DE102015119839A1 - High energy absorbing steel alloy and tubular steel product - Google Patents

Abstract

The present invention relates to a steel alloy having high energy-absorbing capacity and good formability, comprising, in addition to melting-caused unavoidable impurities and iron, the following components in weight percent: C: 0.05-0.6% sum of Cr + 2 · Ti + 3 · (Mo + V + Nb) + 4 · W = 1-7%, wherein the structure of the steel alloy contains martensite constituents of 10-40% by volume of retained austenite, wherein the energy absorption capacity expressed by the product of tensile strength (Rm) and uniform elongation (Ag) is greater than 12,000 MPa% and the steel alloy has a minimum tensile strength of 1000 MPa. In addition, the invention relates to a steel tube product with high energy absorption capacity and good formability, which is characterized in that it consists at least partially of such a steel alloy.

Description

The present invention relates to a high energy absorbing steel alloy and a tubular steel product.

For the production of, for example, motor vehicle components, it is known to use so-called TRIP (Transformation Induced Plasticity) steels. As a rule, TRIP steels consist of ferrite, bainite and retained austenite, which transforms into martensite due to deformation. Deformation-induced martensite transformation of retained austenite is also referred to as the TRIP effect.

To stabilize the residual austenite and to increase the TRIP effect, it is known to subject TRIP steels to a heat treatment. In particular, a so-called quenching and partitioning (Q & P) is used. By this method, a structure is set with tempered martensite with retained retained austenite, which is stabilized.

The martensite increases the strength and the retained austenite ensures due to the TRIP effect further good elongation properties.

A disadvantage of low-alloy martensitic / austenitic steels is that the formation of bainite and / or cementite (iron carbide) must be prevented in order to obtain a sufficient amount of retained austenite in the steel. From the prior art it is known, for example, to add silicon to suppress the formation of bainite and cementite in large quantities. A disadvantage of this type of suppression of bainite formation and cementite formation is that the addition of silicon worsens the surface finish of the steel product during hot forming. In addition, in these martensitic / austenitic steels, significant proportions of bainite and cementite and thus inadequate austenite stabilization are generally observed.

The object of the present invention is thus to provide a solution by means of which the energy absorption capacity and the formability of a steel can be improved in a simple and reliable manner.

The invention is based on the finding that this object can be achieved by adding carbide formers to the steel alloy.

According to the invention the object is therefore achieved by a steel alloy with high energy absorption capacity and good formability, which in addition to melting unavoidable impurities and iron comprises the following components in percent by weight:
C: 0.05-0.6%
Sum of Cr + 2 · Ti + 3 · (Mo + V + Nb) + 4 · W = 1-7%,
wherein the structure of the steel alloy contains, in addition to martensite constituents of 10-40 vol .-% retained austenite, the energy absorption capacity expressed by the product of tensile strength Rm and uniformity Ag Ag is greater than 14,000 MPa% and the steel alloy has a minimum tensile strength of 1000 MPa.

The present invention thus provides a low-alloy steel alloy which has a high energy absorption capacity and good formability. The steel alloy is also referred to below as alloy, steel or material. Salary data for alloying elements are given as weight percentages, but may be referred to only as percentages.

Carbon (C) is necessary for the production of martensitic / austenitic microstructures. According to the invention carbon is added in an amount of at least 0.05%. It has been found that with a carbon content of less than 0.05%, there is not enough carbon in the steel to achieve significant retained austenite stabilization. However, the carbon content is limited according to the invention to a maximum of 0.6%. Above 0.6%, the martensitic matrix of the material, in particular of the final material, is too brittle after a tempering treatment at relatively low temperatures to represent a technically usable material. In the present invention, the carbon content of the steel alloy is in a range of 0.05% to 0.6%, and preferably in the range of 0.1-0.6%, and more preferably in the range of 0.15 to 0.5%.

According to the invention, the steel alloy additionally comprises at least one carbide former. Carbide formers are in particular alloying elements which form carbon carbides with carbon. The formation of iron carbide (Fe ₃ C cementite) is thereby suppressed.

