WO2017013193A1 - Formable lightweight steel with improved mechanical properties and method for producing semi-finished products from said steel - Google Patents

Abstract

The invention relates to a formable lightweight steel having improved mechanical properties and high resistance to delayed hydrogen-induced cracking and hydrogen embrittlement, comprising the following 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, particularly advantageously to 0.5, as well as at least one or more of the following carbide-forming elements in the specified proportions (in wt.%): Al ≤15; Cr >0.1 to 8; Mo 0.05 to 2; Ti 0.01 to 2; V 0.005 to 1; Nb 0.00 to 1; W 0.005 to 1; Zr 0.001 to 0.3; with the remainder consisting of iron including the usual steel-accompanying elements, with the optional addition of the following elements in wt.%: Ni max. 5, Co max. 10, Ca max. 0.005, B max. 0.01 and Cu 0.05 to 2. The invention also relates to a method for producing said lightweight steel.

Description

 REFORMABLE LIGHTWEIGHT BLADE WITH IMPROVED MECHANICAL PROPERTIES AND METHOD FOR PRODUCING SEMI-FINISHED STEEL FROM THIS 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. Furthermore, the invention 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 far below the specific gravity of iron achieves a weight reduction advantageous to the automobile industry while retaining the previous design construction.

The deformable lightweight structural steel known from the published patent application DE 10 2004 061 284 A1 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, the remainder being iron including conventional steel accompanying elements. Optionally, depending on

Requirement Cr, Cu, Ti, Zr, V and Nb are added.

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

Residual stresses in the material as a function of the microstructure and the strength of a delayed release caused by hydrogen embrittlement and as a result, cracking occur.

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

This suggestion is helpful but not sufficient, because even with low set hydrogen levels still the effect of delayed

Cracking can occur. Moreover, steelmaking may, for various reasons, exceed the established maximum value for hydrogen which, while tolerated by alloy, increases the risk of hydrogen embrittlement.

Publication WO 201 1/154153 A1 discloses an austenitic steel which is said to have outstanding 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 AI, 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.

Publication WO 2009/142362 A1 discloses a steel alloy for a high-strength cold-rolled steel sheet, which should also have improved resistance to delayed cracking. 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 AI, 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, published specification EP 2 128 293 A1 discloses a lightweight structural steel with improved elongation, comprising iron and impurities in wt.%: 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, at most 0.01 N and a ratio Mn / Al of 0.4 to 1.0.

Furthermore, the published patent application US 2009/0050622 A1 already describes a method for the continuous heat treatment of strip steel whose strip thickness varies along its length. This steel strip with varying thickness will

continuously produced by so-called flexible rolling. Here, a roll gap of a rolling mill during the production of the steel strip is selectively varied.

The object of the invention is to provide a lightweight steel of the generic type, while maintaining very good mechanical properties

(Ductility, strength) does not have the effect of delayed cracking or

Having hydrogen embrittlement.

This object is based on the preamble in connection 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 teachings of the invention, the deformable lightweight steel with TRIP and

TWIP properties 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):

 AI <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 slower and thus finer and less precipitated. This will be

Alloy elements used more effectively, resulting in lower cost

Alloying concepts result in improved mechanical properties and significant improvement with respect to avoiding delayed hydrogen-induced cracking (delayed fracture) and hydrogen embrittlement.

It has proved to be favorable if the ratio of Sb / C does not exceed a value of 1.5. Values above 1.5 produce no further advantage within the meaning of the invention and, above all, cause negative effects such as loss of ductility and toughness due to precipitation 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 markedly 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 advantageously heat-treated 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

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.

Furthermore, the invention by a method for producing the

Steel realized according to the invention with the following steps:

 - casting the steel by continuous casting or

Thin-slab casting or a near-horizontal horizontal or vertical strip casting process,

 - Hot rolling the cast slab or the cast strip with a thickness of more than 5 mm to a uniform thickness or flexible

Hot rolling the cast slab or the cast strip with a thickness of more than 5 mm to different thicknesses,

Optional cold rolling of the hot rolled strip rolled to a uniform thickness, or a maximum of 5 mm thick cast strip produced by means of casting close to the final dimensions, to a uniform thickness or optional flexible cold rolling of the hot strip rolled to a uniform thickness or by

close to the size of the casting process, cast to a maximum thickness of 5 mm, 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 casting process close to the final dimensions, 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 to 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 to 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 alloy is higher

Manganese-containing steels are possible, which have a better tensile strength and elongation at break than non-Sb-alloyed higher manganese-containing steels with 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 conjunction 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 grain refining in the microstructure, which leads to an improvement in the behavior with respect to hydrogen-related negative effects (delayed cracking,

Hydrogen embrittlement), as well as an increase in strength and improvement in toughness and elongation properties. By the inventive addition of antimony in small amounts up to max. 0.8% by weight is the behavior of the material compared to hydrogen

Influences therefore significantly improved.

