FR2864108A1 - Stainless steel with high mechanical strength and good elongation with an austenitic microstructure and limited martensite pockets for the fabrication of motor vehicle structural components - Google Patents

Abstract

A stainless steel sheet with a high mechanical strength and good elongation has a microstructure that is essentially austenitic incorporating some 0.1 to 0.2 % by volume of martensitic pockets distributed in a homogeneous manner. An independent claim is also included for the fabrication of this stainless steel sheet.

Description

Stainless steel sheet with high mechanical strength and a high

  good elongation, and method of manufacture.

  The present invention relates to a stainless steel sheet having both high mechanical strength and good elongation, as well as its manufacturing process. The stainless steel sheets according to the present invention are more particularly intended for the manufacture of automobile structural parts.

  New legislation put in place to reduce carbon dioxide emissions compels manufacturers to reduce the weight of motor vehicles to reduce their fuel consumption. To this obligation to reduce the weight of the vehicles, is added the obligation to increase the active and passive safety of the vehicles. For this purpose, car manufacturers require materials with very high mechanical characteristics while keeping a good formability. By way of example, these materials must have both a yield strength Re greater than 650 MPa, a resistance elasticity ratio Re / Rm of between 0.7 and 0.9, and a elongation at break A greater than at 30%.

  These requirements are difficult to obtain with metals conventionally used in the automotive field such as carbon steels, and especially dual-phase ferrito- martensitic steels or ferritic steels with a high elastic limit, the mechanical characteristics of which are considered insufficient for the manufacture of certain parts, such as the longitudinal members. For example, the TRIP 800 steel grade conventionally used to manufacture the spars has Rm values of the order of 800 MPa, Re of the order of 350 MPa and an elongation of the order of 18%.

  In addition, in order to protect them against corrosion, the carbon steels must be coated with a coating generally consisting of a metal layer, for example based on zinc or zinc alloy, which further increases the weight of the coatings. parts made from these steels.

  The object of the present invention is therefore to overcome the disadvantages of the materials of the prior art, by providing a material 20 having both a yield strength Re greater than 650 MPa, a yield stress ratio on load at break (strength elasticity) Re / Rm between 0.7 and 0.9 and elongation at break A greater than 30%, and which furthermore does not require subsequent coating before use.

  For this purpose, the inventors have demonstrated that a stainless steel sheet having a composition close to that of metastable austenitic steels and comprising a very low proportion of martensite made it possible to meet these requirements.

  The invention relates to a stainless steel sheet having a high mechanical strength and a good elongation, whose composition comprises in% by weight: 0.025 <C <0.05% 0.3Si <_1 1 Mn5.2% Cr 18.5% 5Ni10% Mo3% 0.1 N0.16% Cu5.0.5% P <0.05% S 0.015%, optionally 0.1 V 0.5% and 0.1 <Nb 0 , 5%, with 0.1 Nb + Ti + V 0.5%, the balance being iron and any impurities resulting from the preparation, and whose microstructure is an essentially austenitic microstructure comprising from 0.1 to 0 2% by volume of martensitic islands distributed homogeneously.

  The stainless steel sheet may also have the following characteristic: the average size of the austenite grains of this stainless steel sheet is between 0.1 and 3 μm.

  The invention also relates to a method of manufacturing a stainless steel sheet as defined above, comprising the steps of: hot rolling a slab of stainless steel comprising, in weight: 0.0255C <- 0,05% 0,3Si <_1 0/0 1 Mn2% Cr_ 18,5% 55Ni10% Mo3% 0,1 N0,16% Cu0,5% P 0,5% S0, 015%, optionally 0,1 V 0.5%, and 0.1 Nb 0.5%, with 0.1 _ <Nb + Ti + V 0.5% the complement being iron and any impurities resulting from the elaboration, to obtain a hot-rolled sheet, which is pickled, possibly cold-rolled said hot-rolled sheet at a reduction ratio between 40 and 80%, and heat-treat the cold-rolled sheet. The process according to the invention may also have the following characteristics: the heat treatment is a partial recrystallization annealing comprising a heating phase at a heating rate, a holding phase at a temperature and during a holding time, followed by a cooling phase at a cooling rate, the temperature is between 730 and 860 C, the heating rate is greater than or equal to 20 C / s, the heating rate is greater than 150 C / s, the speed The heating time is greater than 300 C / s, the temperature holding time is less than 70 s, the cooling rate is greater than or equal to 20 C / s, and the cooling rate is greater than 700 C / s.

