DE102015111866A1 - Formable lightweight structural steel with improved mechanical properties and process for the production of semi-finished products from this steel - Google Patents

Abstract

The invention relates to a malleable lightweight structural steel having improved mechanical properties and high resistance to retarded hydrogen-induced cracking and hydrogen embrittlement with the elements (in weight percent): C 0.02 to ≤ 1.0; Mn 3 to 30; Si ≤ 4; P max. 0.1; S max. 0.1; N max. 0.03; Sb 0.003 to 0.8, in particular advantageously up to 0.5 and at least one or more of the following carbide-forming elements in the stated contents (in% by weight): Al ≤ 15; Cr is 0.1 to 8; Mo is 0.05 to 2; Ti is 0.01 to 2; V 0.005 to 1; Nb 0.005 to 1; W 0.005 to 1; Zr 0.001 to 0.3; Remainder of iron including conventional steel-accompanying elements, with optional addition the following elements in% by weight: max. 5 Ni, max. 10 Co, max. 0.005 Ca, max. 0.01 B and 0.05 to 2 Cu. The invention also relates to a method for producing the aforementioned lightweight structural steel.

Description

The invention relates to a deformable lightweight structural steel with improved mechanical properties and a high resistance to delayed hydrogen-induced cracking, according to claim 1. The invention further relates to a process for the production of semi-finished products from this steel.

The term semi-finished product is understood below to mean hot or cold strip produced from this steel or an intermediate or end product made therefrom, such as, for example, pipes.

In recent years, there has been great development progress in the field of so-called lightweight steels, which are characterized by a low specific weight while high strength and toughness and have a high ductility and thus are of great interest to the vehicle.

In these austenitic steels in the initial state, the high proportion of alloying constituents (Si, Al) having a specific gravity well below the specific gravity of iron achieves a weight reduction advantageous to the automobile industry while retaining the previous design construction.

The from the published patent DE 10 2004 061 284 A1 Known lightweight formable steel has, for example, the following alloy composition (in% by weight): C 0.04 to ≦ 1.0, Al 0.05 to <4.0, Si 0.05 to ≦ 6.0, Mn 9, 0 to <18.0, remainder iron including common steel components. Optionally, Cr, Cu, Ti, Zr, V and Nb can be added as required.

This lightweight steel has a partially stabilized γ-mixed crystal structure with defined stacking fault energy with a z. T. multiple TRIP effect, which allows the stress- or strain-induced conversion of a face-centered γ-mixed crystal (austenite) in an ε-martensite (hexagonal closest packing) and then further deformation in a body-centered ε-martensite and retained austenite.

The high degree of deformation is achieved by TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) properties of the steel.

In the case of this and comparable steels, however, in the presence of internal stresses in the material, depending on the microstructure and the strength, a hydrogen-induced delayed embrittlement and as a result cracking may occur.

To overcome this problem is in the published patent application DE 10 2004 061 284 A1 It has already been proposed to limit the hydrogen content to <20 ppm, preferably to <5 ppm.

While this suggestion is helpful but not sufficient, the effect of delayed cracking may still occur even at low hydrogen levels. Moreover, steelmaking may, for various reasons, exceed the established maximum value for hydrogen which, while tolerated by alloy, increases the risk of hydrogen embrittlement.

From the publication WO 2011/154153 A1 is an austenitic steel is known, which should have excellent resistance to delayed cracking. The steel contains, in addition to iron and impurities in wt .-%: 0.5 to 0.8 C, 10 to 17 Mn, at least 1.0 Al, at most 0.5 Si, at most 0.020 S, at most 0.050 P, 50 to 200 ppm N and 0.050 to 0.25 V.

From the publication WO 2009/142362 A1 For example, a steel alloy is known for a high strength cold rolled steel sheet, which should also have improved delayed crack resistance. The steel contains, in addition to iron and impurities in wt .-%: 0.05 to 0.3 C, 0.3 to 1.6 Si, 4.0 to 7.0 Mn, 0.5 to 2.0 Al, 0 , 01 to 0.1 Cr, 0.02 to 0.1 Ni, 0.005 to 0.03 Ti, 5 to 30 ppm B, 0.01 to 0.03 Sb and 0.008 or less S.

