WO2014040585A1

WO2014040585A1 - Steel alloy for a low-alloy, high-strength steel

Info

Publication number: WO2014040585A1
Application number: PCT/DE2013/000519
Authority: WO
Inventors: Philippe SCHAFFNIT; Jürgen KLABBERS-HEIMANN; Joachim Konrad
Original assignee: Salzgitter Mannesmann Precision Gmbh; Ilsenburger Grobblech Gmbh
Priority date: 2012-09-14
Filing date: 2013-08-28
Publication date: 2014-03-20
Also published as: AU2018201165A1; PE20151042A1; PL2895635T3; US20200131608A1; JP2015533942A; RU2620216C2; UA116111C2; BR112015005216A2; US20150267282A1; EP2895635B1; EP2895635A1; SI2895635T1; CA2881686A1; TR201903460T4; CL2015000634A1; RU2015113522A; MX2015003103A; AR092556A1; AU2013314787A1; KR102079612B1

Abstract

The invention relates to a low-alloy, high-strength, carbide-free bainitic steel for producing bands, sheets and tubes having the following chemical composition (in % by weight): 0.10 - 0.70 C; 0.25 - 4.00 Si; 0.05 - 3.00 Al; 1.00 - 3.00 Mn; 0.10 - 2.00 Cr; 0.001 - 0.50 Nb; 0.001 - 0.025 N; max. 0.15 P; max. 0.05 S; remainder iron having steel production-related impurities to one or more elements from Mo, Ni, Co, W, Nb, Ti or V and Zr are optionally added and rare earths provided that in order to avoid primary precipitations of Al the condition Al x N < 5 x 10^-3 (% by weight) and in order to suppress cementite formation the condition Si + Al > 4 x C (% by weight) are satisfied.

Description

Steel alloy for a low-alloy, high-strength steel Description

The invention relates to a steel alloy for a low alloy, high strength and at the same time tough steel with excellent wear resistance according to claim 1.

In particular, the invention relates to tubes made of this alloy, strips, and sheets of which z. B. components for the automotive industry, such as body panels,

Components of support structures or airbag tubes and cylinder tubes are produced. In the field of the construction equipment industry, e.g. with high wear requirements

Wear plates made of this alloy can be used for excavator buckets. Also, such steels are used for applications where suddenly occurring

Impact energies must be absorbed, e.g. as bulletproof armor.

Tubes made from this alloy can be designed as welded, hot or cold strip or seamless tubes, which may occasionally have deviating from the circular cross-sections.

Construction tubes or sheets of this steel alloy can also be used for highly stressed welded steel structures, for example in crane, bridge, ship, hoist and truck construction.

The demands for ever higher strength and improved processing and component properties while reducing weight and / or cost, u.a. led to the development of ultrafine-grained duplex steels, which are also known as carbide-free bainitic steels under the term "super bainite." The formation of such a structure, consisting of bainitic ferrite with retained austenite lamellae, is known in the art

Difference to the upper and lower Bainitgefüge, schematically outlined in Figure 1.

Characteristic of these steels is e.g. a strength of 1000 to about 2000 MPa, depending on the strength of an elongation at break of at least 5% and a very finely (nano-) structured bainitic structure with shares of retained austenite.

The approach to creating this finest microstructure is in the phase transformation at low temperatures in the bainite while avoiding the excretion of cementite and a Martensitbildung. Suppression of carbides precipitated in bainite, such as cementite, is necessary because, on the one hand, these act as a possible crack trigger strongly embrittling and thus the required toughness can not be achieved and on the other hand can not be adjusted to achieve the properties of the invention shares of stabilized austenite.

However, the economic use of these steels is hampered by the fact that at these low transition temperatures the kinetics of transformation is greatly slowed down, which in particular increases with increasing alloying composition

Carbon content leads to longer isothermal hold times of many hours to one or more days. Such long process times, however, are unsustainable for cost-effective component manufacturing, so that solutions of an alloyed nature have been sought to accelerate the conversion.

