ES2870648T3 - An object comprising a duplex stainless steel and the use thereof - Google Patents

Abstract

An object made from duplex stainless steel, wherein the duplex stainless steel comprises the following composition, in % by weight: C less than or equal to 0.040; If less than or equal to 0.60; Mn 0.80 - 10.0; Cr 21.0-28.0; Neither 4.0-9.0; Mo 0.9 - 4.5; N 0.10-0.45; Cu less than or equal to 0.50; V less than or equal to 0.10; P less than or equal to 0.010; S less than or equal to 0.006; remainder Fe and unavoidable impurities; optionally small amounts of other alloying elements that are As, Ca, Co, Ti, Nb, W. Sn, Ta, Mg, B, Pb and Ce, where the amounts of one or more of these elements are at most 0, 5% by weight; and wherein the duplex stainless steel consists of 55-70 vol% austenite phase and 30-45 vol% ferrite phase; and wherein the object has alternating layers of ferrite phase and austenite phase, said alternating layers are essentially parallel to the plane of the object and said alternating layers have an average layer thickness between about 0.01 and about 4.5 µm, and where "approximately" means ± 5%, and where "essentially parallel" means that the deviation from the plane is less than 10%.

Description

DESCRIPTION

An object comprising a duplex stainless steel and the use thereof

Technical field

The present description refers to an object comprising a duplex stainless steel, in particular the object is suitable for use in spring applications or as a spring as such. The present description also refers to a method for producing the object.

Background

Wire or strip spring applications can be statically or dynamically loaded. The most important properties of steel grades for static spring applications are high strength or elastic limit, well defined elastic modulus, high corrosion resistance and high resistance to stress relaxation. The most important properties of steel grades for dynamic spring applications are high strength or elastic limit, well-defined elastic modulus, high corrosion resistance, high resistance to stress relaxation, and high resistance to fatigue failure.

Document JP 2010059541 describes a process where the final stage is an annealing process that aims to obtain the maximum elongation of a stainless steel that is a low-alloy duplex steel. Austenite phase of such quality is unstable and will partially transform into martensite after plastic deformation.

Spring stainless steel grades of austenitic or martensitic origin typically possess excellent combinations of most of the above properties. However, a major drawback of austenitic steel grades is that the elastic modulus tends to decrease almost linearly with increasing load up to the test stress (Rp0.2) and, as noted above, steel grades Intended for spring applications, they must have an elastic modulus that is maintained at a high level also with increasing load and that does not decrease linearly. Martensitic steel grades can have an elastic modulus that does not decrease linearly with increasing load. However, a major drawback of martensitic steel grades is that these steels have problems with their corrosion resistance.

Therefore, one aspect of the present disclosure is to provide an object which is suitable for spring applications and which will solve or at least reduce the aforementioned drawbacks.

Compendium

Therefore, one aspect of the present disclosure is to provide an object made from duplex stainless steel according to the appended claims.

As used herein, the term "about" means plus or minus 5% of the numerical value of the number with which it is being used. Furthermore, in the present description, the acronym "FCC" means austenite phase and the acronym "BCC" means ferrite phase. Furthermore, the term "essentially parallel" as used herein is intended to mean that the plane deviation is less than 10%.

Furthermore, the object comprising the duplex stainless steel as defined above or hereinafter defined, will not have content of sigma phase and / or precipitated chromium nitride. Furthermore, said object will have an elastic modulus that will remain relatively high with increasing load compared to the performance of pure austenitic stainless steel.

Another aspect of the present description provides a method of manufacturing an object as defined above or hereinafter defined in accordance with the appended claims.

The final stage of the method should be a cold working process because this process will influence the microstructure of the duplex stainless steel more and therefore have a great impact on the elastic modulus. In addition, the present method will provide the object with greater strength after cold working and cold working will also ensure that a strain hardening occurs in the object.