Furthermore, from the published patent application EP 2 128 293 A1 a lightweight steel having improved elongation, comprising, in addition to iron and impurities in weight%: 0.2 to 0.8 C, 2 to 10 Mn, 0.2 or less P, at most 0.015 S, 3.0 to 15 Al , 0.01 N or less and Mn / Al ratio of 0.4 to 1.0.

The object of the invention is to provide a lightweight steel of the generic type, which does not have the effect of a delayed cracking or hydrogen embrittlement while maintaining very good mechanical properties (ductility, strength).

This object is achieved on the basis of the preamble in conjunction with the characterizing features of claim 1 and with respect to a method by the features of claim 6. Advantageous developments are the subject of dependent claims.

According to the teaching of the invention, the transformable lightweight structural steel with TRIP and TWIP properties has the following elements in% by weight:
C 0.02 to ≤ 1.0
Mn 3 to 30
Si ≤ 4
P max. 0.1
S max. 0.1
N max. 0.03
Sb 0.003 to 0.8, advantageously up to max. 0.5
and at least one or more of the following carbide-forming elements in the stated contents (in% by weight):
Al ≤ 15
Cr> 0.1 to 8
Mo 0.05 to 2
Ti 0.01 to 2
V 0.005 to 1
Nb 0.005 to 1
W 0.005 to 1
Zr 0.001 to 0.3

Remainder of iron including conventional steel-accompanying elements, with optional addition of the following elements in% by weight: max. 5 Ni, max. 10 Co, max. 0.005 Ca, max. 0.01 B and 0.05 to 2 Cu.

Surprisingly, it has been found in extensive investigations that by alloying antimony (Sb) within the specified limits, the diffusion of elements, in particular C, N and O, is hindered and thus in connection with a targeted heat treatment, the material behavior can be favorably influenced.

The addition of antimony causes the carbides to grow more slowly and thus be finer and less precipitated. As a result, alloying elements are used more effectively, resulting in less expensive alloying concepts with improved mechanical properties and a significant improvement in terms of avoiding delayed hydrogen-induced cracking (delayed fracture) and hydrogen embrittlement.

It has turned out to be favorable if the ratio of Sb / C does not exceed 1.5. Values above 1.5 bring no further advantage in the sense of the invention and cause above all negative effects such as a loss of ductility and toughness by excretion of antimony at the grain boundaries.

According to the invention, the evaluation of the mechanical properties is based on the product of tensile strength and elongation at break, which is a measure of the performance of the material.

In tests it has been found that in the alloys according to the invention the tensile strength and elongation at break are significantly higher by the addition of antimony in comparison with steel alloys in which no antimony is added, whereby less expensive and higher quality steels can be produced.

It has also been recognized that the above-described effect of antimony can be significantly increased by heat treatment of the steel.

In order to achieve a further improvement of the required properties, the product or semifinished product produced from the alloy according to the invention by deformation, which may be, for example, hot strip, cold strip, flexibly rolled hot or cold strip, a pipe or a body component, is therefore advantageous a heat treatment at 480 to 770 ° C for 1 minute to 48 hours, for example, in a bell annealer with predominantly long annealing times or in a continuous annealing with predominantly short annealing times.

However, such annealing can already take place before the final shaping to a finished product, for example on the cold strip, which is subsequently processed. The timing of the annealing can therefore be flexibly adapted to the production process. An annealing of the end product in addition to an already performed annealing of the semifinished product can lead to a further improvement of the material properties.