Particularly preferably, chromium is contained in the steel alloy. Chromium serves as a carbide former. In addition to chromium, one or more other carbide formers may also be included in the steel alloy.

As carbide formers, alloying elements of subgroups 4 (titanium group), 5 (vanadium group) and 6 (chromium group) of the periodic table can be used.

Carbide formers are generally considered to counteract the stabilization of retained austenite, since they consume carbon to form the carbides, which would be necessary in the retained austenite for stabilization. According to the invention, however, it has been found that the amount of retained austenite in the steel can be increased by targeted addition of carbide formers.

When alloying carbide formers to iron-carbon alloys, at temperatures above the start temperature of the interstitial structure, bainite, also referred to as Bs (bainite start temperature), is an area where no conversions occur. In the time-temperature conversion diagram, this can be seen by a complete separation of the ferrite / perlite and bainite transformation ranges. This area, where no conversions take place, is internationally referred to as Bay. It has been shown that both the unwanted bainite formation and the formation of cementite at these temperatures are made more difficult if carbide formers are added in a targeted manner.

Baybildner, ie carbide, are therefore alloyed according to the invention. In particular, chromium is added as a carbide former. Preferably, the chromium content is in the range of 2-4%, and more preferably 3%.

In addition to chromium, in particular molybdenum is alloyed. Chromium and molybdenum suppress the formation of bainite and thus allow a redistribution of carbon into the retained austenite.

In addition, according to the invention at least one additional carbide former can be alloyed. The carbide formers according to the invention are the elements Ti (subgroup 4), V, Nb (subgroup 5) and also Cr, Mo, W (subgroup 6). These alloying elements are important from an economic point of view for use in steelmaking. These alloying elements, which serve as carbide formers, have a different effect than Baybildner on. In descending order W, Mo, V, Nb and Ti are suitable as well as chromium as Baybildner. These alloying elements are added according to the invention in such a combination that the requirement:
Cr + 2 · Ti + 3 · (Mo + V + Nb) + 4 · W = 1-7%
is satisfied.

In this way, the bay area is reliably increased and a temperature control to achieve a higher proportion of retained austenite simplified.

More preferably, the carbide formers in the steel alloy satisfy the requirement that the sum of Cr + 2 · Ti + 3 · (Mo + V + Nb) + 4 · W be in the range of 2-6%. As a result, bainite formation and cementite formation can be reliably prevented and a structure can be produced in which the proportion of residual austenite is high.

According to the invention, the structure of the steel alloy, in addition to martensite constituents of 10-40 vol .-% retained austenite on. According to a preferred embodiment, the structure of the steel alloy, in addition to martensite constituents of 15-35 vol .-% retained austenite on. With this high proportion of retained austenite in the martensite matrix, the TRIP effect can be used particularly and thus the advantageous material properties can be achieved.

Although not preferred, in addition to the martensitic structure with retained austenite, a small amount of bainite may also be present in the structure of the steel alloy. However, the bainite content is limited to a maximum of 30% by volume. With this bainite content, a sufficient content of retained austenite in the martensitic structure can still be achieved.

The energy absorption capacity of the steel alloy according to the invention, expressed by the product of tensile strength (Rm) and uniform elongation (Ag), according to the invention is greater than 12,000 MPa% and can also be greater than 14,000 MPa% according to the invention.

The product of yield strength and elongation at break and the product of tensile strength and elongation at break can also be higher in the steel alloy according to the invention than in known steels.

In addition, the product of tensile strength (Rm) and elongation at break (A ₅₎ in the inventive steel alloy according to the invention is so high that the formability, especially the cold workability of a steel product made of the steel alloy is guaranteed.

The steel alloy according to the invention has a minimum tensile strength of 1000 MPa.

The steel alloy according to the invention is thus superior to conventional tempered steels in terms of formability and elongation at break with the same tensile strength.