An alloy composition requiring such a long isothermal conversion time of up to 48 hours is known from WO 2009/075494 A1. In addition, it is disadvantageous that in addition to carbon and iron, this steel has expensive additions, e.g. on nickel, molybdenum, boron and titanium and the achievable toughness for the described

Application areas are not sufficiently high.

Carbide-free bainitic steels for rails are e.g. known from DE 696 31 953 T2. The steel alloy disclosed therein, in addition to additions of manganese, chromium and other elements such as molybdenum, nickel, vanadium, tungsten, titanium and boron, a silicon content between 1 and 3%.

It is also mentioned in this document that in addition to silicon, an addition of aluminum can reduce or suppress the formation of carbides in bainite and stabilize the remaining retained austenite. Also, with this steel, the disadvantage of a long conversion time can be overcome, with a corresponding bainitic structure can be produced only with a continuous cooling in air (air hardening).

This steel is designed for the requirements of highly wear-stressed rails, but for tapes, sheets and tubes for the stated application uneconomical or not applicable, since in addition to the requirements for wear resistance, both the strength and toughness requirements are met. In addition, the cross-sectional dimensions of the rails differ significantly from those of the strips, sheets and tubes due to their compact cross-section, which means that the alloy concept can be adapted to the material properties to be achieved after the Air cooling of the steel required. A disadvantage of the known steel is also the expensive addition of titanium and other alloying elements such as nickel, molybdenum and tungsten.

Another problem with the known steel is that no information is made on the nitrogen content, which exerts a negative influence on the material properties, in particular with aluminum additions by the formation of aluminum nitrides.

Through the addition of aluminum, coarse aluminum nitrides are formed by the great affinity for the nitrogen present in the steel during solidification, which precipitate primarily in the steel, which is very detrimental to the ductility, impact energy, the bursting behavior and the fatigue strength of the steel affects and thus the

mechanical properties deteriorated significantly. Higher levels of nitrogen and aluminum in the steel add to this effect.

This has the consequence that this known steel alloy, in place of silicon

Aluminum or additional aluminum is alloyed, for the practice

becomes unusable because the excretion amount and size of harmful

Aluminum Nitrides depends on the existing in the steel nitrogen and aluminum content and since the nitrogen content is disregarded, the concrete

Material properties are unpredictable. In addition, the achievable toughness for the described application of the invention are not sufficiently high in this steel.

The requirements to be fulfilled for the mechanical properties of the steel alloy can be summarized as follows.

Strength: 1250 to 2500 MPa

Elongation at break: over 12%

Notched impact strength at -20 ° C: at least 15 J

The object of the invention is to provide a steel alloy for a low-alloy, high-strength, tough and wear-resistant carbide bainitic steel for the production of strips, sheets and tubes, on the one hand cheaper than the known steel alloys and on the other hand uniform, the requirements of the material properties such Strength, elongation at break, toughness etc. guaranteed. moreover These material properties should be achieved by air hardening even when cooling to still air.

This object is achieved starting from the preamble in conjunction with the characterizing features of claim 1. Advantageous developments are the subject of dependent claims.

According to the teachings of the invention, a steel alloy with the following chemical

Composition proposed (in% by weight):

0, 10 - 0.70 C

0.25 - 4.00 Si

0.05 - 3.00 AI

1, 00 - 3,00 Mn

0, 10 - 2.00 Cr

0.001-0.50 Nb

0.001-0.025 N

Max. 0, 15 p

Max. 0,05 S

Remaining iron with melting impurities with optional addition of one or more elements of Mo, Ni, Co, W, Nb, Ti, or V as well as Zr and rare earths with the proviso that to avoid primary excretions of AIN the condition

Al x N <5 x 10 ^-3 (wt%) and for suppressing cementite formation, the condition of Si + Al> 4 x C (wt%) is satisfied.

Optionally, even rare earths and reactive elements such as Ce, Hf, La, Re, Sc and / or Y can be alloyed with a total of up to 1 wt .-%.

