KR20180033202A - A moldable lightweight steel having improved mechanical properties and a method for producing a semi-finished product from said steel - Google Patents

Abstract

The present invention relates to a formable lightweight steel having improved mechanical properties and a high resistance to delayed hydrogen-induced cracking and hydrogen embrittlement, wherein the lightweight steel comprises the following components: C: 0.02 to 1.0 wt.% 3 to 30% by weight of Mn, 4% by weight or less of Si, 0.1% by weight of P, 0.1% by weight or less of S, 0.003 to 0.8% by weight of S, At least one of the following percentages of the following carbide forming components: Al: 15 wt% or less, Cr: 0.1 to 8 wt%, Mo: 0.05 to 2 wt%, Ti: 0.01 to 2 wt% 0.005 to 1% by weight of V, 0.005 to 1% by weight of Nb, 0.005 to 1% by weight of W and 0.001 to 0.3% by weight of Zr wherein the remainder is iron containing typical steel- Up to 5 wt% Ni, up to 10 wt% Co, up to 0.005 wt% Ca, up to 0.01 wt% B, and from 0.05 to 2 wt% Cu is optionally added.

Description

A moldable lightweight steel having improved mechanical properties and a method for producing a semi-finished product from said steel

The present invention relates to a moldable lightweight steel according to claim 1 having improved mechanical properties and a high resistance to delayed hydrogen-induced cracking. The present invention also relates to a method for producing semi-finished products from such steels.

Hereinafter, "semi-finished product" is understood to mean a hot or cold strip made from such steel or an intermediate or final product made therefrom, such as, for example, a tube.

In recent years, there has been a lot of development in the field of so-called lightweight steels, which are characterized by having a high specific strength and high ductility while maintaining high strength and toughness, and thus are of interest in automobile manufacturing.

In these steels where the initial state is austenite, the weight loss advantageous to the automotive industry is due to the high content of alloy components (Si and Al) having a specific weight much lower than the specific weight of iron, And is implemented.

Variable lightweight steels known from the published publication DE 10 2004 061 284 A1 include, for example, the following alloy compositions: 0.04 to 1.0 wt% C, 0.05 to 4.0 wt% Al, 0.05 to 6.0 wt% Or more of Si and 9.0 to 18.0% by weight or more of Mn, the remainder being iron including typical steel-related components. Optionally, Cr, Cu, Ti, Zr, V and Nb may be added if desired.

These lightweight steels have partially stabilized γ-mixed crystal microstructures with limited multi-TRIP effects and limited stacking fault energies, and this multi-TRIP effect can be attributed to the elliptic martensite martensite and retained austenite at further strains after permitting stress-induced or expansion-induced transformation of face-centered γ-mixed crystals (austenite) into hexagonal densest sphere packing .

A high degree of deformation is achieved by TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) properties of the steel.

However, in these steels and their corresponding steels, delayed embrittlement initiated by hydrogen and, as a consequence, crack formation can occur in the presence of residual stress in the material depending on its microstructure and strength.

To overcome this problem, it has already been proposed in the published publication DE 10 2004 061 284 A1 to limit the hydrogen content to less than 20 ppm, preferably to less than 5 ppm.

Although this suggestion may be helpful, the delayed crack formation effect may still be present even if the hydrogen content is set low, so far not enough. Moreover, it is possible to exceed the maximum fixed value of hydrogen for various reasons in steel manufacture, which can be tolerated in terms of alloys, but increases the risk of hydrogen embrittlement.

Austenite steels are known in the published publication WO 2011/154153 Al and are known to have good resistance to delayed crack formation. In addition to iron and impurities, the steel may contain 0.5 to 0.8 wt% C, 10 to 17 wt% Mn, at least 1.0 wt% Al, at least 0.5 wt% Si, at least 0.020 wt% S, P, 50 to 200 ppm N, and 0.050 to 0.25 wt% V.

