JP6513568B2 - Alloy steel for low alloy high tensile steel - Google Patents

Description

  The invention relates to an alloy steel for low alloy high strength steels, which according to claim 1 has tenacious strength and at the same time excellent wear resistance.

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

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

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

  Due to the demands for higher strength and improved processing properties and part properties at the same time as reducing weight and / or cost, two-phase ultrafine particles, also known as "super bainite", especially as carbide free steels Steel has been developed. The formation of this type of microstructure consisting of bainitic ferrite with lamellae of retained austenite 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 (break elongation) of at least 5% according to the tensile strength, a very fine (nano) structure having a retained austenite part The fine structure of the bainite system.

  The method of producing this hyperfine structure is based on transformation at low temperature in the bainite region, avoiding the precipitation of cementite and the formation of martensite. It is necessary to suppress the carbides which precipitate in bainite, for example cementite, because, on the one hand, they have a strong embrittlement effect as a fracture-inducing substance and the toughness required thereby At the same time, and on the other hand, the proportion of stabilized austenite necessary to achieve the properties according to the invention is not obtained.

  However, the economic utilization of these steels is delayed. At these low transformation temperatures, depending on the alloy composition, the transformation kinetics are strongly decelerated, and in particular from several hours to a day or several days, with the increase of the carbon content, the holding time at constant temperature Because they can be made longer. However, such long processing times are not accepted as economic production of parts, so the alloy concept was explored as a solution to accelerate transformation.

  Alloy compositions that require such long isothermal transformation times of up to 48 hours are known from WO 2009/075494 A1. In addition to carbon and iron, this steel is also disadvantageous in that it contains expensive nickel, molybdenum, boron and titanium, and the achievable toughness is not yet sufficient for the field of application described here.

  A carbide-free bainitic steel for railways is known, for example, from DE 69631953 T2. Besides manganese, chromium and further elements such as molybdenum, nickel, vanadium, tungsten, titanium and boron, the alloy steels disclosed above have a silicon content of between 1 and 3%.

  The publication also mentions that, besides silicon, the addition of aluminum can reduce or suppress the formation of carbides in bainite and stabilize the remaining retained austenite. The steel can also overcome the disadvantages of long transformation times, but here the corresponding bainitic microstructure can be produced by continuous cooling with air (air quench) alone.

  While this steel is configured for the demand for railway lines exposed to high wear stress, it can not be used or is uneconomical for strip, sheet and pipe for the above mentioned applications . In these cases, in addition to the requirements for wear resistance, the requirements for strength and toughness must also be fulfilled. Furthermore, due to their compact cross section, the dimensions of the cross section of the line differ significantly from the dimensions of the steel strip, sheet and pipe where the alloying concept needs to be adjusted with respect to the material properties to be achieved after air cooling of the steel. . The disadvantage of the known steels is also the addition of expensive titanium and other alloying elements such as nickel, molybdenum and tungsten.

  A further problem of the known steels is the lack of information on the nitrogen content which adversely affects the material properties, in particular by the formation of aluminum nitride, when aluminum is added.

  As a result of the addition of aluminum, due to the high affinity with the nitrogen present in the steel, coarse aluminum nitride is formed during solidification and precipitates 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 life, which significantly degrades the mechanical properties.

  As a result, the deposition amount and size of harmful aluminum nitrides depend on the respective nitrogen and aluminum content in the steel, but because nitrogen is not taken into account, the concrete material properties can not be predicted, so instead of silicon This known alloy steel, to which aluminum or further aluminum is added, is rendered unusable in practice. Furthermore, the toughness achievable in the field of the described application according to the invention is not high enough.

The requirements for the mechanical properties to be met by the alloy steels can be summarized as follows.
Strength: 1250 to 2500 MPa
Elongation at break: Notch impact toughness at 12% or more -20 ° C: At least 15J

  The object of the present invention is to reveal a low-alloy, high-strength, carbide-free bainitic steel alloy steel for the production of strip, sheet and pipe, with toughness and wear resistance. . This steel, on the one hand, is more cost-effective than the known alloy steels, and on the other hand ensures certain material properties which fulfill the requirements, for example tensile strength, elongation at break, toughness etc. In addition, 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. Advantageous refinements are the inventions of the dependent claims.