According to one embodiment, the steel alloy has a silicon content of less than 1.1%. Silicon can be used as a deoxidizer due to its high oxygen affinity and is therefore present in most tempered steel alloys. In low-alloyed steels, silicon is known for suppressing cementite formation in steels. However, it has been shown that with Silicon alloyed steels with increasing content in the hot forming tend to form more adherent oxide layers, which deteriorate the surface quality, which in particular also has a detrimental effect on subsequent coating processes, such as galvanizing.

By limiting the silicon content to 1.1% in the steel alloy according to the invention, the negative effects of silicon on the properties of the steel, in particular the poor surface properties, can be minimized.

In one embodiment, the yield ratio (yield strength to tensile strength) is Rp0.2 / Rm ≤ 0.8 of the steel alloy. With conventional tempered steels, this ratio is higher, for example> 0.9. With the present steel alloy, therefore, a better deformability of the steel product produced from the steel alloy, in particular tubular steel product can be ensured.

The uniform elongation (Ag) of the steel alloy according to the invention can be ≥ 5%. In conventional tempered steels, in particular steels with greater than 1000 MPa, the uniform elongation is lower and is in particular <5%.

According to a preferred embodiment, the steel alloy is in a heat-treated state, in particular in a state after heat treatment of quenching and partitioning (Q & P).

According to the invention, it has been shown that alloyed steels are outstandingly suitable for quenching and partitioning heat treatment with carbide formers, in particular chromium, but also titanium, molybdenum, vanadium, niobium and tungsten. This finding is in contrast to the prior art opinion that the carbide formers are generally counterproductive to Q & P.

Quenching and Partitioning Heat Treatment (Q & P) (quenching and redistribution) produces a two-phase microstructure consisting of low-carbon martensite and retained austenite.

During the quenching step, the steel is first fully austenitized and then quenched to a temperature that is between the martensite start temperature and the martensite finish temperature. Due to the suppressed cementite precipitation, the carbon diffuses from the supersaturated martensite to the retained austenite during the partitioning step. Carbon stabilizes the austenite, locally lowering the martensite start temperature of the carbon-enriched austenite to below room temperature. Therefore, in a final quench to room temperature, no high-carbon martensite is formed and carbon-enriched austenite remains. The martensite, which is preferably tempered, increases the strength and the retained austenite still ensures good elongation properties due to the TRIP effect.

• a. Abschrecken nach einer Warmformgebung bei mindestens Ac3 Temperatur zunächst in einem ersten Kühlschritt mit einer Kühlrate größer der kritischen Abkühlgeschwindigkeit auf T1 größer Ms.
• b. Abschrecken in einem zweiten Kühlschritt auf eine Temperatur T2 a. Wobei T2 < Ms – 50°C und größer Raumtemperatur ist; b. Und der zweite Kühlschritt mit einer im Vergleich zum ersten Kühlschritt um den Faktor 3 bis 20 niedrigeren Kühlrate erfolgt
• c. Erwärmung mit einer Heizrate und Haltephase zur Stabilisierung polyederförmigen Restaustenits, die der folgenden Gleichung genügt: (T3 – T2)/10× Heizrate in K/s > Haltezeit (in sec.) bei T3 Wobei T3 > Bainit Start Temperatur
• d. Weiterer Kühlschritt auf eine Rohrtemperatur < Ms.

According to one example, the product of the steel alloy, for example a tubular element, is subjected to the following process steps:

• a. Quenching after hot forming at least Ac3 temperature first in a first cooling step with a cooling rate greater than the critical cooling rate on T1 greater than Ms.
• b. Quenching in a second cooling step to a temperature T2 a. Where T2 <Ms - 50 ° C and greater than room temperature; b. And the second cooling step takes place with a cooling rate that is lower by a factor of 3 to 20 than the first cooling step
• c. Heating with a heating rate and holding phase to stabilize polyhedron retained austenite satisfying the following equation: (T3 - T2) / 10 × heating rate in K / s> holding time (in sec.) At T3 Where T3> bainite start temperature
• d. Further cooling step to a tube temperature <Ms.

Step c represents the step of partitioning. The longer the rewarming (heating rate) takes, the shorter the hold time for partitioning. At a moderate heating rate of, for example, 10K / s with inductive heating and a temperature difference (dT) of 200K, the retention time is thus <200s.