• – Vergießen des Stahls nach dem Stranggussverfahren oder Dünnbrammengießverfahren oder einem endabmessungsnahen horizontalen oder vertikalen Bandgießverfahren,
• – Warmwalzen der gegossenen Bramme beziehungsweise des gegossenen Bandes mit einer Dicke von mehr als 5 mm auf eine einheitliche Dicke oder flexibles Warmwalzen der gegossenen Bramme beziehungsweise des gegossenen Bandes mit einer Dicke von mehr als 5 mm auf unterschiedliche Dicken,
• – Optionales Kaltwalzen des auf eine einheitliche Dicke gewalzten Warmbandes oder mittels endabmessungsnahem Gießverfahren hergestellten maximal 5 mm dicken gegossen Bandes auf eine einheitliche Dicke oder optionales flexibles Kaltwalzen des auf eine einheitliche Dicke gewalzten Warmbandes oder mittels endabmessungsnahem Gießverfahren hergestellten maximal 5 mm dicken gegossen Bandes auf unterschiedliche Dicken,
• – Optionales Glühen des Warm- oder Kaltbandes mit folgenden Parametern: Glühtemperatur: 480 bis 770°C, Glühdauer: 1 Minute bis 48 Stunden.

Furthermore, the invention is realized by a method for producing the steel according to the invention with the following steps:

• Casting the steel by continuous casting or thin slab casting or a horizontal or vertical band casting process close to the final dimensions,
• Hot rolling of the cast slab or cast strip with a thickness of more than 5 mm to a uniform thickness or flexible hot rolling of the cast slab or of the cast strip with a thickness of more than 5 mm to different thicknesses,
• Optional cold rolling of the rolled to a uniform thickness hot strip or produced by near-final casting method a maximum of 5 mm thick cast strip to a uniform thickness or optional flexible cold rolling of rolled to a uniform thickness hot strip or near-final casting method produced maximum 5 mm thick cast strip to different thicknesses .
• - Optional annealing of the hot or cold strip with the following parameters: annealing temperature: 480 to 770 ° C, annealing time: 1 minute to 48 hours.

With regard to the maximum 5 mm thick cast strip produced by means of a close-to-casting casting process, it is particularly advantageous to cold roll to a uniform thickness or to flexibly cold roll to different thicknesses and then to anneal the cold strip with the following parameters: annealing temperature: 480-770 ° C, Annealing time: 1 minute to 48 hours.

For alloys with Al contents of> 1 wt .-%, the annealing is preferably carried out at temperatures of 670-770 ° C at annealing times of 1 minute to 12 hours, as lower temperatures and longer annealing times lead to lower tensile strength and elongation at break.

For the annealing itself, a continuous annealing is preferably used for hot-rolled strip, cold strip and flexibly rolled strips for short annealing times, and preferably for long annealing times a crown annealing. For other semi-finished products and products, other annealing devices can be used with the given parameters, such as a muffle furnace.

With the invention, the production of low-cost Sb-alloyed steels with higher manganese content is possible, which have better tensile strength and elongation at break than non-Sb-alloyed, higher manganese-containing steels having the same chemical composition.

In addition, the addition of antimony also significantly improves the behavior towards hydrogen (delayed cracking and hydrogen embrittlement).

The reason for the improvement in the material properties is that antimony inhibits the diffusion of carbon and aluminum. Furthermore, antimony lowers the interfacial energy, resulting in a finer distribution of the carbides. The reduced carbon diffusion thus delays the local accumulation of carbon at the grain boundaries and in the microstructure and in connection with aluminum the formation of kappa carbides or in particular with V, Nb, Mo, Cr, W, Zr and Ti, the formation of local larger carbides. The homogeneity of the material is thereby improved with the described positive effects on the mechanical properties and the resistance to delayed cracking and hydrogen embrittlement. The precipitation of finely divided carbides leads to a grain refining in the microstructure, which is accompanied by an improvement in the behavior against hydrogen-related negative effects (delayed cracking, hydrogen embrittlement) and an increase in the strength and improvement of the toughness and elongation properties.

By the inventive addition of antimony in small amounts up to max. 0.8% by weight, the behavior of the material against hydrogen-induced influences is therefore significantly improved.

The addition of large amounts of antimony, on the other hand, causes an undesirably high precipitation of antimony at the grain boundaries and thereby reduces the toughness and elongation properties. For antimony to be effective, at least levels of 30 ppm are necessary. However, antimony contents of more than 0.8 wt.% Embrittle the material and should therefore be avoided. Optimal, the maximum content of antimony is 0.5 wt .-%.