Detailed description

The present description refers to an object manufactured from duplex stainless steel, wherein the duplex stainless steel comprises the following composition, in% by weight:

C less than or equal to 0.040;

If less than or equal to 0.60;

Mn 0.80-10.0;

Cr 21.0-28.0;

Ni 4.0-9.0;

Mo 0.9 - 4.5;

N 0.10-0.45;

Cu less than or equal to 0.50;

V less than or equal to 0.10;

P less than or equal to 0.010;

S less than or equal to 0.006;

remaining Fe and unavoidable impurities;

optionally small amounts of other alloying elements such as, Ca, Co, Ti, Nb, W, Sn, Ta, Mg, B, Pb and Ce. The amounts of one or more of these elements are a maximum of 0.5 % in weigh.

and wherein the duplex stainless steel consists of 55-70% by volume of austenite phase and 30-45% by volume of ferrite phase;

and wherein the object has alternating layers of ferrite phase and austenite phase, said alternating layers are essentially parallel to the plane of the object and said alternating layers have an average thickness between about 0.01 and about 4.5 µm.

Duplex stainless steel as defined above or hereinafter defined will provide the object with high corrosion resistance. Furthermore, the alternating layers of ferrite phase and austenite phase will provide the object with a well defined elastic modulus that will remain relatively high with increasing load. A well defined elastic modulus means that the elastic modulus will remain at a high level with increasing load on the material and will not decrease almost linearly with increasing load up to the test stress (Rp0.2). Therefore, the object will be suitable for spring applications.

According to one embodiment, duplex stainless steel has a PRE greater than 28. PRE is defined herein as PRE = Cr + 3.3 * Mo + 16 * N (factors to be multiplied by the respective weight percentage of the alloying element respective). Duplex stainless steel as defined above or hereinafter defined will provide the object with high resistance to corrosion, especially pitting corrosion due to its high PRE value both in the ferrite phase and in that of austenite, that is, the PRE value for both ferrite and austenite is greater than about 28. Therefore, the respective amounts of Cr, Mo, and N are chosen so that PRE is greater than about 28 in phase austenite and ferrite, respectively.

According to one embodiment, duplex stainless steel as defined above or hereinafter defined consists of 55-70 volume% austenite phase and 30-45 volume% ferrite phase, such as 65-70% by volume of austenite phase and 30-35% by volume of ferrite phase. This means that strain-induced martensite will not be present in the duplex stainless steel of the present disclosure and therefore in the object composed of the duplex stainless steel. This is possible because duplex stainless steel as defined above or hereinafter defined is highly alloyed and therefore the object will have the ability to undergo cold deformation generated by cold working without transformation. from its austenitic structure into a martensitic structure.

According to one embodiment, the object as defined above or hereinafter defined is in the form of a plate or a strip or a wire. The plate or the strip or the wire can be used to make a spring, whereby the present description also refers to a spring.

According to the invention, the at least one hot working process is hot rolling. The hot work process is carried out at a temperature of 1050 to 1300 ° C. Furthermore, according to one embodiment, the at least one hot working process is performed once or more than once, for example, in one embodiment, the hot rolling may be performed on the body several times such as, for example , 6 times or until the desired reduction in hot work of the workpiece is obtained. Hot work will also form austenite phase and ferrite phase layers, but the thickness of these layers is greater than in the final object. According to yet another embodiment, the workpiece can be heated between the hot work stages.

According to the invention, the at least one cold working process is cold rolling

According to the invention, the cold working process is carried out once or more than once. In one embodiment, the cold work process can be performed on the workpiece several times, for example 4 times or until the desired cold deformation of the final object is obtained. According to one embodiment, the cold deformation of the final object, that is, the deformation of the object, is at least 10%, such as at least 25%, such as at least 50%, such as at least 75%. , such as 75 to 95%. According to the invention, the thickness of the final object obtained in its cold rolled state is from 20 gm to 5 mm.

According to one embodiment, the method comprises a hot work process, a cold work process, a hot work process and a cold work process. According to another embodiment, the method comprises a hot work process, a cold work process, a hot work process, a cold work process and a cold work process.