The heat treatment, in particular the step of partitioning, is preferably carried out according to the invention with inductive heating. In this way, the desired heating rates and holding phases can be set specifically.

The steel alloy according to the invention has, in addition to the high energy absorption capacity and the good formability, also a good machinability. The machinability, that is to say the ability to machine the steel product produced from the steel alloy, is determined in particular by the strength and toughness. With the inventively achievable strength and toughness, the machinability of the steel alloy, in particular of the product produced from the steel alloy can be ensured.

With the steel alloy according to the invention, different steel products can be manufactured. Preferably, the inventive Steel alloy used to produce a tubular steel product.

According to a further aspect, the present invention therefore relates to a tubular steel product with high energy absorption capacity and good formability. The tubular steel product is characterized in that it at least partially consists of the steel alloy according to the invention. Particularly preferably, the raw steel product comprises a pipe element which consists at least partially and preferably completely of the steel alloy according to the invention. Alternatively, it is also possible that the steel pipe product consists exclusively of such a tubular element.

In the oil industry, a steel tube product of the present invention may be used, for example, as a drill pipe or as a so-called perforation gun.

Thus, according to one embodiment, the tubular steel product forms at least part of a perforation gun. In particular, the tubular steel product may be the hollow carrier (hollow carrier) of a perforation gun. A perforation gun is a unit used in the oil extraction industry. The perforation gun is used to open or reopen wellbores for gas or crude oil storage. A perforation gun in this case preferably comprises a hollow carrier, which is also referred to as a hollow carrier. In the hollow carrier ignition units are introduced. As the perforation gun is brought into position, for example, to a depth in which an oil reservoir is located, and positioned relative thereto, the hollow carrier must withstand high mechanical stresses, particularly due to the prevailing pressure and temperature. A hollow carrier produced from the steel alloy according to the invention can meet these requirements.

The tubular steel product, which is at least part of a perforation gun, in particular a hollow carrier, has a plurality of, in particular locally limited, sections of reduced wall thickness. These localized sections preferably represent punctiform or circular sections. The sections are provided in the hollow carrier for forming wall openings on the hollow carrier upon ignition of ignition charges introduced into the hollow carrier. Due to the high energy absorption capacity of the steel alloy according to the invention, of which the hollow carrier preferably consists, it can be ensured when igniting the ignition charges that the hollow carrier does not burst. Only the areas of reduced wall thickness are destroyed, thus allowing the perforation of the surrounding rock.

The introduction of sections with a smaller wall thickness is possible in a simple manner in the case of the steel alloy according to the invention, since it gives a good machinability.

According to an alternative embodiment, the tubular steel product is a tubular steel product for a high energy consuming motor vehicle.

The tubular steel product in this embodiment may be, for example, a shaft, a stabilizer, an injection pipe, or an impact protection unit such as an air bag module, a door impact beam, a roll bar, or an A pillar.

According to one embodiment, the steel tube product forms at least part of an airbag gas pressure vessel. As an airbag gas pressure vessel, the part of an airbag module is referred to, is stored in the medium, in particular gas, under elevated pressure or in which a high pressure of the medium is generated. By means of the medium, the actual airbag is filled. In the embodiment as an airbag gas pressure vessel, the steel tube product has at least two longitudinal sections of different outer circumference. In particular, at least one of the pipe ends can have a smaller outer circumference. The outer circumference of a longitudinal section in this embodiment is preferably at least 5 percent smaller than the outer circumference of the further longitudinal section.

According to a preferred embodiment, the steel alloy in the steel tube product has volume-related constituents of 10-40% of film and polyhedron-shaped retained austenite.

According to a further embodiment, the steel tube product, at least in the region in which it is formed by the steel alloy according to the invention, a metallic corrosion-protective coating at least on its outer surface. The metallic corrosion protection layer may be, for example, a zinc layer that is applied by galvanizing. Such a type of coating is possible when using the steel alloy according to the invention, since their surface finish can be improved due to the possible low silicon content.

