JP2015533942A - Alloy steel for low alloy high strength steel - Google Patents

Abstract

The present invention is a bainite steel free of low alloy high tensile carbides for the production of steel strips, sheets and pipes, with the following chemical composition in weight percent: 0.10 to 0.70 C, 0 .25 to 4.00 Si, 0.05 to 3.00 Al, 1.00 to 3.00 Mn, 0.10 to 2.00 Cr, 0.001 to 0.50 Nb, 0 0.001 to 0.025 N, at most 0.15 P, at most 0.05 S, the balance being trump elements of iron and steel, Mo, Ni, Co, W, Nb, Ti Or optionally having one or more elements of V and Zr and rare earths, but satisfying the condition Al? N <5-10-3 (wt%) and cementite to avoid primary precipitation of AlN Condition to satisfy the condition Si + Al> 4? C (weight%) to suppress formation To, on the bainitic steel.

Description

  According to claim 1, the present invention relates to an alloy steel for low-alloy high-strength steel that has tenacious tenacity and at the same time has excellent wear resistance.

  In particular, the present invention relates to pipes, strips and sheets made from this alloy, from which, for example, parts for the automotive industry such as vehicle bodies, parts of support structures, or airbag tubes and cylinder tubes. Manufactured. In the field of construction-machine industry, wear-resistant plates made from this alloy can also be used when high wear-resistance requirements arise, for example for hydraulic excavators. Such steels are also used in applications where sudden impact energy must be absorbed, such as bullet proof armor.

  Pipes made from this alloy can be configured either hot or cold, from steel strip or as welded pipes that are seamlessly produced, and in some cases can have a cross-section that differs from a circular shape.

  Construction pipes or plates made from this alloy can also be used in welded steel constructions that are exposed to particularly high stresses, such as crane structures, bridge structures, hoist structures and heavy vehicle structures.

  Due to the demand for higher strength and improved processing and part properties while reducing weight and / or cost, the ultra-fine grained two-phase, also known as “superbainite”, especially as a carbide-free steel Steel has been developed. The formation of this type of microstructure consisting of bainitic ferrite with residual austenite lamellae is shown schematically in FIG. 1 in contrast to the microstructure of the upper and lower bainite.

  The characteristics of these steels are, for example, a strength (tensile strength) of 1000 to about 2000 MPa, an elongation at break (breaking elongation) of at least 5% depending on the tensile strength, and a very fine (nano) structure having a retained austenite portion. This is a bainite-based microstructure.

  The method of generating this ultrastructure is based on transformation at low temperatures in the bainite region, avoiding the precipitation of cementite and the formation of martensite. It is necessary to suppress the carbides that precipitate in bainite, for example cementite, on the one hand because they have a strong embrittlement effect as a material that can be fracture-inducible and thereby the tenacity required. This is because the proportion of stabilized austenite necessary to achieve the properties according to the invention is not obtained.

  However, the economic use of these steels has been delayed. At these low transformation temperatures, depending on the alloy composition, the transformation kinetics are strongly decelerated, especially with increasing carbon content, from a few hours to a day or days. This is because it can be made longer. However, since such long processing times are not acceptable for economic production of parts, the alloy concept has been explored as a solution to accelerate transformation.

  An alloy composition which requires such a long constant transformation time up to 48 hours is known from WO2009 / 075494A1. This steel is also disadvantageous in that it contains expensive nickel, molybdenum, boron and titanium in addition to carbon and iron, and the toughness that can be achieved is not yet sufficient for the field of application described here.

  Bainitic steel free from carbides for railway tracks is known, for example, from DE 69631953T2. In addition to manganese, chromium and further elements such as molybdenum, nickel, vanadium, tungsten, titanium and boron, the alloy steel disclosed above has a silicon content between 1 and 3%.

  This publication also mentions that in addition to silicon, the addition of aluminum can reduce or inhibit the formation of carbides in bainite and stabilize the remaining retained austenite. With this steel, the disadvantages of long transformation times can be overcome, but here the corresponding bainite-based microstructure can be produced by continuous cooling with air (air quenching) alone.

