is greater than 70%, the elongation tends to decrease.

These characteristics are obtained for steels having a niobium content of between 0.15 and 0.31%, a nitrogen content of between 0.12 and 0.16%, the contents of niobium and of nitrogen being such that: / 8 + 0.1% ≤ N ≤ Nb / 8 + 0.12%, the end-of-rolling temperature being less than 990 ° C and the cumulative reduction ratio ε being greater than 30%. For steels having between 0,20% and 0,31%, a nitrogen content between 0,12 and 0,16%, the contents of niobium and nitrogen being such that: Nb / 8 + 0,1% ≤ N ≤ Nb / 8 + 0.12%, these characteristics are obtained when the end-of-rolling temperature is less than 970 ° C and when the cumulative reduction rate ε is greater than 30% (tests 12-1, I2-2)

When niobium is not completely precipitated (tests 11-3, 11-4, I2-4, I2-5), the distributed elongation is greater than 45%. For the steel compositions according to the invention, this result is obtained when the end-of-rolling temperature is greater than 1000 ° C. In comparison, reference steel can not offer such characteristics.

We will therefore choose more particularly certain manufacturing conditions (end of rolling temperature, cumulative reduction rate) depending on whether we are looking for a steel sheet with a particularly high yield strength, or rather offering a high elongation capacity. .

Furthermore, the tensile curves of the steels according to the invention do not show any bearing testifying to a location of the deformation and whatever the hot rolling conditions, unlike the reference steel which has a localization as soon as possible. when partially recrystallized (R-1, R-2, R-3). This point is particularly advantageous for shaping, ensuring the absence of vermiculures.

Thus, because of their particularly high mechanical characteristics, and in particular their very advantageous combination of distributed elasticity-elongation ratio, hot-rolled steel sheets according to the invention will be advantageously used for applications requiring good shaping and a high resistance to corrosion. In the case of their use in the automobile industry, their advantages will be exploited for the economic manufacture of structural elements.

Claims (9)

1. Hot-rolled sheet of austenitic stainless steel, the product P (Rp_0.2 (Mpa) x uniform elongation (%)) of which is greater than 21000 Mpa.% and the chemical composition of which comprises - the contents being expressed in terms of weight:

   0.015% ≤ C ≤ 0.030%

   0.5% ≤ Mn ≤ 2%

   Si ≤ 2%

   16.5% ≤ Cr ≤ 18%

   6% ≤ Ni ≤ 7%

   S ≤ 0.015%

   P ≤ 0.045%

   Al ≤ 0.050%

   0.15% ≤ Nb ≤ 0.31%

   0.12% ≤ N ≤ 0.16%
   the contents of Nb and N being such that:

   Nb/8+0.1% ≤ N ≤ Nb/8+0.12%,
   optionally:

   0.0005% ≤ B ≤ 0.0025%

   Mo ≤ 0.6%,

   the rest of the composition being composed of iron and inevitable impurities resulting from manufacture.
2. Hot-rolled sheet of austenitic stainless steel according to claim 1, characterised in that the contents of niobium and nitrogen of said steel, expressed in terms of weight, are such that:

   0.20% ≤ Nb ≤ 0.31%

   0.12% ≤ N ≤ 0.16%.
3. Hot-rolled sheet of austenitic stainless steel according to claim 1 or 2, the proof strength Rp_0.2 of which is greater than 650 Mpa, characterised in that the average austenite grain size of said steel is less than 6 microns; in that the non-recrystallised surface fraction is from 30 to 70%; and in that the niobium is entirely in the form of precipitates.
4. Hot-rolled sheet of austenitic stainless steel according to claim 1 or 2, the uniform elongation of which is greater than 45%, characterised in that the niobium is not entirely precipitated.
5. Method of producing a hot-rolled sheet of austenitic stainless steel, the proof strength Rp_0.2 of which is greater than 650 Mpa, according to which:

   - a half-finished steel product having a composition according to claim 1 or 2 is provided, then

   - said half-finished product is heated to a temperature of from 1250°C to 1320°C, and then

   - said half-finished product is rolled with an end-of-rolling temperature of less than 990°C and a cumulative reduction rate ε over the final two finishing stands of more than 30%.
6. Production method according to claim 5, characterised in that a half-finished steel product having a composition according to claim 2 is provided; and in that said half-finished product is rolled with an end-of-rolling temperature of less than 970°C.
7. Method of producing a hot-rolled sheet of austenitic stainless steel, the uniform elongation of which is greater than 45%, according to which:

   - a half-finished steel product having a composition according to claim 1 or 2 is provided, then

   - said half-finished product is heated to a temperature of from 1250°C to 1320°C, and then

   - said half-finished product is rolled with an end-of-rolling temperature of more than 1000°C.
8. Method of producing a hot-rolled sheet of austenitic stainless steel, the product P (Rp_0.2 (MPa) x uniform elongation (%)) of which is greater than 21000 Mpa.%, according to which:

   - a half-finished steel product having a composition according to claim 1 or 2 is provided, then

   - said half-finished product is heated to a temperature of from 1250°C to 1320°C, and then

   - said half-finished product is hot-rolled.
9. Use of a hot-rolled sheet of stainless steel according to any one of claims 1 to 4 or produced by a method according to any one of claims 5 to 8 in the production of structural elements in the automotive sector.