The small carbides (mainly Cr, Mo, Ti, Nb, V, W, Zr and kappa carbides) which are much more finely distributed compared to the prior art improve the degree of utilization of the corresponding alloying elements, which is potentially allows a reduction in the amount added. Furthermore, due to the reduced carbon diffusion and the reduced grain growth due to the addition of antimony, the process window for the heat treatments required according to the invention is increased, that is to say the steel is less sensitive to process fluctuations (temperature, time) with respect to the resulting mechanical properties.

The following describes the positive effects of the alloying elements used according to the invention:
Al: Improves the strength and elongation properties, lowers the specific gravity, and influences the conversion behavior of the alloys of the present invention. Al contents of more than 15% by weight deteriorate the elongation properties, therefore a maximum content of 15% by weight is determined. High Al contents of greater than or equal to 4% by weight act in combination with high C contents of greater than or equal to 0.6% by weight as carbide formers for kappa carbides. Below 4% by weight, Al delays the precipitation of carbides.
B: Improves strength and stabilizes austenite. Contents of more than 0.01 wt .-% lead to embrittlement of the material, which is why a maximum content of 0.01 wt .-% is set.
C: Needed to form carbides, stabilizes austenite and increases strength. Contents of more than 1% by weight of C deteriorate the welding properties and lead to precipitation of undesirably large carbides and thus deterioration of the elongation and toughness properties, therefore, a maximum content of 1% by weight is determined. In order to achieve a sufficient strength of the material, a minimum addition of 0.01 wt .-% is required.
Ca: Used to modify non-metallic oxide inclusions, which can lead to inhomogeneities and unwanted material failure. Due to its high vapor pressure in liquid steel, the content is limited to a maximum of 0.005 wt .-%.
Co: Increases the strength of the steel, stabilizes the austenite and improves the heat resistance. Contents of more than 10% by weight deteriorate the elongation properties, for which reason a maximum content of 10% by weight is determined.
Cr: Improves strength and reduces corrosion rate, retards ferrite and pearlite formation and forms carbides. The maximum salary is with. 8 wt .-%, since higher contents have a deterioration of the elongation properties.
Cu: Reduces the corrosion rate and increases the strength. Contents above 2 wt .-% deteriorate the manufacturability by forming low-melting phases during casting and hot rolling, which is why a maximum content of 2 wt .-% is set.
Mn: Stabilizes austenite, increases strength and toughness, and allows for strain-induced martensite and / or twinning formation in the alloys of the present invention. Contents less than 3 wt .-% are not sufficient to stabilize the austenite and thus worsen the elongation properties, while at levels greater than 30 wt .-% no further advantages are expected and the production is difficult due to the low Mn vapor pressure.
Mo: acts as a strong carbide former and increases strength. Contents of Mo exceeding 2% by weight deteriorate the elongation properties, and therefore a maximum content of 2% by weight is determined.
Nb + V: Grain-refining, in particular through the formation of carbides, which simultaneously improves the strength, toughness and elongation properties. Contents of over 1 wt .-% bring no further advantages.
Ni: Stabilizes austenite and improves elongation properties, especially at low application temperatures. More than the addition of 5 wt .-% Ni brings no further advantage.
Si: hinders carbon diffusion, reduces specific gravity and increases strength and elongation and toughness properties. Furthermore, an improvement in cold rollability by alloying Si could be observed. Contents of more than 4 wt .-% lead to embrittlement of the material and affect the hot and cold rollability negative, which is why a maximum content of 4 wt .-% is set.
Ti: Grain-refining as a carbide former, which simultaneously improves strength, toughness, and elongation properties, and reduces intergranular corrosion. Contents of Ti exceeding 2 wt% deteriorate the elongation properties, and therefore a maximum content of 2 wt% is determined.
W: acts as a carbide former and increases strength and heat resistance. Contents of W of more than 1 wt% deteriorate the elongation properties, therefore, a maximum content of 1 wt% is determined.
Zr: acts as a carbide former and improves strength. Contents of Zr of over 0.3 wt% deteriorate the elongation properties, and therefore a maximum content of 0.3 wt% is determined.