  This steel is configured for the demands on railway tracks exposed to strong wear stresses, but cannot be used for the steel strips, sheets and pipes for the above applications or is uneconomical . In these cases, in addition to the requirements for wear resistance, the requirements for strength and toughness must also be met. In addition, because of their compact cross-section, the cross-section dimensions of the track are significantly different from the dimensions of steel strips, sheets and pipes where the alloying concept needs to be adjusted with respect to the material properties to be achieved after steel air cooling. . A disadvantage of the known steel is also the addition of expensive titanium and other alloying elements such as nickel, molybdenum and tungsten.

  A further problem with the known steels is that there is no information about the nitrogen content which adversely affects the material properties, especially by the formation of aluminum nitride, when aluminum is added.

  As a result of the addition of aluminum, the affinity with nitrogen present in the steel is high, so that rough aluminum nitride is formed during solidification and precipitates primary (primarily) in the steel, and the ductile notch impact toughness of the steel It has a large negative effect on (ductility notch impact toughness), bursting behavior and lifetime, thereby significantly degrading mechanical properties.

  As a result, the deposition and size of harmful aluminum nitride depends on the respective nitrogen and aluminum content in the steel, but since nitrogen is not taken into account, the specific material properties cannot be predicted, so that This known alloy steel, to which aluminum or further aluminum is added, is rendered unusable in practice. Furthermore, the toughness that can be achieved in the field of application described according to the invention is not sufficiently high.

The requirements for the mechanical properties that the alloy steel should satisfy can be summarized as follows.
Strength: 1250-2500 MPa
Elongation at break: 12% or more Notch impact toughness at −20 ° C .: at least 15 J

  The object of the present invention is to reveal a tough and wear-resistant, low alloy, high strength, carbide-free bainite steel for steel strips, sheets and pipes. . This steel, on the one hand, is more cost-effective than known alloy steels, and on the other hand ensures certain material properties that meet requirements such as tensile strength, breaking elongation, toughness and the like. Furthermore, these material properties are also achieved when cooling with stationary air by air quenching.

  This object is solved on the basis of its preamble together with the features of claim 1. An advantageous refinement is the invention of the dependent claims.

It is a figure which shows the classification | category of the fine structure of a bainite. It is the schematic of transformation behavior. It is a figure which shows the temperature course (cooling and quenching by residual air) in the cooling of the sheet | seat of the melting 17 of a test. It is a figure which shows the mechanical characteristic value of the test alloy tested compared with the conventional high strength steel materials. FIG. 2 is a schematic diagram of a fine structure, that is, a structure of the austenite particles having Nb (C, N) -precipitation and sub-particles having different orientations. It is a figure which shows the calculation result which estimates the primary precipitation of aluminum nitride (AlN). It is a figure which shows the X-ray spectrum of the alloy of this invention.

According to the present invention, the following chemical composition in weight percent:
C of 0.10 to 0.70,
0.25 to 4.00 Si,
0.05 to 3.00 Al,
1.00 to 3.00 Mn,
0.10 to 2.00 Cr,
0.001 to 0.50 Nb,
0.001 to 0.025 N,
A maximum of 0.15 P,
S of up to 0.05
And the balance is iron with smelting related contaminations (in the claims, iron and steel tramp elements), Mo, Ni, Co, W, Nb, Ti or V and Zr and Optionally contains one or more rare elements, but meets the condition Al x N <5 x 10 ^-3 (wt%) to prevent primary precipitation of AlN and suppresses the formation of cementite Therefore, an alloy steel is provided that satisfies the condition Si + Al> 4 × C (% by weight).

  Optionally, a total of up to 1% by weight of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and / or Y can be added.