According to one embodiment, the method as defined above or hereinafter defined comprises a heat treatment step, wherein the heat treatment consists in annealing the object obtained after a cold working step. Annealing can be performed to reduce any intermetallic phase formed, such as sigma phase and chromium nitrides, or to reduce the strength of the object or to change the content of austenite or ferrite phase in the object. The annealing temperature will depend on both the composition and the thickness of the object. Normally, the annealing temperature is above 1000 ° C. According to another embodiment, the object is subjected to an annealing stage at least between the penultimate and the last cold working stage. Furthermore, according to another embodiment, several annealing stages (such as more than one) may be applied between the respective cold working stages (such as more than one cold working stage). According to one embodiment, the object is annealed in a temperature range of 1050 ° C to 1250 ° C for a period of about 1 to 600 seconds. During the heating of the object in this temperature range, it is important to avoid exposing said product to a temperature of 750 ° C to 1000 ° C for too long, since this is the temperature range where the sigma phase and / or nitrides Chromium forms more quickly. Therefore, the temperature ramp can be such that the time to pass said interval is less than about 2 minutes. Also, the last stage of the process is a cold work stage.

According to one embodiment, the present method also comprises a stage of aging the object obtained after the cold working stage or after an annealing stage. This stage will provide a further increase in the test stress of the object and also a further improvement of the elastic modulus performance. Before being subjected to aging, the object can undergo a forming operation in which it is formed into a spring. Aging can be carried out for 0.25 to 4 hours at a temperature of 400 to 450 ° C. As the aging stage is carried out at low temperatures, it can be carried out after the final stage of the cold working process.

Hereinafter, the alloying elements of duplex stainless steel are discussed as defined above or defined hereinafter. The amounts are given in% by weight (% w):

Carbon, C is a representative element to stabilize the austenitic phase and is an important element to maintain mechanical strength. However, if there is a high carbon content, carbides will precipitate, reducing corrosion resistance.

Therefore, the carbon content is limited to less than 0.040% by weight.

Manganese, Mn, has a strain hardening effect and counteracts the transformation from austenitic to martensitic structure after deformation. To have these effects, Mn must be present in at least 0.80% by weight or equal percentage. Furthermore, Mn has an austenite stabilizing effect up to a content of about 10.0% by weight. Above that level, the stabilization of the ferrite will increase and therefore it will be difficult to add more stabilizing ferrite elements, such as Cr and Mo, without obtaining too much ferrite. Therefore, the maximum Mn content should not be more than 10.0% by weight. According to one embodiment, the Mn content is equal to or less than 6.0% by weight. According to yet another embodiment, it is equal to or less than 5.0% by weight. According to one embodiment, the Mn content is in the range of 2 to 5% by weight. When Mn is present in amounts such as those suggested above, it will increase the strain hardening capacity of duplex stainless steel and also prevent the austenite phase from becoming so unstable, that is, it will prevent the transformation of the austenitic structure into a martensitic structure after deformation.

Nitrogen, N, has a positive effect on the corrosion resistance of duplex stainless steel as defined above or hereinafter defined and also has a strong effect on the PRE equivalent pitting corrosion resistance as well. which PRE is defined as Cr 3.3Mo 16. In addition, N contributes greatly to solid solution strengthening and strain hardening of duplex stainless steel. N also has a strong austenite stabilizing effect and counteracts the transformation of austenitic structure to martensitic structure after plastic deformation. To contribute to all these positive effects, N is added in an amount of 0.10% by weight or more. However, at too high levels, N tends to form chromium nitrides, which should be avoided due to negative effects on ductility and corrosion resistance. Therefore, the N content must therefore be equal to or less than 0.45% by weight. According to one embodiment, the N content is 0.30 to 0.42% by weight.

Molybdenum, Mo, has a strong influence on the corrosion resistance of duplex stainless steel as defined above or hereinafter defined and strongly influences PRE and contributes strongly to both solid solution strengthening and to strain hardening. Therefore, Mo is added in an amount equal to or greater than 0.9% by weight.