It is a figure which shows the classification of the fine structure of bainite. It is the schematic of transformation behavior. It is a figure which shows the temperature progress (cooling and hardening by residual air) in cooling of the sheet | seat of the fusion | melting 17 of a test. FIG. 6 shows mechanical property values of the tested test alloys compared to conventional high strength steels. FIG. 2 is a schematic view of the microstructure, ie the structure of the aforementioned austenite particles with Nb (C, N) -deposition and the sub-particles with different orientation. It is a figure which shows the calculation result which predicts primary precipitation of aluminum nitride (AlN). FIG. 2 shows the X-ray spectrum of the alloy of the invention.

According to the invention, the following chemical compositions in weight%:
0.10 to 0.70 C,
0.25 to 4.00 Si,
0.05 to 3.00 Al,
Mn of 1.00 to 3.00,
0.10 to 2.00 Cr,
0.001 to 0.50 Nb,
0.001 to 0.025 N,
Up to 0.15 P,
Up to 0.05 S
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 have one or more elements of the rare element, but satisfy the condition Al x N <5 x 10 ^-3 (wt%) to avoid primary precipitations of AlN and suppress cementite formation An alloy steel is provided, provided that the condition Si + Al> 4 × C (% by weight) is satisfied.

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

In puddle lumps or slabs, the steel according to the invention already has a strength (R _m ) of at least 1250 MPa after air cooling, an elongation at break of at least 12%, and at −20 ° C. of at least 15 J Toughness (KBZ) (see Table 1). The microstructure consists of carbide free bainite and retained austenite, at least 75% bainitic ferrite, at least 10% retained austenite, and at most 5% martensite (or martensitic phase and / or decomposed austenite) Have a proportion of

  The alloy steels according to the invention are based on the development of carbide-free bainitic steels from DE6906953T2 and WO2009 / 075494A1.

  Tests carried out in accordance with the present invention, in comparison with known alloy steels to achieve the required material properties, surprisingly target within the range of 0.05 to 3.0% by weight In addition to excellent material strength and wear resistance, it has been shown that very good toughness can be achieved by air quenching with the addition of aluminum and niobium in the range of 0.001 to 0.5% by weight. . In particular, the addition of niobium significantly improves the toughness properties by grain refinement, so that the alloy meets the high requirements of 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, which 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. While aluminum accelerates the ferrite and bainite transformation, the addition of chromium delays the ferrite transformation (see FIG. 2). The kinetics of ferrite and bainite formation can be controlled by the combination of these two elements targeted.

  Tests have shown that, in addition to the known beneficial effects of aluminum addition to avoid the precipitation of carbides in bainite, the kinetics of the bainite transformation are significantly accelerated by the addition of aluminum compared to silicon. ing. The latter also increases with the increase of the content of aluminum. This means that the toughness and strength of the steel according to the invention are significantly improved after continuous cooling compared to steels alloyed with silicon only, ie higher toughness and strength values Can be achieved. Even at thicker sheets (e.g. greater than 10 mm), cooling rates greater than 10 DEG C / s are advantageous in order 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. The kinetics of transformation of ferrite, pearlite and bainite, and the effect of C, Si, Al, Mn, Cr and Mo on the martensitic start temperature are shown schematically.

According to the 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 deposition in the steel compared to known steels. Not more than 0.015% by weight, or more preferably not more than 0.010% by weight, is strictly necessary to achieve these advantageous properties, in which case further Al x The condition of N <5 × 10 ⁻³ must be satisfied. Otherwise, the lower limit of the nitrogen content is 0.001% by weight, optimally allowing the formation of niobium carbonitride, which is required to increase toughness by grain refinement. It is necessary to be 0.0020% by weight.

  The composition of the tested alloy and the measured mechanical properties are shown in Table 1. All samples were heated to about 950 ° C. and then either residual air cooled or accelerated cooled. The required cooling rate is selected according to the sheet thickness and composition. As the mechanical sampling results show, the sample melt 14 could not achieve the required properties because the Cr content was too low. 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 residual air or air quench cooling 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. The steels developed are found to be in the area of higher strength materials with improved stretch properties.