In Luppen-, or slab state steels according to the invention already after

Cooling in air has a strength (R _m ) of more than 1250 MPa, an elongation at break of more than 12% and a toughness (KBZ) at -20 ° C of at least 15 J (see Table 1). The structure consists of carbide-free bainite and retained austenite with a content of at least 75% bainitic ferrite, at least 10% retained austenite and up to a maximum of 5% martensite (or martensite phase and / or decomposed austenite).

The steel alloy according to the invention is based on the development of the

DE 696 31 953 T2 and WO 2009/075494 A1 known carbide bainitic steel on. The experiments carried out in the course of the present invention have surprisingly shown that in comparison with the known steel alloy to achieve the required material properties even at an air hardening by a targeted addition of aluminum in the range 0.05 to 3.0 wt .-% and niobium in Range 0.001 to 0.5 wt .-% in addition to excellent material and wear resistance very good toughness properties can be achieved. In particular, the addition of niobium brings about a significant improvement of the toughness properties by grain refining, so that this alloy optimally meets both high demands on the mechanical properties and on the wear resistance.

By an advantageous addition of chromium in the range from 0.10 to 2.00 wt.%, Moreover, the kinetics of ferrite formation can be decisively controlled so that the formation of coarse polygonal ferrite grains, which can negatively influence the material properties, is effectively avoided. Crucial here is the interaction of aluminum and chrome. While aluminum accelerates ferritic and bainitic transformation, the addition of chromium retards ferritic transformation (see Figure 2). Through a specific combination of these two elements, both the kinetics of ferrite and bainite formation can be controlled.

In addition to the known beneficial effects of adding aluminum to the prevention of carbide precipitations in bainite, experiments have also shown that the addition of aluminum compared to silicon significantly reduces the kinetics of bainitic

Transformation accelerated. This also increases with increasing aluminum contents, which means that the toughness and strength of the steel according to the invention after continuous cooling are markedly improved in comparison with steels alloyed with silicon only, ie higher toughness and strength values can be achieved. Cooling rates greater than 10 ° C / s are advantageous in order to achieve the required combination of mechanical properties even with thicker sheets (for example, from 10 mm); The required mechanical properties can also be advantageous by cooling in still air with thinner sheets or by adjusting the

Achieve alloy concept. The influence of the various alloying elements on the kinetics of the transformation is shown in FIG. 2. The effects of C, Si, Al, Mn, Cr and Mo on the transformation kinetics of ferrite, perlite and bainite and on the martensite start temperature are shown schematically.

According to the invention, however, in comparison to the known steel to achieve these advantageous properties, it is imperative that the nitrogen content be as specified Upper limit of 0.025 wt .-%, better still 0.015 wt .-%, or optimally 0.010 wt .-% does not exceed the number and size of harmful aluminum nitrides as

In addition to minimize primary precipitation in the steel, in addition the condition

AI x N <5 x 10 ^"3 (wt .-%) must be fulfilled. On the other hand, a minimum nitrogen content of 0.001 wt .-%, optimally 0.0020, required for a

Toughening by grain refinement to allow necessary niobium carbonitride formation.

The investigated alloy compositions and the determined mechanical characteristics are given in Table 1. All samples were heated to about 950 ° C and then cooled in still air or accelerated. The required cooling rate is made dependent on the sheet thickness and the composition. As the results of the mechanical sampling show, the required properties could not be achieved with the test melt 14 because of the too low Cr content. The test melt 6 according to the invention fulfilled the requirements because of the larger sheet thicknesses of 12 mm only by accelerated cooling. Typical temperature profiles for cooling in still air or with quenching are shown in FIG.

In FIG. 4, some of the investigated experimental melts and their mechanical characteristics and cooling conditions are compared to common and high-strength ones

Steel materials shown. It becomes clear that with the developed steel alloy the range of high-strength materials at the same time significantly improved

Stretching properties is included.

Table 1: Alloy compositions in wt .-% and mechanical properties of the alloys studied

The results impressively confirm the excellent mechanical properties (strength and toughness of the steel alloy according to the invention)

Semi-finished products such as slabs or slabs) in the cured state (Table 1).