However, Mo also increases the temperature at which the unwanted sigma phase is stable and promotes its generation rate and therefore the Mo content must be equal to or less than 4.5% by weight. According to one embodiment, the Mo content is 2 to 4% by weight.

Chromium, Cr, has a strong impact on the corrosion resistance of duplex stainless steel as defined above or hereinafter defined, especially pitting corrosion. Furthermore, Cr improves the elastic limit and counteracts the transformation of the austenitic structure into a martensitic structure after the deformation of duplex stainless steel. Cr also has a ferrite stabilizing effect on duplex stainless steel. Therefore, the Cr content must be equal to or greater than 21.0% by weight. At high levels, increasing Cr content will result in a higher temperature for the stable sigma phase and unwanted chromium nitrides, and faster generation of the sigma phase. Therefore, the Cr content is equal to or less than 28.0% by weight. According to one embodiment, the Cr content is 24 to 28% by weight, such as 26 to 28% by weight.

Copper, Cu, has a positive effect on corrosion resistance. However, it is optional to add Cu to the duplex stainless steel as defined above or defined hereinafter. Cu is often present in discarded products used for steel production and is allowed to remain in the steel at moderate levels. The Cu content is equal to or less than 0.50% by weight. According to one embodiment, the Cu content is equal to or less than 0.02% by weight.

Nickel, Ni, has a positive effect on general corrosion resistance. Ni also has a strong austenite stabilizing effect and counteracts the transformation of the austenitic to martensitic structure upon deformation of duplex stainless steel. Therefore, the Ni content is equal to or greater than 4.0% by weight. At levels above 9.0% by weight, Ni will result in austenite levels above 70%. Therefore, the Ni content should not be greater than or equal to 9.0% by weight. According to one embodiment, the Ni content is 7.0 to 9.0% by weight.

Silicon, El Si, is almost always present in duplex stainless steels, as it may have been used for deoxidization or is found in scrap used for duplex stainless steels, although the goal is to have as little as possible. It has a stabilizing effect of ferrite and, at least in part for that reason, the Si content should be less than or equal to 0.60% by weight, such as between 0.40 and 0.60% by weight.

Vanadium, V, can be present as an impurity element in duplex stainless steel and since it tends to follow scrap and therefore the content is difficult to control. Duplex stainless steel should preferably contain amounts as low as possible due to carbide precipitation and for current duplex stainless steel, the content of should be equal to or less than 0.10% by weight, such as equal to or less than 0 .01% by weight.

Phosphorous (P) may be an impurity and is contained in duplex stainless steel as defined above or defined hereinafter in an amount less than or equal to 0.010% by weight.

Sulfur (S) may be an impurity contained in duplex stainless steel as defined above or defined hereinafter. S can impair hot workability at low temperatures. Therefore, the allowable content of S is less than or equal to 0.006% by weight.

Optionally, small amounts of other alloying elements may be added to the duplex stainless steel as defined above or defined hereinafter to improve, e.g. ex. machinability or hot work properties, such as hot ductility. Said elements are As, Ca, Co, Ti, Nb, W, Sn, Ta, Mg, B, Pb and Ce. The amounts of one or more of these elements are a maximum of 0.5% by weight, such as a maximum of 0.1% by weight.

According to one embodiment, the present object comprises a duplex stainless steel consisting of all the elements mentioned above or below in the present specification.

When the terms "maximum" or "less than or equal to" are used, it is known to the person skilled in the art that the lower limit of the range is 0% by weight, unless another number is specifically indicated.

The remaining elements of duplex stainless steel as defined above or hereinafter defined are iron (Fe) and normally occurring impurities.

Examples of impurities are elements and compounds that have not been added on purpose, but cannot be completely avoided, as they normally occur as impurities, e.g. For example, in the raw material or additional alloying elements used to make the duplex stainless steel.

According to one embodiment, the duplex stainless steel consists of the aforementioned alloying elements and ranges.

The step of providing a duplex stainless steel body as defined above or hereinafter defined may include providing a melt of said duplex stainless steel and melting said melt to obtain a duplex stainless steel body. as defined above or defined hereinafter. Casting may include continuous casting of the melt.