Advantageous alloy combinations are given in claims 3 to 5.

An alloy according to claim 3, using optimized heat treatment parameters (see Table 1 to 4) on a product of tensile strength and elongation at break of at least 20,000 MPa% and a tensile strength of at least 800 MPa. The product of tensile strength and elongation at break is a measure of the material's performance during forming.

Although the heat treatment 680 ° C 10 min from Table 2 not yet optimal values for the product of tensile strength and elongation at break of at least 20,000 MPa%, but you can already see here the positive effect of the addition of antimony.

An alloy according to claim 4 has a product of tensile strength and elongation at break of at least 30,000 MPa% and a tensile strength of at least 800 MPa.

An alloy according to claim 5 comprises finely divided kappa-carbide precipitates and a product of tensile strength and elongation at break of at least 30,000 MPa% and a yield strength of at least 700 MPa and a tensile strength of at least 800 MPa.

Table 1 lists the alloy compositions tested. With otherwise approximately the same chemical composition, the content of Sb and additions of Nb were varied.

2 mm thick hot strips were produced from these steels and these were cooled in air after hot rolling. Samples were taken from these hot strips and the tensile strength and elongation at break were determined therefrom.

The results from the product of tensile strength and elongation at break are shown in Tables 2 to 4, wherein that heat treatment with the maximum product of tensile strength and elongation at break is considered to be the most favorable for the respective alloy. It becomes clear that the steels alloyed with Sb according to the invention always have a higher product of tensile strength and elongation at break than the comparative alloys. Table 1: Alloy composition alloy C Mn al Si Cr V sb miscellaneous L1 0.19 7.1 2 0.55 1 0.05 0 L2 0.19 7.1 2 0.55 1 0.05 0,012 L3 0.19 7.1 2 0.55 1 0.05 0.027 L4 0.19 7.1 2 0.55 1 0.05 0,041 L5 0.21 6.3 2 0.2 1 0.06 0 Nb 0.05 L6 0.21 6.3 2 0.2 1 0.05 0,039 Nb 0.05 L7 0.25 7.9 1 0.5 0.9 0.08 0 L8 0.25 7.9 1 0.5 0.9 0.08 0.04 Table 2: Determined products of tensile strength and elongation at break L1 to L4 heat treatment TS · EI L1 L2 L3 L4 650 ° C, 24 h 22453 23195 23772 22633 680 ° C, 10 min 15263 16695 16830 16111 680 ° C, 5 h 27162 27997 28258 29000 680 ° C, 24 h 26660 28985 30546 29720 Table 3: Determined products of tensile strength and elongation at break L5 and L6 heat treatment TS · EI L5 L6 690 ° C, 3 h 19368 21449 750 ° C, 10 min 22751 25502 500 ° C, 10 min 23525 26737 Table 4: determined product of tensile strength and elongation at break L7 and L8 heat treatment TS · EI L7 L8 650 ° C, 24 h 18378 20457

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102004061284 A1 [0005, 0009]
• WO 2011/154153 A1 [0011]
• WO 2009/142362 A1 [0012]
• EP 2128293 A1 [0013]

Claims (8)