  To obtain a stainless steel sheet according to the invention, it is first necessary to develop and then cast in the form of a slab a stainless steel which comprises the following elements: carbon with a content between 0.025 and 0.05% in weight.

  Carbon promotes the formation of austenite, and controls the amount and hardness of deformation martensite. In addition, its solid solution solution hardens the steel and increases its resistance. If the carbon content is less than 0.025%, the steel becomes unstable and a lot of martensite is formed, resulting in insufficient elongation at break. On the other hand, if the carbon content is greater than 0.05%, the steel becomes stable, the formation of deformation martensite is insufficient and the steel no longer has enough energy to recrystallize. As a result, the minimum annealing temperature for initiating recrystallization is high and the size of the austenite grains becomes too large to achieve high mechanical characteristics.

  silicon with a content of between 0.3 and 1% by weight. Silicon is used as the deoxidizer of liquid steel, and it participates in hardening in solid solution. Its content is limited to 1% by weight, since it has a tendency to disturb the manufacturing process of the steel sheet by posing problems of segregation during the slab casting of the steel.

    manganese at a content of between 1 and 2% by weight. Manganese promotes the formation of austenite. If the manganese content is greater than 2%, the austenite being too stable, the formation of deformation martensite is insufficient and this does not allow to reach the required levels of elastic limit.

  However, if the manganese content is less than 1%, the deoxidation of the steel is insufficient.

    chromium at a content of between 15 and 18.5%. Chromium promotes the formation of deformation martensite, and is an essential element in giving steel good corrosion resistance. If the chromium content is greater than 18.5%, too much deformation martensite is generated, which makes it necessary to increase the content of the elements favoring the formation of austenite such as carbon, nitrogen, nickel and manganese. If the chromium content is less than 15%, the corrosion resistance of the steel is insufficient.

    nickel at a content of between 5 and 10%. Nickel stabilizes the austenite and promotes re-passivation. If the nickel content is less than 5%, the corrosion resistance of the steel is insufficient. If the nickel content is greater than 10%, the austenite over-stabilizes, there is not enough deformation martensite, and the mechanical characteristics of the steel are insufficient.

  molybdenum at a content of not more than 3%. Molybdenum promotes the formation of deformation martensite and increases corrosion resistance, especially when combined with nitrogen. Beyond a content of 3%, the corrosion resistance of the steel is not improved.

    nitrogen at a content of between 0.1 and 0.16%. Nitrogen promotes the formation of austenite, delays the precipitation of carbides, stabilizes austenite, and improves formability. In addition, it plays a role in the adjustment of grain size in the structure. However, if added at a level greater than 0.16%, it may deteriorate the hot ductility of the steel.

    copper at a level not exceeding 0.5%. Copper promotes the formation of austenite and contributes to resistance against corrosion. However, beyond a content of 0.5%, the proportion of copper that is not in solid solution in the austenite increases and the hot formability of the steel is degraded.

  phosphorus at a level not exceeding 0,5%. Phosphorus is a segregating element. It promotes the solid solution hardening of steel, however its content must be limited to 0.5 because it increases the fragility of the steel and its weldability.

  sulfur at a level of less than or equal to 0.015%. Sulfur is also a segregating element whose content must be limited in order to avoid cracking during hot rolling.

  In addition, the composition may optionally comprise: vanadium at a content of between 0.1 and 0.5%. Vanadium promotes the weldability of steel, and inhibits the growth of austenite grains in the heat affected zone. Above 0.5%, vanadium does not contribute to the improvement of the weldability, and below 0.1%, the weldability of the steel is insufficient.

  niobium at a content of between 0.1 and 0.5%. Niobium promotes the weldability of steel, however, beyond 0.5%, it degrades the hot formability of the steel strip.

    with a total content of niobium, titanium and vanadium of between 0.1 and 0.5% to guarantee the weldability of the steel with no adverse effect on the hot ductility.

  The remainder of the composition is iron and other elements usually expected to be found as impurities resulting from the processing of stainless steel, in proportions that do not affect the properties sought.

  After being cast, the slab is hot rolled in a strip train to form a hot rolled sheet which is annealed and optionally etched.

  The hot-rolled sheet is then cold-rolled at room temperature at a reduction ratio between 40 and 80%.

  During the cold rolling operation at a reduction rate of between 40 and 80%, 60 to 90% of deformation martensite is formed in the form of laths. If the cold rolling is carried out at a reduction rate of less than 40%, the deformation martensite and dislocation rates are insufficient to give the stainless steel according to the invention the required characteristics. However, if the cold rolling is carried out at a reduction rate greater than 80%, excessive stresses are imposed on the rolls of the mill, and it is no longer possible to roll the steel sheet.