As an essential element aluminum plays a role, which in addition to the acceleration of the conversion kinetics in combination with silicon suppresses the carbide precipitation in bainite, thereby the retained austenite is stabilized, since carbon has only a low solubility in the ferrite. A high proportion of retained austenite of at least 10% in bainite causes, in addition to the extremely fine lamellar microstructure, the excellent mechanical properties. Scanning electron micrographs were the various

Structural constituents determined, with a mean fin spacing of 300 nm could be determined. A schematic representation of a former Austenitk grain with

Substructure (such as subgrains) with fine lamellar microstructure is outlined in FIG. Nb (C, N) precipitation stabilizes the former austenite grain structure.

With appropriate proportions of retained austenite, the so-called TRIP effect can also be used to advantage. Steels, commonly referred to as TRIP ("Transformation Induced Plasticity"), are steels that also have very high strength and high ductility, making them particularly suitable for cold forming. These properties are retained thanks to their special microscopic structure, which inhibits the strain-induced martensite formation and the associated solidification and increases the ductility. The effect of the TRIP effect is optimal at a residual austenite content of about 10 to 20% in the microstructure.

The alloying concept according to the invention will be explained in more detail below.

Carbon: for reasons of sufficient strength of the material, the minimum content should not be less than 0.10% by weight. In view of a sufficiently low martensite start temperature and thus the setting of a very fine microstructure but still good weldability, the carbon content should not exceed 0.70 wt .-%. Carbon contents between 0.15 and 0.60% by weight have been found to be favorable, optimal properties being achieved when the carbon content is between 0.18 and 0.50% by weight. Aluminum / silicon: the essential element to achieve the required

Material properties after continuous cooling is aluminum, which enormously accelerates the transformation kinetics.

In order to achieve this effect, the aluminum content should be at least 0.05% by weight, but not more than 3.00% by weight, since otherwise coarse polygonal ferrite grains may be produced which again impair the mechanical properties. Is the

Aluminum content too low, the bainitic transformation is too slow, so that increased martensite is formed, which is unfavorable to the elongation at break and the

Impact energy. For sufficient suppression of carbides in bainite, silicon may additionally be added in amounts of from 0.25 to 4.00% by weight. Good material properties are achieved at aluminum contents of between 0.07 and 1.55% by weight and optimally between 0.09 and 0.75% by weight. Corresponding silicon contents are from 0.50 to 1.75% by weight or between 0.75 and 1.50% by weight.

The selective addition of chromium of at least 0.10 to 2.00 wt .-%, the ferritic conversion can be delayed and controlled by combining with aluminum, both the kinetics of ferrite and bainite formation targeted.

Advantageous chromium contents are 0.10 to 1.75% by weight or between 0.10 and 1.50% by weight.

Manganese: the manganese addition in the range of 1.00 to 3.00% by weight results, depending on the respective requirements of the steel alloy, from a compromise between strength which can be achieved by higher additions and one

sufficient toughness that can be achieved at lower levels. With regard to a very good or optimal property combination, the manganese content should be between 1.50 and 2.50 wt.% Or between 1.70 and 2.50 wt.%.

Niobium / nitrogen: adjust the niobium content from 0.001 to 0.50 wt.% To ensure the formation of Nb (C, N). The resulting grain refining contributes to a significant improvement in toughness properties. In addition, one is

Nitrogen content of 0.001 to 0.025 wt .-% to Nb (N) recommended, since NbN is more stable than NbC and thus lead to an increased grain refining ungsbeitrag. Advantageous niobium contents are from 0.001 to 0.10 or 0.001 to 0.05 wt .-% with advantageous nitrogen contents of 0.001 to 0.015 or 0.002 to 0.010 wt .-%.