As a result of the steps of the method being used, as mentioned above, they will be seen in the object alternating layers of ferrite and austenite, said layers being essentially parallel to the plane of the object. The thickness of the layers will affect the test stress of the product. In order to obtain sufficient test stress, the mean FCC thickness and BCC thickness of each layer should be between 0.01 and about 4.5 pm, such as about 0.5 to about 4.5 pm, such as about 1.0 to about 4.5 pm, such as about 1.0 to about 4.2 pm, such as 2.0 to 4.2 pm. The thickness of the product in its final cold worked state (after the last cold working stage) can be from 20 pm to 5 mm. Before being cold worked, the body comprising the duplex stainless steel as defined above or hereinafter defined is subjected to a hot work, in which, according to one embodiment, the thickness of the body it is reduced from approximately 100 to 200 mm to 2-15 mm.

Measurement of the thickness of the BCC and FCC phases, respectively, is performed by taking a perpendicular cross section of the object (the strip, sheet or wire) and then polishing and etching in acid (such as HNO3) to obtain a contrast between the two phases. . The measurement is then made in an optical light microscope using a suitable magnification (100-1000 times) so that each phase is visible and so that a sufficiently large number of phase limits can be counted to obtain a reasonable statistical certainty (plus 30 phase limits). A suitable transverse position for measurement on a wire is 25% of its diameter. A strip or sheet should be measured 25% of the width from the edge, in the center of the thickness. The thickness of each BCC and FCC phase is measured, in the direction of the diameter of a wire or along the direction of the thickness of a strip or sheet, and from this the average thickness of BCC and FCC, respectively, is calculated.

The present description is further illustrated by the following non-limiting examples.

Examples

The alloys having the chemical composition shown in Table 1 were cast and cast into 1 kg ingots. After melting and casting, the obtained ingots were hot rolled into strips at a temperature of about 1250 ° C using 9 rolling passes. The samples were reheated 3 times during hot rolling to keep the temperature above 1050 ° C. The final thickness of the strips ranged from 3.7 to 4.0 mm.

The hot rolled strips were then cold rolled until a cold reduction of about 75% was obtained. 5 passes were used in the cold laminator.

Ferrite content was determined using magnetic scale measurements. The magnetic scale measurement was carried out according to IEC 60404-1. The magnetic phase content was assumed to be equal to the ferrite content and the remainder was assumed to be austenite. The values are found in Table 2.

To measure the thickness measurement of the BCC and FCC phases, a sample was taken in a perpendicular cross section of the strip at 25% of the width from the edge and then the sample was polished and etched (1M HNO3). The measurement was carried out in an optical light microscope (Nikon) using a suitable magnification (1000 times), that is, each phase was visible and more than 30 phase boundaries were observed. The thickness of each BCC and FCC phase was measured along the thickness direction and the mean thickness of BCC and FCC was calculated, respectively. The values obtained are shown in table 2.

The strength of the cold rolled strip was determined by tensile tests according to SS EN ISO 6891-1 in the rolling direction. Two tensile test specimens were waterjet cut from each cold rolled strip specimen. The results are collected in Table 3. As can be seen from the table, all the samples had good tensile strength. Examples 10 and 12 are reference examples that do not form part of the invention.

Table 1: Chemical composition of the samples: all values are given in% by weight (% w).

Table 2 Phase thickness of samples and phase content of samples

Table 3. Tensile test results

As can be seen, the obtained objects will have a stress test Rp0.2 above 1200 MPa after cold rolling.

The resistance to pitting corrosion of the experimental grades was evaluated using the PRE formula (defined as PRE = Cr 3,3Mo 16N) as described above. By introducing the total compositions (from table 1) into Thermo-Calc®, it was possible to deduce the equilibrium compositions in each of the phases to the specific content of BCC (from table 2) and, therefore, it could be calculated the PRE of each phase according to the table. Four.