Steel alloys for high strength cold-rolled steel are known from published publication WO 2009/142362 A1 and are likewise known to have improved resistance to delayed crack formation. In addition to the iron and the impurities, the steel may contain 0.05 to 0.3 wt% C, 0.3 to 1.6 wt% Si, 4.0 to 7.0 wt% Mn, 0.5 to 2.0 wt% Al, 0.01 to 0.1 wt% Cr, 0.1% Ni, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb, and 0.008% S or less.

In addition, lightweight steels with improved swelling power are known from published publication EP 2 128 293 A1 and include, in addition to iron and impurities, 0.2 to 0.8% by weight of C, 2 to 10% by weight of Mn, up to 0.2% , At least 0.015 wt.% S, 3.0 to 15 wt.% Al and a minimum of 0.01 wt.% N and a Mn / Al ratio of 0.4 to 1.0.

In addition, a method of successively heat treating a steel strip is disclosed in the published publication US 2009/0050622 A1, wherein the thickness of the strip depends on its length. These steel strips with various thicknesses are produced continuously by so-called flexible rolling. In this regard, the nip of the roller system is changed in an intended manner during the manufacture of the steel strip.

It is an object of the present invention to provide a general type of lightweight steel which does not have delay cracking or hydrogen embrittlement effects while maintaining very good mechanical properties (ductility and strength).

This object is implemented from the premise together with the characterization features of claim 1 and the method is implemented by the features of claim 6. Advantageous developments are disclosed in the dependent claims.

According to the teachings of the present invention, a formable lightweight steel having TRIP and TWIP properties comprises the following components:

C: 0.02 to 1.0 wt%

Mn: 3 to 30% by weight,

Si: 4% by weight or less,

P: at most 0.1% by weight,

S: at most 0.1% by weight,

N: at most 0.03% by weight,

0.003 to 0.8% by weight, advantageously at most 0.5% by weight of Sb, and

At least one of the following proportions of the following carbide forming components:

Al: 15 wt% or less,

Cr: more than 0.1 to 8 wt%

Mo: 0.05 to 2% by weight,

Ti: 0.01 to 2% by weight,

V: 0.005 to 1% by weight,

0.005 to 1% by weight of Nb,

W: 0.005 to 1% by weight, and

Zr: 0.001 to 0.3% by weight,

Wherein the remainder is iron containing typical steel-related components and contains the following components: up to 5 wt% Ni, up to 10 wt% Co, up to 0.005 wt% Ca, up to 0.01 wt% B, Cu by weight is optionally added.

Surprisingly, it has been found that the alloying of antimony (Sb) with the specified limits can delay the diffusion of the components, especially C, N and O, resulting in favorable effect of the material behavior with the intended heat treatment.

The addition of antimony results in the growth of slower carbides, thus the carbides are more finely distributed and precipitated into smaller sizes. As a result, alloying elements are used more effectively, leading to a more cost-effective alloy concept of improved mechanical properties and a clear improvement in terms of avoiding delayed hydrogen-induced cracking (delayed fracture) and hydrogen embrittlement.

It has proved advantageous that the ratio of Sb / C does not exceed a value of 1.5. Values in excess of 1.5 do not provide any additional advantage in terms of the present invention and predominantly cause negative effects such as loss of ductility and toughness due to deposition of antimony at grain boundaries.

According to the present invention, the mechanical properties are evaluated by determining the tensile strength and the fracture elongation of the product, and the tensile strength and the fracture elongation are dimensions for the performance of the material.

According to the test, in the case of the alloy according to the present invention, the addition of antimony results in a significantly higher tensile strength and fracture elongation due to the addition of antimony as compared with a steel alloy to which no antimony has been added, which makes it possible to produce more cost- Respectively.

It has also been demonstrated that the effect of the antimony described above can be significantly increased by heat treating the steel.

Products which are made from alloys according to the invention by modification and which can be, for example, hot strips, cold strips, flexibly hot rolled or cold strips, tubes or body components, to achieve further improvements in the desired properties Or the semi-finished product is advantageously applied to the heat treatment at 480 to 770 캜 for 1 minute to 48 hours, for example, in a batch type annealing process at a very long annealing time, or continuously annealing at a very short annealing time Process.