In addition, the addition of nitrogen does not bind too much C over Nb, since otherwise the austenite-stabilizing effect of C could be lost. If necessary, for further increase in strength, for example molybdenum (bis

1, 00 wt .-%), nickel (up to 5.00 wt .-%), cobalt (up to 2.00 wt .-%) or tungsten (up to 1, 50 wt .-%) are alloyed as a mixed crystal hardener. Alternatively or additionally, micro-alloying elements based on vanadium can be added to 0.20% by weight and / or titanium to 0.10% by weight. It should be a Summengehalt at Ti, V of max. 0.20 wt .-% and Ni, Mo, Co, W, Zr a Summengehalt of max. 5.50 wt .-% are maintained. In order to be able to exploit the effect of these alloying elements, a minimum content of 0.01% by weight should be maintained in each case.

Rare earths and reactive elements: the optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and / or Y can be used to set a targeted fin spacing and thus to further strength / and

Toughening in amounts of up to 1 wt .-% take place. If necessary, a total amount of 20 ppm should be added.

In the alloy composition should be to achieve the required

Material properties, in particular the mechanical-technological properties, the following conditions for the transformation kinetics and the conversion behavior

(Figure 2), the stabilization of the retained austenite and the Martensitstartternperatur are observed taking into account the cooling rate or respected, wherein in said. empirically determined formulas, the contents of C, Mn, Si, Al, Cr and Mo in wt .-% and j are used as a cooling rate in ° C / sec. The units of the coefficients used in the formulas should be chosen according to the variables used in the formulas.

Kinetics of ferritic transformation:

to comply with or adjust the mechanical and technological properties and in particular to avoid the formation of coarse polygonal ferrite grains, which adversely affect the material properties, the following condition must be met:

(35 × C) + (10 × Mn) - Si - (5 Al) + Cr> 13 / + 10 kinetics of the bainitic transformation:

the following equation for the kinetics of the bainitic transformation is to be fulfilled so that a favorable microstructure for the mechanical-technological properties can be set with very finely formed bainitic ferrite / retained austenite lamellae: 400 exp [(-7 × C) - (4 × Mn) + 8 Al + 3] /> 1 martensite starting temperature (° C):

In order to avoid larger martensitic microstructures which may degrade the mechanical properties, the martensite starting temperature shall be determined as follows:

525 - (350 x C) - (45 x Mn) - (16 x Mo) - (5 Si) + (15 x AI) 400

To stabilize the retained austenite, cementite formation must be suppressed. This is achieved by a targeted alloy with Si and Al, since both elements have a very low solubility in cementite. For this, the following condition must be observed:

Si + Al> 4 × C

To avoid harmful primary AIN excretions, the following requirement must be met:

AI N <5 x 10 ^"3

In Figure 6, this relationship is shown again graphically. Conversion capability:

for adjusting the properties of the invention based on the described

Microstructure must complete before the final heat treatment

Austenitization of the steels of the invention can be achieved (see Figure 1).

In order to achieve the required combination of mechanical properties (strength, ductility and toughness), the following ratio of ferrite and austenite formers must be observed:

C + Si / 6 + Mn / 4 + (Cr + Mo) / 3> 1

The microstructure of the steel according to the invention consists of bainitic ferrite and retained austenite lamellae. It may have fractions of up to 5% martensite (or martensite / austenite phase and / or decomposed austenite). The two most important parameters of the microstructure, which significantly influence the mechanical properties of the steel, are the fin spacing and the proportion of retained austenite. The smaller the fin spacing and the higher the proportion of retained austenite, the higher the strength and elongation at break of the material become. In order to achieve the required high strength of the material of at least 1250 up to 2500 MPa, the average fin spacing should be less than 750 nm, advantageously less than 500 nm.

In order to achieve the required elongation values of at least 12% (elongation at break), a residual austenite content of at least 10% and a martensite proportion of at most 5% should be present.

In order to achieve the required high toughness by grain refining by niobium carbonitride formation, the average former Austenitkorngröße should not exceed a value of 100 μιτι.

Since the microstructure is very fine, the microstructural constituents can hardly be differentiated by light microscopy, so that a combination of electron microscopy and X-ray diffraction can be used on a case-by-case basis.

By scanning electron microscopy, the structural components can be distinguished. In this way, a mean fin spacing of about 300 nm was determined.