Table 4 Composition and resistance to pitting corrosion of each of the two phases

Therefore, as can be seen from the above experiments, the objects of the present description will have a high yield strength in combination with good ductility and also good corrosion resistance and high tensile strength due to the strengthening of the solution. solid and strain hardening, thickness and phase content.

As can be seen from the above results, when the average phase thickness is less than 4.5 pm, the extremely fine microstructure (shown in table 5), which is obtained in the present cold worked duplex stainless steel , will have an impact on the mechanical properties.

Claims (15)

1. An object manufactured from duplex stainless steel, wherein the duplex stainless steel comprises the following composition, in% by weight: C less than or equal to 0.040; If less than or equal to 0.60; Mn 0.80-10.0; Cr 21.0-28.0; Ni 4.0-9.0; Mo 0.9 - 4.5; N 0.10-0.45; Cu less than or equal to 0.50; V less than or equal to 0.10; P less than or equal to 0.010; S less than or equal to 0.006; remaining Fe and unavoidable impurities; optionally small amounts of other alloying elements that are As, Ca, Co, Ti, Nb, W. Sn, Ta, Mg, B, Pb and Ce, where the amounts of one or more of these elements are at most 0, 5% by weight; and wherein the duplex stainless steel consists of 55-70% by volume of austenite phase and 30-45% by volume of ferrite phase; and wherein the object has alternating layers of ferrite phase and austenite phase, said alternating layers are essentially parallel to the plane of the object and said alternating layers have an average layer thickness between about 0.01 and about 4.5 gm, and where "approximately" means ± 5%, and where "essentially parallel" means that the deviation from the plane is less than 10%. The object according to claim 1, wherein the amounts of one or more of these other alloying elements are 0.1% by weight at most. The object according to claim 1 or claim 2, wherein the duplex stainless steel consists of 65-70% by volume of austenite phase and 30-35% by volume of ferrite phase. The object according to any one of claims 1 to 3, wherein said alternating layers have an average layer thickness between about 0.5 and about 4.5 gm, such as from about 1.0 to about 4.5 gm. , such as about 1.0 to about 4.2 gm. 5. The object according to any one of claims 1 to 4, wherein the duplex stainless steel has a PRE greater than 28 and wherein the PRE is defined as PRE = Cr 3,3Mo 16N. The object according to any one of claims 1 to 5, wherein the Mn content in the duplex stainless steel is in the range of 2 to 5% by weight. The object according to any one of claims 1 to 6, wherein the content of N in the duplex stainless steel is in the range of 0.3 to 0.42% by weight. 8. The object according to any one of claims 1 to 7, wherein the content of Mo in the duplex stainless steel is in the range of 2 to 4% by weight. The object according to any one of claims 1 to 8, wherein the Cr content in the duplex stainless steel is in the range of 24 to 28% by weight, such as 26 to 28% by weight. The object according to any one of claims 1 to 9, wherein the Ni content in the duplex stainless steel is in the range of 7.0 to 9.0% by weight. The object according to any one of claims 1 to 10, wherein said object is a sheet, a strip or a wire. 12. A spring comprising the object according to any one of claims 1 to 11. 13. A method for manufacturing an object according to any one of claims 1 to 12, comprising the steps from: - providing a duplex stainless steel body as defined in any one of claims 1 to 11; - one or more hot rolled to transform the body into a 2-15 mm workpiece and the hot rolled are performed at a temperature of about 1050 to about 1300 ° C; - one or more cold rolled to transform the workpiece into an object having a thickness of 20 ± 5 mm; where the final stage of said method must be cold rolled. The method according to claim 13, wherein the method also comprises one or more heat treatment steps wherein the one or more heat treatment steps is annealing that is carried out at a temperature above 1000 to 1250 ° C in an annealing time of 1-600 seconds and where the time spent during heating between 750 ° C - 1000 ° C is less than 2 minutes and where one or more annealing stages are performed between the cold rolling stages. The method according to any one of claims 13 to 14, comprising an additional step of aging the object for 0.25 to 4 hours at a temperature of 400 to 450 ° C, wherein the aging is carried out after the step cold rolling end.