In the state of a puddle lump or slab, the steel according to the invention is already at least 15 J at a strength (R _m ) of 1250 MPa or more, an elongation at break of 12% or more and −20 ° C. after air cooling. Toughness (KBZ) (see Table 1). The microstructure consists of carbide free bainite and residual austenite, at least 75% bainite-based ferrite, at least 10% residual austenite, and up to 5% martensite (or martensite phase and / or decomposed austenite). Have a ratio of

  The alloy steel according to the invention is based on the development of a carbide-free bainite steel from DE 6906953T2 and WO 2009 / 075494A1.

  Compared to known alloy steels to achieve the required material properties, the tests carried out according to the invention surprisingly have a target range of 0.05 to 3.0% by weight. In addition to excellent material strength and wear resistance, very good toughness can be achieved by adding aluminum and niobium in the range of 0.001 to 0.5 wt% and air quenching. . In particular, the addition of niobium significantly improves the toughness properties by grain refinement, so that this alloy meets the high requirements for mechanical properties and wear resistance.

  Also, as a result of the advantageous addition of chromium in the range of 0.10 to 2.00% by weight, the formation of coarse polygonal ferrite bodies that can adversely affect the material properties can be effectively avoided. The kinetics of ferrite formation can be critically controlled. In this regard, the interaction between aluminum and chromium is important. Aluminum accelerates the ferrite and bainite transformation, while the addition of chromium delays the ferrite transformation (see FIG. 2). Depending on the targeted combination of these two elements, the kinetics of ferrite and bainite formation can be controlled.

  In addition to the known beneficial effect of adding aluminum to avoid carbide precipitation in bainite, tests show that the addition of aluminum significantly accelerates the kinetics of bainite transformation compared to silicon. ing. The latter also increases with increasing aluminum content. This means that the toughness and strength of the steel according to the invention is significantly improved after continuous cooling compared to the steel alloyed with silicon alone, i.e. higher toughness and strength values. Can be achieved. Even for thicker sheets (eg, greater than 10 mm), cooling rates greater than 10 ° C./s are advantageous to achieve the required combination of mechanical properties. The required mechanical properties can also be achieved by cooling with residual air in the case of thinner sheets or by adjusting the alloying concept. The effect of different alloying elements on the kinetics of transformation is shown in FIG. Figure 3 schematically shows the kinetics of the transformation of ferrite, pearlite and bainite and the effect of C, Si, Al, Mn, Cr and Mo on the martensite onset temperature.

According to the present invention, the nitrogen content exceeds the specified upper limit of 0.025% by weight in order to minimize the number and size of harmful aluminum nitrides as primary precipitates in the steel compared to known steels. More preferably 0.015% by weight, or even more preferably not exceeding 0.010% by weight, in order to achieve these advantageous properties, in which case Al × The condition of N <5 × 10 ⁻³ must be met. Otherwise, the lower limit of nitrogen content is 0.001% by weight, optimally to allow the niobium carbonitride formation required to increase toughness by grain refinement It must be 0.0020% by weight.

  The composition of the tested alloys and the measured mechanical properties are shown in Table 1. All samples were heated to about 950 ° C. and then cooled with residual air or accelerated cooling. The required cooling rate is selected depending on the sheet thickness and composition. As indicated by mechanical sampling results, the sample melt 14 was too low in Cr content to achieve the required properties. The test melt 16 subsequently met the requirements with accelerated cooling alone due to the larger sheet thickness of 12 mm. A typical temperature profile for cooling with residual air or air quenching is shown in FIG.

  In FIG. 4, the melts of several tested tests and their mechanical properties and cooling conditions are shown in comparison with a conventional high tensile steel material. It can be seen that the developed steel is in the region of higher strength materials with improved stretch properties.

  The results confirm the excellent mechanical properties in the hardened state (strength and toughness of products semi-finished in the present invention, such as paddle agglomerates or flat steel alloys) (Table 1).