However, such annealing may also occur prior to the final shaping to form the finished product on, for example, the cold strip, which will subsequently be further processed. The annealing time can therefore be adjusted in a flexible manner for the manufacturing process. In addition to the initial annealing to the semi-finished product, the annealing of the final product may cause further improvement of the material properties.

Furthermore, the present invention is achieved by a method for manufacturing a steel according to the present invention,

- casting the steel into a continuous casting process or a thin slab casting process or a horizontal or vertical strip casting process which approximates the final dimensions,

Hot rolling a cast slab or casting strip having a thickness of more than 5 mm to a single thickness or hot-rolling a casting slab or casting strip having a thickness of more than 5 mm to different thicknesses,

Hot strips rolled to a single thickness or casting strips produced by a casting process approaching the final dimensions and which have a minimum thickness of 5 mm are optionally cold rolled to a single thickness or rolled to a single thickness or to a final dimension Flexibly cold-rolling the cast strips produced by the casting process in proximity and having a thickness of at least 5 mm to different thicknesses, and

- optionally annealing the hot strip or cold strip at the following parameters: an annealing temperature of 480 to 770 占 폚 and 1 minute to 48 hours.

With regard to the cast strip produced by the casting process approaching to the final dimensions and having a minimum thickness of 5 mm, it can either be cold rolled into a single thickness or flexibly cold rolled to different thicknesses, , I.e. an annealing temperature of 480 to 770 DEG C and a duration of 1 minute to 48 hours.

In an alloy having an Al content of greater than 1% by weight, the annealing treatment is preferably carried out at a temperature of 670 to 770 DEG C for an annealing time of 1 minute to 12 hours, which means that the lower temperature and the longer annealing time, This results in a decrease in fracture elongation.

For annealing itself to hot strips, cold strips and flexibly rolled strips, the continuous annealing process is preferably used for a short annealing time and the batch annealing process is preferably used for a long annealing time. For other products and semi-finished products, other annealing devices, such as muffle furnaces, having predetermined parameters may be used.

According to the present invention, it is possible to produce cost-effective Sb-alloy steels having a higher manganese content, wherein the steels exhibit improved tensile strength and fracture properties compared to non-Sb-alloy steels with the same chemical composition and higher manganese content Lt; / RTI >

Moreover, the behavior toward hydrogen (delayed crack formation and hydrogen embrittlement) is also significantly improved by adding antimony.

The improvement in material properties is caused by antimony which delays the diffusion of carbon and aluminum. In addition, antimony increases the interfacial energy, which makes the carbide more finely distributed. Thus, the reduction of carbon diffusion delays the local floatation of carbon in the grain boundaries and microstructures, and delays the formation of kappa-carbides with aluminum, especially with V, Nb, Mo, Cr, W, Zr and Ti Thereby delaying the local formation of larger carbides. Thus, the homogeneity of the material is improved with the above-described positive effects on mechanical properties and resistance to delayed crack formation and hydrogen embrittlement. Precipitation of finely distributed carbides leads to grain refinement in the microstructure, which results in improved behavior for hydrogen-induced negative effects (delayed crack formation and hydrogen embrittlement), increased strength and toughness And improvement in expansion properties.

The addition of a small amount, i. E. Of up to 0.8% by weight of antimony in accordance with the present invention, significantly improves the behavior of the material for hydrogen-induced effects.

In contrast, the addition of excessive amounts of antimony causes undesirable serious precipitation of antimony in the grain boundary, thus lowering toughness and expansion properties. In order for antimony to be effective, a ratio of at least 30 ppm is required. However, if the antimony content exceeds 0.8% by weight, the material becomes brittle and therefore should be avoided. Optimally, the maximum content of antimony is 0.5% by weight.

Small carbides (mainly Cr-, Mo-, Ti-, Nb-, V-, W-, Zr- and Kappa-carbides) precipitated in a much finer distribution compared to the prior art, ≪ / RTI > which potentially allows a reduction in the amount of addition. In addition, the reduction of carbon diffusion and grain growth due to the alloying of antimony increases the process window for the heat treatment required in accordance with the present invention. That is, the steel responds in a less sensitive manner to process variations (temperature and time) with respect to the resulting mechanical properties.