The addition of large amounts of antimony, however, causes an undesirably high excretion 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 compared to the prior art much finer distributed

Precipitated carbides (predominantly Cr, Mo, Ti, Nb, V, W, Zr and Kappa carbides) improve the utilization of the corresponding alloying elements, potentially allowing a reduction in the amount of addition. 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 necessary according to the invention is increased, that is, the steel is less sensitive to the resulting mechanical properties

Process fluctuations (temperature, time).

Hereinafter, the positive effects of the invention used

Alloy elements described:

AI: Improves the strength and elongation properties, lowers the specific gravity, and influences the conversion behavior of the invention Alloys. Al contents of more than 15% by weight deteriorate the

Elongation properties, which is why a maximum content of 15 wt .-% is set. High Al contents of greater than or equal to 4% by weight, in combination with high C contents of greater than or equal to 0.6% by weight, act as carbide formers for kappa carbides. Below 4% by weight, AI retards 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 wt .-% C worsen the

Welding properties and lead to the elimination of undesirably large carbides and thus to the deterioration of the elongation and toughness properties, which is why a maximum content of 1 wt .-% is set. 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

Temperature strength. Contents of over 10 wt .-% worsen the

 Elongation properties, which is why a maximum content of 10 wt .-% is set.

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 .-% worsen the manufacturability by forming low-melting phases during casting and hot rolling, which is why a maximum content of 2 wt .-% is determined.

Mn: Stabilizes austenite, increases strength and toughness, and allows for deformation-induced martensite and / or twinning in the

alloys according to the 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 of over 2 wt .-% worsen the

Elongation properties, which is why a maximum content of 2 wt .-% is set. W: acts as a carbide former and increases strength and heat resistance. Content 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 more than 0.3% by weight deteriorate the elongation properties, therefore, a maximum content of 0.3% by weight is set.

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) 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% provides, but one recognizes here already the positive effect of

Admixture of antimony.

An alloy according to claim 4 comprises 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. These hot strips became samples taken from it and determines the tensile strength and elongation at break.

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

Table 2: Determined products of tensile strength and elongation at break L1 to L4

Table 3: Determined products of tensile strength and elongation at break L5 and L6

Table 4: determined product of tensile strength and elongation at break L7 and L8

Claims

claims 1 . 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): AI <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. 2. Lightweight steel according to claim 1, characterized in that the ratio Sb / C is less than or equal to 1, 5. 3. Lightweight steel according to claim 1 and 2, characterized by the elements in wt .-%:  C is 0.03 to 0.5, more preferably 0.1 to 0.35 Mn 3 to 10, in particular advantageous 5 to 9 AI is 0.1 to 4, particularly advantageously 1 to 3.5 Si is 0.1 to 3, particularly preferably 0.1 to 1 Sb 0.005 to 0, 3, in particular 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. 4. Lightweight steel according to claim 1 and 2, characterized by the elements in wt .-%:  C 0.4 to 0.9 Mn 12 to 18 AI 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. 5. lightweight steel according to claim 1 and 2, characterized by the elements in wt .-%:  C 0.6 to 1, 4 Mn 10 to 30 AI> 4 to 15 Si is 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 being 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. 6. A process for producing a steel according to claims 1 to 5, comprising the steps of: 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. 7. The method according to claim 6, characterized in that the produced by means of a near-final casting method, a maximum of 5 mm thick cast strip is cold rolled to a uniform thickness or is flexibly cold-rolled to different thicknesses and then annealed the cold strip with the following parameters becomes:  Annealing temperature: 480 to 770 ° C, annealing time: 1 minute to 48 hours. 8. The method according to claim 6 or 7, characterized in that for 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.