The result of an X-ray diffraction measurement is shown in FIG. From the

Intensity distribution of the X-ray spectrum, the crystal structure of the existing structural constituents and their phase components can be determined.

Using the X-ray diffraction method, residual austenite fractions between 10% and 20% were determined.

Claims

claims

1. A steel alloy for a low-alloy high-strength carbide-free bainitic steel for the manufacture of strips, sheets and tubes with the following chemical

Composition (in% by weight):

0.10-0.70 C

0.25-4.00 Si

0.05-3.00 AI

1,00- 3,00 n

0,10-2,00 Cr

0.001-0.50 Nb

0.001-0.025 N

max.0,15 p

max.0,05 S

The remainder is iron with melting impurities with optional addition of one or more elements of Mo, Ni, Co, W, Nb, Ti, or V and Zr and rare earths with the proviso that to avoid primary excretions of AIN the condition AI ^χ N <5 ^χ 10 ³ (wt .-%) and for suppressing the cementite the condition Si + AI> 4 x C (wt .-%) are satisfied.

2. Steel alloy according to claim 1

characterized,

the alloy has the following contents in% by weight:

0.15-0.60 C

0.50-1.75 Si

0.07-1.50 AI

1,50-2,50 Mn

0.10-1.75 Cr

0.001 -0.10 Nb

0.001-0.015 N

3. Steel alloy according to claim 2

characterized,

the alloy has the following contents in% by weight:

0.18-0.50 C

0.75-1.5 Si 0.09-0.75 AI

1, 70 - 2.50 Mn

0, 10 -1, 5% Cr

0.001-0.05 Nb

0.002 - 0.010 N

4. Steel alloy according to claim 2

characterized,

the optionally alloyed elements have the following contents in% by weight: max. 5.00 Ni

Max. 1, 00 mo

Max. 2.00 Co

Max. 1, 50 W

Max. 0.10 Ti

Max. 0.20V

wherein the sum amount of Ti, V max. 0.20% and the sum amount of Ni, Mo, Co, W max. 5.50 wt .-% is.

5. Steel alloy according to one of claims 1 to 4

characterized,

that the structure consists of carbide-free bainite and retained austenite with a content of at least 75% bainite, at least 10% retained austenite and up to a maximum of 5% martensite.

6. Steel alloy according to one of claims 1 to 5

characterized,

that the following conditions for conversion kinetics, martensite start temperature and microstructure are met to achieve the required material properties:

Ferritic transformation kinetics (here C, Mn, Si and AI correspond to the element contents in wt .-% and † the cooling rate in ° C / sec):

(35C) + (10Mn) -Si - ( ^5χAl ) + Cr> 13/1 + 10

Bainitic transformation kinetics (here C, Mn, and Al correspond to the

Element contents in wt .-% and † the cooling rate in ° C / sec):

400 x exp [(-7 x C) - (4 ^x Mn) + 8 AI + 3] / t> 1 Martensite starting temperature (° C, where C, Mn, Si, Al and Mo correspond to the elemental contents in% by weight):

525 - (350 x C) - (45 ^χ Mn) - (16 Mo) - (5 * Si) + (15 x Al) 400 400

Stabilization of retained austenite (where C, Si, and Al correspond to elemental contents in wt%):

Si + Al> 4 × C

Prevention of primary AIN precipitates (where AI and N correspond to element contents in wt%):

AI x N <5 x 10 ³

Fulfillment of the required combination of mechanical properties:

C + Si / 6 + Mn / 4 + (Cr + Mo) / 3> 1

7. Steel alloy according to one of claims 1 to 6

characterized,

the average spacing of the retained austenite lamellae is less than 750 nm.

8. Steel alloy according to claim 7

characterized,

the average spacing of the retained austenite lamellae is less than 500 nm.

9. Use of the steel alloy according to at least one of the preceding

Claims 1 to 7 for hot or cold rolled strips, sheets, tubes, profiles or forgings for the automotive industry, construction industry, and mechanical engineering; as well as rods and wires.

10. Use of the steel alloy according to one of claims 1 to 7 for wearing parts and parts for armor.