Hereinafter, the positive effects of the alloy components used in accordance with the present invention will be disclosed.

Al: improves strength and expansion properties, decreases specific density, and affects the conversion behavior of the alloy according to the present invention. An Al content of greater than 15% by weight impairs the expandability, and for this reason its maximum content is set at 15% by weight. A high Al content of greater than or equal to 4 wt% acts as a carbide former for the kappa-carbide with a high C content of 0.6 wt%. At less than 4% by weight, Al delays the precipitation of carbides.

B: Improves strength and stabilizes austenite. A content of greater than 0.01% by weight results in the embrittlement of the material, and for this reason the maximum content thereof is set at 0.01% by weight.

C: Required to form carbides, stabilize austenite, and increase strength. A C content of more than 1% by weight impairs the weldability and leads to the precipitation of undesirable large carbides, thus causing deterioration of the expansion and toughness properties, and for this reason its maximum content is set at 1% by weight. In order to achieve sufficient strength for the material, a minimum addition of 0.01% by weight is required.

Ca: Used to modify non-metallic oxidic inclusions that can result in non-uniformity and unwanted material failure. Due to its high vapor pressure in liquid steel, the above content is limited to a minimum of 0.005% by weight.

Co: Increases the strength of the steel, stabilizes the austenite, and improves the heat resistance. A content of more than 10% by weight impairs the properties of the expanded material, and for this reason the maximum content thereof is set at 10% by weight.

Cr: improves strength, reduces conversion, delays formation of ferrite and perlite, and forms carbides. The maximum content is set at 8 wt% because higher contents cause deterioration of the expansion properties.

Cu: Reduces the corrosion rate and increases the strength. The content of more than 2% by weight inhibits productivity by forming a low melting point phase during casting and hot rolling, and for this reason its maximum content is set at 2% by weight.

Mn: Stabilizes austenite, increases strength and toughness, and enables strain-induced martensite formation and / or twinning in alloys according to the present invention. A content of less than 3% by weight is not sufficient to stabilize the austenite, thus impairing the swelling properties, whereas a further advantage can not be expected in the content of more than 30% by weight, Becomes more difficult.

Mo: acts as a powerful carbide former and increases strength. The Mo content in excess of 2% by weight impairs the expansion properties, and for this reason the maximum content thereof is set at 2% by weight.

Nb + V: By forming a carbide, it acts in a particle refining manner, thereby improving strength, toughness and expansion properties at the same time. The content of more than 1% by weight does not provide any additional advantage.

Ni: Stabilizes austenite and improves swelling properties especially at low application temperatures. The addition of Ni in excess of 5% by weight does not provide any additional advantage.

Si: Delays the diffusion of carbon, reduces the degree of confidentiality, and increases strength and expansion properties and toughness properties. In addition, improvements in cold-rollability can be seen by alloying Si. The content of more than 4% by weight results in embrittlement of the material and adversely affects the possibility of hot and cold rolling, and for this reason its maximum content is set at 4% by weight.

Ti: acts as a carbide forming agent in a particle refining manner, which simultaneously improves strength, toughness and expansion properties, and reduces intercrystalline corrosion. A Ti content of more than 2% by weight impairs the swelling properties, and for this reason the maximum content thereof is set at 2% by weight.

W: acts as a carbide forming agent and increases strength and heat resistance. A W content of more than 1% by weight impairs the swelling properties, and for this reason the maximum content thereof is set at 1% by weight.

Zr: acts as a carbide forming agent and improves strength. A Zr content of greater than 0.3% by weight damages, and for this reason its maximum content is set at 0.3% by weight.

Advantageous alloy combinations are disclosed in claims 3 to 5.

The alloy claimed in claim 3 has a product of tensile strength and failure elongation of at least 20,000 MPa% and a tensile strength of at least 800 MPa by using optimized heat treatment parameters (see Tables 1 to 4). The product of the tensile strength and the fracture elongation is a measure of the performance of the material at the time of deformation.