  Finally, the cold-rolled sheet undergoes a heat treatment so as to give the stainless steel an austenitic structure comprising from 0.1 to 0.2% by volume of martensitic islands, distributed homogeneously.

  The inventors have demonstrated that if the martensite is not in the form of islets distributed homogeneously in the austenitic phase, but that it is in the form of a strip, the elongation at break and the formability of the stainless steel degrade very significantly.

  In addition, beyond 0.2% by volume of martensite, the elongation at break also deteriorates very significantly, and below 0.1% by volume of martensite, the yield strength becomes insufficient .

  Preferably, to obtain the best characteristics, the average size of the austenitic grains of the stainless steel according to the invention must be between 0.1 and 3 μm.

  The heat treatment according to the invention consists in subjecting the cold rolled steel sheet to a partial recrystallization annealing comprising a heating phase at a heating rate V1, a holding phase at a temperature T and for a time of maintaining M, followed by a cooling phase at a cooling rate V2, under conditions adapted to give the stainless steel the microstructure according to the invention.

  Thus, the cold-rolled sheet is heated to a holding temperature T of between 730 and 860 ° C under suitable conditions so that the recrystallization of the stainless steel is partial.

  When the sheet is heated to a holding temperature T less than 730 C, band martensite is formed and the elongation at break of the stainless steel decreases sharply. At a temperature T greater than 860 ° C., the austenite grains increase in favor of martensite, which disappears completely, and the yield strength Re is insufficient.

  Preferably, the cold-rolled sheet is heated to the holding temperature T, at a heating rate V1 greater than or equal to 20 Cls, more preferably at a speed V1 greater than 150 Cls, and advantageously at a speed V1 greater than 300 C / s.

  When the heating rate V1 of the sheet is less than 20 C / s, the stainless steel can only recrystallize during a very long maintenance time M which is not compatible with the industrial requirements. On the other hand, austenite grains grow in favor of martensite, and the yield strength Re is insufficient to give stainless steel good mechanical properties.

  From 20 C / s, the size of the austenite grains decrease substantially and the yield strength Re is sufficient to give the stainless steel good mechanical properties.

  From 150 C / s, stainless steel sheets with good mechanical properties are obtained with a maintenance time M compatible with the industrial requirements.

  From 300 C / s, the stainless steel sheets obtained have good mechanical properties, even when the holding temperature T is high. In addition, the mechanical characteristics are further improved because the risks of alignment of martensite are avoided and the microstructure composed of martensitic islands whose grain size is less than 3 pm becomes more homogeneous.

  Preferably, the cold-rolled sheet is maintained at the holding temperature T for a period of less than 70 s. Indeed, beyond 70 s, the austenitic grains grow in favor of martensite, and the mechanical characteristics of stainless steel become insufficient.

  The partially recrystallized steel sheet is then cooled to a cooling rate V2 greater than or equal to 20 Cls, for example by air blowing, and preferably greater than 700 C / s, for example by air blowing. under pressure and water spray.

  Indeed, from a cooling rate of 20 C / s, and especially from 700 C / s, the elastic limit and the tensile strength increase. The invention will now be illustrated by examples. implementation, given in a non-limiting manner. i0

  A steel grade, the composition of which is given in Table 1 below, is cast as slabs which are hot-rolled to a thickness of 0.6 mm. Then, 19 samples marked A, B, C ... up to S of the hot-rolled sheet are cold rolled with an is reduction rate (noted LAF rate in Table 2) of between 40 and 80%. These cold-rolled samples are then subjected to a heat treatment, whose conditions are summarized in Table 2, comprising a heating phase at a heating rate VI, a holding phase at a temperature T and during a holding time M followed by a cooling phase at a cooling rate V2.

  The mechanical characteristics, the martensite rate (denoted "mart rate" in Table 2) expressed in percentage of volume and the size of the austenitic grains (denoted "tga" in Table 2) expressed in μm, samples are then measured. and also collected in Table 2.

  In this table, the presence in the sample of band martensite in the steel is indicated by (MB).

  Table 1: Chemical Composition of the Stainless Steel According to the Invention, Expressed in% by Weight C Si Mn Cr Ni Mo N Cu PSV Nb Ti 0.025 0.5 1.5 17.5 6.8 0.1 0.13 0.1 0.02 0.002 - - - The rest of the composition consists of iron and unavoidable impurities resulting from the elaboration. i0

  Table 2: conditions of the heat treatment and mechanical characteristics of the samples tests rate VI T M V2 Re Rm Re A rate t.g.a.