According to a further embodiment, the steel tube product is used for crash-relevant structural components, chassis components, shafts or airbag gas pressure vessel of a motor vehicle.

According to the invention, the steel tube product and in particular the part of the tubular steel product which is made of the steel alloy according to the invention consists, heat-treated, in particular, treated by a quenching and partitioning heat treatment.

By this heat treatment, the retained austenite which is formed in a large amount in the alloy of the present invention can be stabilized, and thus the desired product properties can be tailored.

In the accompanying figures, embodiments of tubular steel products according to the present invention are shown schematically. Hereby shows:

1 a schematic representation of a steel pipe product in the embodiment as an airbag gas pressure vessel;

2 a schematic representation of a steel tube product in an embodiment as a hollow carrier of a perforation gun; and

3 : A schematic representation of a jet pipe product in one embodiment as a stabilizer.

In 1 is an embodiment of the tubular steel product according to the invention 1 shown as a gas pressure vessel, in particular airbag gas pressure vessel. The steel pipe product 1 comprises a tubular element 10 , In the in 1 illustrated embodiment, the pipe ends 101 rejuvenated or confiscated. The rejuvenation of the pipe ends 101 can be generated by cold forming. The pipe ends 101 In the illustrated embodiment, each have a diameter D ₁ , which is less than the diameter D _{0 of} the tubular element 10 in its middle area 102 , The diameter of the pipe ends 101 can also be different. In the in 1 embodiment shown has the steel tube product 1 a combustion chamber 14 on, in which a detonator 12 and the other pyrotechnic components are provided. At the one pipe end 101 is the combustion chamber 14 with a welded-on disc 17 locked. To the combustion chamber 14 closes the cold gas storage 15 at. This is from the combustion chamber 14 through the membrane 11 , which can also be called rupture disk, separated. The cold gas storage 15 lies in the middle area 102 of the tubular element 10 having the larger diameter D ₀ . To the cold gas storage 15 closes the diffuser 13 at. In the 1 is in the area of the diffuser 13 a filling hole 16 shown. The pipe end 101 of the diffuser 13 is with a disc 17 welded, that is closed by this.

In the cold gas storage 15 For example, a pressure of 580 bar prevail. In the combustion chamber 14 For example, the pressure on igniting the igniter may increase from, for example, 580 bar to 1,200 bar. This pressure can the steel tube product according to the invention 1 due to its properties reliably withstand, without that brittle fracture or extension of a brittle crack is to be feared.

In 2 is a schematic view of another embodiment of the tubular steel product 1 shown, which represents a perforation gun. The perforation gun 1 comprises a tubular element 10 , which can also be called a Hollow Carrier. The pipe element 10 preferably represents a seamless pipe element. In the pipe element 10 are local areas 100 introduced with a reduced wall thickness. The local areas 100 each have a circular surface. The areas 100 are about the length of the pipe element 10 distributed. In the pipe element 10 is an ignition unit 18 introduced with Zündladungen. Through the ignition unit 18 the explosive material of the ignition charge is ignited and thereby the areas 100 of the tubular element 10 opened and on the other the surrounding material, such as rock, perforated.

In 3 is another embodiment of the tubular steel product 1 shown. In this embodiment, the steel tube product 1 a stabilizer. The stabilizer 1 In the illustrated embodiment, a tube element comprises 10 whose ends 101 on each of a connection component 2 are attached. As it turned out 3 results are the pipe ends 101 with the connection component 2 via a connection point 19 , For example, connected by a weld.

With the present invention a number of unexpected advantages can be achieved. On the one hand, according to the invention, the austenite content is significantly increased by alloying in carbide formers, which according to the general opinion would have to further reduce the expected austenite contents due to the expected carbide formation, and a temperature regime which intentionally suppresses bainite and carbide formation. On the increased use of silicon, which leads to poorer surfaces in the hot forming, can be omitted according to the invention.