  As an essential element, in addition to accelerating transformation kinetics, aluminum plays an important role in suppressing the precipitation of carbides in bainite in combination with silicon. As a result, the retained austenite is stabilized because the solubility of carbon in ferrite is limited. A high proportion of retained austenite, at least 10% in bainite, results in excellent mechanical properties in addition to extremely fine lamellar microstructure. The different microstructured components were measured with a scanning electron microscope and an average 300 nm lamellar spacing was measured. A schematic diagram of the previous austenite particles having a basic structure with a fine lamellar structure (e.g. sub grains) is shown schematically in FIG. Here, the structure of the austenite particles described above is stabilized through the precipitation of Nb (C, N).

  The so-called TRIP effect can then also be used advantageously depending on the proportion of the corresponding retained austenite. Usually, steels belonging to the term TRIP (Transformation Induced Plasticity) are steels having at the same time very high strength and excellent ductility, thereby being particularly suitable for cold forming. These properties are obtained by their special microstructure, transformation-induced martensite formation and the associated work hardening are inhibited and ductility is increased. The effect of the TRIP effect is optimal at a proportion of retained austenite of about 10 to 20%.

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

carbon:
For sufficient strength of the material, the minimum content should not be less than 0.10% by weight. The carbon content should not exceed 0.70% by weight at a sufficiently low martensite onset temperature and at this temperature to build a very fine structure but still have good weldability. . It has been found that the carbon content is advantageously between 0.15 and 0.60% by weight. Optimal properties are achieved when the carbon content is between 0.18 and 0.50% by weight.

Aluminum / silicon:
An essential element for achieving the material properties required after continuous cooling is aluminum, which strongly accelerates the kinetics of transformation.
In order to achieve this effect, the aluminum content must be at least 0.05% by weight, but the maximum must be 3.00% by weight. Otherwise, a coarse polygonal ferrite body that adversely affects the material properties can be formed. If the aluminum content is too low and the bainite transformation is too slow, martensite formation is promoted, which adversely affects fracture elongation and notch impact toughness. Silicon can be added at a content of 0.25 to 4.00% by weight for sufficient suppression of carbides in bainite. Good material properties are achieved with an aluminum content between 0.07 and 1.50% by weight and an optimum content between 0.09 and 0.75% by weight. The corresponding silicon content is between 0.50 and 1.75% by weight, or between 0.75 and 1.50% by weight.

  As a result of the targeted addition of at least 0.10 to 2.00% by weight of chromium, the ferrite transformation can be slowed down. And, in combination with aluminum, the kinetics of ferrite and bainite formation can be controlled in a targeted manner. An advantageous chromium content is between 0.10 and 1.75% by weight or between 0.10 and 1.50% by weight.

manganese:
The addition of manganese in the range of 1.00 to 3.00% by weight results in the respective demands on the alloy steel as a compromise between the strength that can be achieved with large additions and the sufficient toughness that can be achieved with low contents Depends on. In a very good or optimal combination of properties, the manganese content should be between 1.50 and 2.50% by weight or between 1.70 and 2.50% by weight.

Niobium / Nitrogen:
A niobium content of 0.001 to 0.50% by weight must be specified to ensure the formation of Nb (C, N). The resulting particle refinement contributes to a significant improvement in toughness properties. Furthermore, since NbN is more stable than NbC and thus leads to further refinement of the particles, a nitrogen content of 0.001 to 0.025% by weight is recommended to form Nb (N). The preferred niobium content is 0.001 to 0.10 or 0.001 to 0.05% by weight, whereas the preferred nitrogen content is 0.001 to 0.015 or 0.002 to 0.001. 010% by weight. Furthermore, the addition of nitrogen prevents excessive binding of C via Nb. Otherwise, the austenite stabilizing effect of C can be lost.

  As required, as a solid solution hardener to further increase the strength, for example, molybdenum (up to 1.00 wt%), nickel (up to 5.00 wt%), cobalt (2.00 wt%) %) Or tungsten (up to 1.50% by weight) can be added. Alternatively or additionally, micro-alloying elements can be added, for example up to 0.20% by weight vanadium and / or up to 0.10% by weight titanium. A total content of up to 0.20 wt% Ti, V and up to 5.50 wt% Ni, Mo, Co, W, Zr must be observed. In order to take advantage of the effects of these alloying elements, a minimum content of 0.01% by weight must be observed.