  LAF (C / s) (C) (s) (C / s) (MPa) (MPa) Rm (%) mart. (pm) (%) (%) A comp. 80 20 680 60 20 1198 1286 0.93 4.5 10 -

MB

  B comp. 80 20 700 60 20 1031 1105 0.93 14.5 2 -

MB

  C inv. 80 20 730 60 20 960 1068 0.90 34 0.2 1.6 D inv. 80 20 750 60 20 722 878 0.82 35 0.2 1.8 E inv. 80 20 780 60 20 668 885 0, 75 40.5 0.1 2.2 F inv. 80 20 800 60 20 651 881 0.74 41.5 0.1 3.0 G comp. 80 20 830 60 20 535 852 0.62 41 0 3.3 H comp. 80 20 850 60 20 507 849 0, 60 40 0 3.6 comp. 80 20 850 15 569 866 0.65 37 3.5 J Comp. 80 20 850 8 20 595 898 0.66 42.5 0 3.0 K comp. 80 20 850 4 20 626 911 0, 69 43 0 2.7 L inv. 80 20 850 2 20 686 925 0.74 39.5 0.1 2.4 M comp. 80 700 850 15 20 491 864 0.57 39 0 2.8 N comp. 80 700 850 8 20 538 878 0, 61 42 0 2.5 O inv. 80 700 850 2 20 704 994 0.71 38.5 0.1 2.4 P inv. 80 700 850 0 20 730 1001 0.73 38.5 0.1 2.2 Q inv. 40,500 770 0 20,833 1006 0.82 30.5 0.2 2.5 R inv. 55,500 790 0 20,650 892 0.72 34 0.15 2 S inv. 65 500 760 0 20 685 899 0, 76 35 0.2 2 According to the results grouped together in Table 2, it can be seen that when the stainless steel has an austenitic microstructure comprising no martensite at all, or conversely when it has an austenitic microstructure comprising band martensite, the yield strength, the formability and elongation of the steel do not meet the specifications required by the car manufacturers. i0

Claims (11)

  1. Stainless steel sheet having high mechanical strength and good elongation, the composition of which comprises in% by weight: 0.0255C_50.05% 0.35 Si <_1% Mn 52% 155Cr518.5% io 55Ni510% Mo <0.1% N 50.0% Cu 50.5% P <0.5% S50.015%, optionally 0.1 <V <0.5% and 0.1 <Nb <0, 5%, with 0.1 <_ Nb + Ti + V <0.5% the balance being iron and any impurities resulting from the preparation, and whose microstructure is an essentially austenitic microstructure comprising from 0.1 to 0.2% by volume of martensitic islands distributed homogeneously.   2. A stainless steel sheet according to claim 1, characterized in that the average size of the austenite grains is between 0.1 and 3 pm.   3. A method of manufacturing a stainless steel sheet according to one of claims 1 or 2, comprising the steps of: hot rolling a slab of stainless steel comprising, by weight: 0.025 5 C 5 0.05% 0.35 Si_51% 1 5Mn <_2% 155Cr518,5% 55__Ni <_10% Mo <_3% 0,1 <_N <_0,16% Cu <_0,5% P <_0,5% S <_0,015% optionally 0.1% by 0.5%, and 0.1% by 0.5%, with 0.1% <Nb + Ti + V <0.5%, the balance being iron and any impurities resulting from the preparation, to obtain a hot-rolled sheet, which is pickled, possibly cold rolled said hot-rolled sheet, at a reduction rate between 40 and 80%, and perform a heat treatment of the cold-rolled sheet.   4. Method according to claim 3, characterized in that the heat treatment is a partial recrystallization annealing comprising a heating phase at a heating rate VI, a holding phase at a temperature T and during a holding time M, followed by a cooling phase at a cooling rate V2.   5. Method according to claim 4, characterized in that the temperature T is between 730 and 860 C.   6. Method according to claim 4, characterized in that the heating rate V1 is greater than or equal to 20 C / s.   7. Method according to claim 6, characterized in that the heating rate V1 is greater than 150 Cls.   8. The method of claim 7, characterized in that the heating rate VI is greater than 300 Cls.   9. The method of claim 4, characterized in that the holding time M of the temperature T is less than 70 s.   10. Process according to claim 4, characterized in that the cooling rate V2 is greater than or equal to 20 C / s.   11. The method of claim 10, characterized in that the cooling rate V2 is greater than 700 C / s. io