Convertible lightweight structural steel with improved mechanical properties and high resistance to retarded hydrogen-induced cracking and hydrogen embrittlement with the elements (in wt%): C 0.02 to ≤ 1.0 Mn 3 to 30 Si ≤ 4 P max. 0.1 S max. 0.1 N max. 0.03 Sb 0.003 to 0.8, in particular advantageous to 0.5 and at least one or more of the following carbide-forming elements in the stated contents (in% by weight): Al ≤ 15 Cr> 0.1 to 8 Mo 0.05 to 2 Ti 0.01 to 2 V 0.005 to 1 Nb 0.005 to 1 W 0.005 to 1 Zr 0.001 to 0.3 Remainder of iron including conventional steel-accompanying elements, with optional addition the following elements in% by weight: max. 5 Ni, max. 10 Co, max. 0.005 Ca, max. 0.01 B and 0.05 to 2 Cu. Lightweight steel according to claim 1, characterized in that the ratio Sb / C is less than or equal to 1.5. Light-weight steel according to claim 1 and 2, characterized by the elements in% by weight: C is 0.03 to 0.5, more preferably 0.1 to 0.35 Mn 3 to 10, in particular advantageous 5 to 9 Al is 0.1 to 4, more preferably 1 to 3.5 Si is 0.1 to 3, particularly preferably 0.1 to 1 Sb 0.005 to 0.3, particularly advantageously 0.01 to 0.1 Cr> 0.1 to 5, particularly advantageously 0.5 to 4 V 0.005 to 1, particularly advantageously 0.02 to 0.1 wherein the steel has a product of tensile strength and elongation at break of at least 20,000 MPa% and a tensile strength of at least 800 MPa. Light-weight steel according to claim 1 and 2, characterized by the elements in% by weight: C 0.4 to 0.9 Mn 12 to 18 Al 0.5 to 4 Si 0.5 to 3 Sb 0.005 to 0.4 and at least one or more of the following carbide-forming elements in the stated contents (in% by weight): Cr max. > 0.1 to 4 Mo max. 0.05 to 1 Ti max. 0.01 to 0.1 V max. 0.005 to 0.3 Nb max. 0.005 to 0.3 W max. 0.005 to 0.5 Zr max. 0.001 to 0.3 Remainder of iron including conventional steel-accompanying elements, with optional addition the following elements in% by weight: max. 5 Ni, max. 10 Co, max. 0.005 Ca, max. 0.01 B and 0.05 to 2 Cu, wherein the steel has a product of tensile strength and elongation at break of at least 30,000 MPa% and a tensile strength of at least 800 MPa. Light-weight steel according to claim 1 and 2, characterized by the elements in% by weight: C 0.6 to 1.4 Mn 10 to 30 Al> 4 to 15 Si 0.05 to 0.5 Sb 0.005 to 0.5 and at least one or more of the following carbide-forming elements in the stated contents (in% by weight) ): Cr max. > 0.1 to 4 Mo max. 0.05 to 1 Ti max. 0.01 to 0.1 V max. 0.005 to 0.3 Nb max. 0.005 to 0.3 W max. 0.005 to 0.5 Zr max. 0.001 to 0.3 remainder of iron including conventional steel-accompanying elements, with optional addition the following elements in% by weight: max. 5 Ni, max. 10 Co, max. 0.005 Ca, max. 0.01 B and 0.05 to 2 Cu, the steel having finely divided kappa-carbide precipitates and a product of tensile strength and elongation at break of at least 30,000 MPa% and a yield strength of at least 700 MPa and a tensile strength of at least 800 MPa. Process for producing a steel according to claims 1 to 5, comprising the steps: Casting of the steel by the continuous casting method, thin slab casting method or a horizontal or vertical strip casting process close to the final dimensions, Hot rolling of the cast slab or cast strip with a thickness of more than 5 mm to a uniform thickness or flexible hot rolling to different thicknesses, Optional cold rolling of the strip, which has been hot-rolled to a uniform thickness, or a maximum thickness of 5 mm, produced by casting close to the final dimensions, to a uniform thickness or flexible cold-rolling to different thicknesses, - Optional annealing of the hot or cold strip with the following parameters: Annealing temperature: 480 to 770 ° C, annealing time: 1 minute to 48 hours. A method according to claim 6, characterized in that the produced by means of a casting near final casting maximum 5 mm thick cast strip is cold rolled to a uniform thickness or cold rolled to different thicknesses flexible and then the cold strip is annealed with the following parameters: annealing temperature: 480 to 770 ° C, annealing time: 1 minute to 48 hours. A method according to claim 6 or 7, characterized in that in alloys with Al> 1 wt .-% preferably anneals are carried out at temperatures of 670 to 770 ° C with annealing times of 1 minute to 12 hours.