Rare earths and reactive elements:
Rare earths and reactive elements such as Ce, Hf, La, Re, Sc and / or Y can optionally be added to achieve optimal lamellar spacing, thus further increasing strength and toughness Therefore, the total content can be up to 1% by weight. If necessary, 20 ppm should be added as a total content.

  In order to achieve the required material properties, especially mechanical properties, in the alloy composition, the kinetics and transformation behavior of the transformation (Fig. 2), the stabilization of the retained austenite and the martensite onset temperature, taking into account the cooling rate. The following conditions must be observed: Here, in the above-mentioned empirically determined formula, the contents of C, Mn, Si, Al, Cr and Mo are expressed by weight%, and T as a cooling rate is expressed in ° C / s. The unit of coefficients used in the formula is selected according to the variables used in the formula.

Ferrite transformation kinetics:
In order to support or set the mechanical properties, in order to avoid the formation of coarse polygonal ferrite particles, in particular adversely affecting the material properties, the following conditions must be met:
(35 * C) + (10 * Mn) -Si- (5 * Al) + Cr> 13 / T + 10

The kinetics of bainite transformation:
In order to achieve a suitable microstructure with very finely structured bainite-based ferrite / residual austenite lamellae for mechanical properties, the following equation of bainite transformation kinetics must be established: .
400 × exp [(− 7 × C) − (4 × Mn) + 8Al + 3] / T> 1

Martensite start temperature (℃):
In order to avoid an increase in the proportion of martensite microstructure that degrades the mechanical properties, the martensite onset temperature must be determined as follows.
525- (350 × C) − (45 × Mn) − (16 × Mo) − (5 × Si) + (15 × Al) << 400

In order to stabilize the retained austenite, the formation of cementite must be suppressed. This is accomplished by targeted alloying with Si and Al. This is because the solubility of both elements in cementite is extremely small. For this purpose, the following conditions must be satisfied.
Si + Al> 4 × C

In order to avoid detrimental primary precipitation of AlN, the following conditions must be met:
Al × N <5 × 10 ⁻³
FIG. 6 schematically illustrates this relationship again.

Transformation ability:
In order to obtain the properties according to the invention based on the described microstructure, complete austenitization of the steel according to the invention must be achieved before the final heat treatment (see FIG. 1).

In order to achieve the required combination of mechanical properties (strength, ductility and toughness), the following ferrite and austenite formation relationships must be met.
C + Si / 6 + Mn / 4 + (Cr + Mo) / 3> 1.

  The microstructure of the steel according to the invention consists of lamellar ferrite and residual austenite. It can have a proportion of martensite up to 5% (or martensite / austenite phase and / or decomposed austenite). The two most important features of the microstructure that significantly affect the mechanical properties of the steel are the lamellar spacing and the percentage of retained austenite. The smaller the lamella spacing and the higher the percentage of retained austenite, the greater the material strength and elongation at break.

  In order to achieve the required high strength of the material, at least 1250 to 2500 MPa, the average lamellar spacing must be less than 750 nm, preferably less than 500 nm.

  To achieve an elongation (and elongation at break) of at least 12%, the retained austenite ratio must be at least 10% and the martensite ratio must be at most 5%.

  In order to achieve high toughness by grain refinement by niobium carbonitride formation, the average size of the austenite particles described above should not exceed a value of 100 μm.

  Since the microstructure is so fine that the components of the microstructure can hardly be distinguished from each other by the microscope, a combination of electron microscopy and X-ray diffraction must be used depending on the case.

  Microstructured components can be distinguished by scanning electron microscopy. In this way, an average lamella spacing of about 300 nm was determined.

  The result of the X-ray diffraction measurement is shown in FIG. From the intensity distribution of the X-ray spectrum, the crystal structures of the microstructured components of the invention and the proportions of their phases can be determined.