The positive effects of alloying antimony are already known in the art, although the heat treatment at 680 ° C for 10 minutes in Table 2 still does not provide an optimal value for the product of tensile strength and failure elongation of at least 20,000 MPa%.

The alloy claimed in claim 4 has a product of a tensile strength and a fracture elongation of at least 30,000 MPa% and a tensile strength of at least 800 MPa.

The alloy claimed in claim 5 has finely distributed kappa-carbide precipitation and has a product of tensile strength and failure elongation of at least 30,000 MPa%, a yield strength of at least 700 MPa and a tensile strength of at least 800 MPa.

The alloy compositions discussed above are provided in Table 1. The content of Sb and the addition of Nb are changed, and the remaining chemical compositions are approximately the same.

Hot strips having a thickness of 2 mm were produced from these steels and then cooled in the atmosphere after hot rolling. The test specimens were removed from these hot strips and their tensile strength and failure elongation were determined.

The results for the product of the tensile strength and the fracture elongation are shown in Tables 2 to 4 in which the heat treatment with the product of the highest tensile strength and the fracture elongation is considered to be most desirable for the individual alloys. It is apparent that steels alloyed with Sb according to the present invention always have a product of tensile strength and fracture elongation higher than comparative alloys.

Alloy composition alloy C Mn Al Si Cr V Sb Other L1 0.19 7.1 2 0.55 One 0.05 0 L2 0.19 7.1 2 0.55 One 0.05 0.012 L3 0.19 7.1 2 0.55 One 0.05 0.027 L4 0.19 7.1 2 0.55 One 0.05 0.041 L5 0.21 6.3 2 0.2 One 0.06 0 Nb: 0.05 L6 0.21 6.3 2 0.2 One 0.05 0.039 Nb: 0.05 L7 0.25 7.9 One 0.5 0.9 0.08 0 L8 0.25 7.9 One 0.5 0.9 0.08 0.04

The product of the determined tensile strength and the fracture elongation of L1 to L4 Heat treatment TS * Hand L1 L2 L3 L4 650 ° C, 24 hours 22,453 23,195 23,772 22,633 680 ° C, 10 minutes 15,263 16,695 16,830 16,111 680 ° C, 5 hours 27,162 27,997 28,258 29,000 680 ℃, 24 hours 26,660 28,985 30,546 29,720

L5 and L6 times the determined tensile strength and fracture elongation Heat treatment TS * Hand L5 L6 690 ° C, 3 hours 19,368 21,449 750 ° C, 10 minutes 22,751 25,502 500 ° C, 10 minutes 23,525 26,737

L7 and L8 times the determined tensile strength and failure elongation Heat treatment TS * Hand L7 L8 650 ° C, 24 hours 18,378 20,457

Claims (8)