The present invention thus creates a new material group of high-strength Q & P steels, which can be used excellently for pipes. In particular, it is possible to produce steel products, in particular tubes, which have high strength and simultaneously high ductility. In particular, the steel products according to the invention have a higher energy absorption capacity compared to conventional tempered pipes. Due to the lower yield ratio of the steel alloy better ductility is achieved. Due to the low content or absence of silicon in the steel alloy, a better surface quality of the steel product is achieved. In particular, in the steel alloy according to the invention, a coating of the surface, for example galvanizing, is possible even after hot forming.

Finally, due to the low content of alloying elements, the cost of the steel alloy is low.

LIST OF REFERENCE NUMBERS

1

 Tubular steel product

10

 tube element

101

 pipe end

102

 middle area

11

 membrane

12

 fuze

13

 diffuser

14

 combustion chamber

15

 Cold gas storage

16

 fill hole

17

 disc

100

 Area of lesser wall thickness

18

 ignition unit

19

 access point

2

 connection component

Claims (19)

High energy absorption steel alloy with good formability, comprising, in addition to melting inevitable impurities and iron, the following components in weight percent: C: 0.05-0.6% Sum of Cr + 2 · Ti + 3 · (Mo + V + Nb) + 4 · W = 1-7%, wherein the structure of the steel alloy contains, in addition to martensite, constituents of 10-40% by volume of retained austenite, the energy absorbing capacity expressed by the product of tensile strength (Rm) and uniform elongation (Ag) being greater than 12,000 MPa% and the steel alloy having a minimum tensile strength of 1000 MPa having. Steel alloy according to claim 1, characterized in that the carbon content is in the range between 0.1-0.6% and preferably in the range between 0.15-0.5%. Steel alloy according to one of claims 1 or 2, characterized in that the steel alloy has a silicon content of <1.1%. Steel alloy according to one of claims 1 to 3, characterized in that the chromium content is in the range of 2-4%, preferably 3% Steel alloy according to one of claims 1 to 4, characterized in that the sum of Cr + 2 · Ti + 3 · (Mo + V + Nb) + 4 · W = 2-6%. Steel alloy according to one of claims 1 to 5, characterized in that the structure of the steel alloy in addition to martensite constituents of 15-35 vol .-% has retained austenite. Steel alloy according to one of claims 1 to 6, characterized in that the structure of the steel alloy in addition to martensite and retained austenite has at most 30 vol .-% bainite. Steel alloy according to one of claims 1 to 7, characterized in that the energy absorption capacity expressed by the product of tensile strength Rm and uniform elongation Ag is greater than 14,000 MPa%. Steel alloy according to one of claims 1 to 8, characterized in that the yield ratio Rp0.2 / Rm ≤ 0.8. Steel alloy according to one of claims 1 to 9, characterized in that it has a uniform elongation of ≥ 5%. Steel alloy according to one of claims 1 to 10, characterized in that the steel alloy is subjected to a heat treatment, in particular a quenching and partitioning treatment. Steel tube product with high energy absorption capacity and good formability, characterized in that it at least partially consists of a steel alloy according to one of claims 1 to 11. Tubular steel product according to claim 12, characterized in that it forms at least part of a perforation Gun, wherein the tubular steel product has a plurality, in particular locally limited sections of reduced wall thickness. Tubular steel product according to claim 12, characterized in that it is a tubular steel product for a motor vehicle with high energy absorption capacity. Steel tube product according to claim 12 or 14, characterized in that it forms at least part of an airbag gas pressure vessel having at least two longitudinal sections of different outer circumference, wherein the outer circumference of a longitudinal section is smaller by at least 5 percent than the outer circumference of the further longitudinal section. Steel tube product according to one of claims 12 to 15, characterized in that the steel tube product has volume-related constituents of 10-40% of film and polyhedron-shaped retained austenite. Tubular steel product according to one of claims 12 to 16, characterized in that it has a metallic corrosion-protective coating at least on its outer surface. Tubular steel product according to one of claims 13 to 17, characterized in that the tubular steel product is at least a part of a crash relevant structural component of the motor vehicle, in particular suspension components or waves. Tubular steel product according to one of claims 12 to 18, characterized in that the steel tube is heat-treated, in particular by a quenching and partitioning heat treatment treated.

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