  The proportion of retained austenite between 10% and 20% was determined using X-ray diffractometry.

Claims (10)

Alloy steel for bainite steel free of low-alloy high-strength carbides for the manufacture of strip steel, sheets and pipes, with the following chemical composition in weight percent:
C of 0.10 to 0.70,
0.25 to 4.00 Si,
0.05 to 3.00 Al,
1.00 to 3.00 Mn,
0.10 to 2.00 Cr,
0.001 to 0.50 Nb,
0.001 to 0.025 N,
A maximum of 0.15 P,
S of up to 0.05
Have
The balance is trump elements of iron and steel, optionally having one or more elements of Mo, Ni, Co, W, Nb, Ti, or V and Zr and rare earth, provided that the primary precipitation of AlN Alloy steel satisfying the condition Al × N <5 × 10 ⁻³ (wt%) in order to avoid and satisfying the condition Si + Al> 4 × C (wt%) in order to suppress the formation of cementite. The alloy steel has the following content in% by weight:
C of 0.15 to 0.60,
0.50 to 1.75 Si,
0.07-1.50 Al,
1.50 to 2.50 Mn,
0.10 to 1.75 Cr,
0.001 to 0.10 Nb,
0.001 to 0.015 N
The alloy steel according to claim 1, comprising: The alloy steel has the following content in% by weight:
C from 0.18 to 0.50,
0.75 to 1.5 Si,
0.09-0.75 Al,
1.70-2.50 Mn,
0.10 to 1.5% Cr,
0.001 to 0.05 Nb,
0.002 to 0.010 N
The alloy steel according to claim 2, comprising: Optionally, the element to be alloyed contains the following content in weight percent:
Ni up to 5.00,
Mo up to 1.00,
Up to 2.00 Co,
1.50 W maximum,
Ti of up to 0.10,
V of 0.20 at maximum
Have
The alloy steel according to claim 2, wherein the total content of Ti and V is at most 0.20%, and the total content of Ni, Mo, Co and W is at most 5.50% by weight. .   5. The microstructure according to claim 1, wherein the microstructure is composed of carbide-free bainite and residual austenite and has a proportion of at least 75% bainite, at least 10% residual austenite and at most 5% martensite. Alloy steel as described in. In order to achieve the required material properties, in the formation of transformation kinetics, martensite onset temperature and microstructure, the following conditions:
Ferrite transformation kinetics (where C, Mn, Si and Al are the element content in weight%, T corresponds to the cooling rate in ° C./s):
(35 * C) + (10 * Mn) -Si- (5 * Al) + Cr> 13 / T + 10,
Bainitic transformation kinetics (where C, Mn and Al are elemental contents in weight%, T corresponds to the cooling rate in ° C / s):
400 × exp [(− 7 × C) − (4 × Mn) + 8Al + 3] / T> 1,
Martensite start temperature (° C., where C, Mn, Si, Al and Mo correspond to element content in weight%):
525- (350 × C) − (45 × Mn) − (16 × Mo) − (5 × Si) + (15 × Al) << 400,
Stabilization of residual austenite (here C, Si and Al correspond to element content in weight%):
Si + Al> 4 × C,
In order to avoid the main AlN precipitation (where Al and N correspond to the element content in weight%),
Al × N <5 × 10 ⁻³ ,
To meet the required combination of mechanical properties,
C + Si / 6 + Mn / 4 + (Cr + Mo) / 3> 1
The alloy steel according to any one of claims 1 to 5, wherein   The alloy steel according to any one of claims 1 to 6, wherein an average distance of lamellae of the retained austenite is less than 750 nm.   The alloy steel according to claim 7, wherein the average distance of the residual austenite lamella is less than 500 nm.   Hot or cold rolled steel strip, sheet metal, pipes, profiles or forged parts for the automotive industry, construction industry and construction machinery of the alloy steel according to at least one of claims 1-7, and Use on rods and wires.   Use of the alloy steel according to any one of claims 1 to 7 for wear parts and parts for protective clothing.

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