As a formable lightweight steel with improved mechanical properties and high resistance to delayed hydrogen-induced cracking and hydrogen embrittlement,
Having the following components:
C: 0.02 to 1.0 wt%
Mn: 3 to 30% by weight,
Si: 4% by weight or less,
P: at most 0.1% by weight,
S: at most 0.1% by weight,
N: at most 0.03% by weight,
0.003 to 0.8% by weight of Sb, particularly advantageously at most 0.5% by weight, and
At least one of the following proportions of the following carbide forming components:
Al: 15 wt% or less,
Cr: more than 0.1 to 8 wt%
Mo: 0.05 to 2% by weight,
Ti: 0.01 to 2% by weight,
V: 0.005 to 1% by weight,
0.005 to 1% by weight of Nb,
W: 0.005 to 1% by weight, and
Zr: 0.001 to 0.3% by weight,
Wherein the remainder is iron containing typical steel-related components and contains the following components: up to 5 wt% Ni, up to 10 wt% Co, up to 0.005 wt% Ca, up to 0.01 wt% B, Lightweight steel in which Cu in weight% is optionally added. The lightweight steel according to claim 1, wherein the ratio of Sb / C is 1.5 or less. 3. The composition according to claim 1 or 2, having the following components:
0.03 to 0.5% by weight of C, particularly advantageously 0.1 to 0.35% by weight,
Mn: 3 to 10% by weight, particularly advantageously 5 to 9% by weight,
Al: 0.1 to 4% by weight, particularly advantageously 1 to 3.5% by weight,
Si: 0.1 to 3% by weight, particularly advantageously 0.1 to 1% by weight,
0.005 to 0.3% by weight of Sb, particularly advantageously 0.01 to 0.1% by weight,
Cr: more than 0.1 to 5% by weight, particularly advantageously 0.5 to 4% by weight, and
V: 0.005 to 1% by weight, particularly advantageously 0.02 to 0.1% by weight,
Wherein the steel has a product of a tensile strength and a failure elongation of at least 20,000 MPa and a tensile strength of at least 800 MPa. 3. The composition according to claim 1 or 2, having the following components:
0.4 to 0.9% by weight of C,
Mn: 12 to 18% by weight,
Al: 0.5 to 4% by weight,
0.5 to 3% by weight of Si,
0.005 to 0.4% by weight of Sb, and
At least one of the following proportions of the following carbide forming components:
Cr: at most 0.1 to 4% by weight,
Mo: at most 0.05 to 1% by weight,
Ti: at most 0.01 to 0.1% by weight,
V: at most 0.005 to 0.3% by weight,
Nb: at most 0.005 to 0.3% by weight,
W: at most 0.005 to 0.5% by weight, and
Zr: at most 0.001 to 0.3% by weight,
Wherein the remainder is iron containing typical steel-related components and contains the following components: up to 5 wt% Ni, up to 10 wt% Co, up to 0.005 wt% Ca, up to 0.01 wt% B, By weight Cu is optionally added,
Wherein the steel has a product of a tensile strength and a fracture elongation of at least 30,000 MPa and a tensile strength of at least 800 MPa. 3. The composition according to claim 1 or 2, having the following components:
0.6 to 1.4% by weight of C,
Mn: 10 to 30% by weight,
Al: more than 4 to 15% by weight,
Si: 0.05 to 0.5% by weight,
0.005 to 0.5% by weight of Sb, and
At least one of the following proportions of the following carbide forming components:
Cr: at most 0.1 to 4% by weight,
Mo: at most 0.05 to 1% by weight,
Ti: at most 0.01 to 0.1% by weight,
V: at most 0.005 to 0.3% by weight,
Nb: at most 0.005 to 0.3% by weight,
W: at most 0.005 to 0.5% by weight, and
Zr: at most 0.001 to 0.3% by weight,
Wherein the remainder is iron containing typical steel-related components and contains the following components: up to 5 wt% Ni, up to 10 wt% Co, up to 0.005 wt% Ca, up to 0.01 wt% B, By weight Cu is optionally added,
Characterized in that the steel has finely distributed kappa-carbide precipitation and has a product of a tensile strength and a fracture elongation of at least 30,000 MPa, a yield strength of at least 700 MPa and a tensile strength of at least 800 MPa. . A method for manufacturing a steel as claimed in any one of claims 1 to 5,
Casting the steel into a continuous casting process, a thin slab casting process or a horizontal or vertical strip casting process, bringing it closer to final dimensions,
- hot rolling a cast slab or casting strip having a thickness of more than 5 mm into a single thickness or by hot rolling to a different thickness,
Optionally cold-rolling the cast strip to a single thickness, or cold-rolling the cast strip to a thickness of at least 5 mm, to a different thickness, and cold-rolling the cast strip to a thickness of at least 5 mm,
- optionally annealing the hot strip or cold strip at the following parameters: an annealing temperature of 480 to 770 占 폚 and 1 minute to 48 hours. 7. The method of claim 6, wherein the cast strips produced by a casting process approaching final dimensions and having a thickness of at least 5 mm are either cold rolled to a single thickness or flexibly cold rolled to different thicknesses, Lt; RTI ID = 0.0 > 770 C < / RTI > and an annealing period of 1 minute to 48 hours. The method according to claim 6 or 7, wherein annealing in an alloy with 1 wt.% Al is preferably carried out at a temperature of 670 to 770 DEG C for an annealing time of 1 minute to 12 hours.