  The results support the excellent mechanical properties in the hardened state (semi-finished products according to the invention, such as the strength and toughness of the paddle block or flat alloy steels) (Table 1).

  As an essential element, aluminum plays an important role in suppressing the precipitation of carbides in bainite in combination with silicon, in addition to accelerating the kinetics of transformation. As a result, retained austenite is stabilized because the solubility of carbon in ferrite is limited. The high proportion of retained austenite, which is at least 10% in bainite, leads to excellent mechanical properties in addition to the very fine lamellar microstructure. Components of different microstructures were measured with a scanning electron microscope, and an average lamellar spacing of 300 nm was measured. A schematic view of the previous (previous) austenite particles having a substructure (for example, subgrains) having a fine lamellar structure is schematically shown in FIG. Here, the above-described structure of austenite particles is stabilized through the precipitation of Nb (C, N).

  The so-called TRIP effect can then be used advantageously, depending on the proportion of the corresponding retained austenite. Usually, the steels belonging to the term TRIP (Transformation Induced Plasticity) are steels which simultaneously have very high strength and good ductility, whereby they are 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 TRIP effect is optimal for a retained austenite fraction of about 10 to 20%.

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

carbon:
For sufficient strength of the material, the minimum content should not be less than 0.10% by weight. At a sufficiently low martensitic start temperature and at this temperature a very fine structure is built, but in still having good weldability, the carbon content should not exceed 0.70% by weight . The carbon content proved to be advantageous between 0.15 and 0.60% by weight. The optimum properties are achieved when the carbon content is between 0.18 and 0.50% by weight.

Aluminum / silicon:
An essential element to achieve the required material properties after continuous cooling is aluminum, which strongly accelerates the kinetics of transformation.
To achieve this effect, the aluminum content should be at least 0.05% by weight, but the maximum should be 3.00% by weight. Otherwise, coarse polygonal ferrite bodies may be formed which adversely affect the material properties. If the aluminum content is too low and the bainite transformation is too slow, the formation of martensite is promoted which adversely affects the breaking elongation and the notch impact toughness. For sufficient control of carbides in bainite, silicon can be added at a content of 0.25 to 4.00% by weight. Good material properties are achieved with an aluminum content of between 0.07 and 1.50% by weight, with an optimum content of 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 moderated. And, in combination with aluminum, the kinetics of ferrite and bainite formation can be controlled in a targeted manner. Preferred 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 by large additions and the sufficient toughness that can be achieved at low contents. Depends on. In a combination of very good or optimal 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 has to be specified to ensure the formation of Nb (C, N). The resulting grain refinement contributes to a significant improvement in toughness properties. Furthermore, a nitrogen content of 0.001 to 0.025% by weight is recommended to form Nb (N), since NbN is more stable than NbC and thus leads to further grain refinement. The preferred niobium content is 0.001 to 0.10 or 0.001 to 0.05% by weight, while the preferred nitrogen content is 0.001 to 0.015 or 0.002 to 0. It is 010% by weight. Furthermore, the addition of nitrogen prevents the excessive binding of C via Nb. Otherwise, the austenite stabilization effect of C may be lost.

  If necessary, as a solid solution hardener to further increase the strength, for example, molybdenum (up to 1.00% by weight), nickel (up to 5.00% by weight), cobalt (2.00% by weight) %) Or tungsten (up to 1.50 wt.%) Can be added. Alternatively, or in addition, micro-alloying elements such as up to 0.20% by weight vanadium and / or up to 0.10% titanium may be added. A total content of up to 0.20% by weight of Ti, V and up to 5.50% by weight of Ni, Mo, Co, W, Zr should 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 the strength and toughness To a total content of up to 1% by weight. If necessary, a total content of 20 ppm should be added.

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

Kinetics of ferrite transformation:
In order to support or set the mechanical properties, in particular in order to avoid the formation of coarse polygonal ferrite particles which adversely affect the material properties, the following conditions have to be fulfilled:
(35 × C) + (10 × Mn) -Si- (5 × Al) + Cr> 13 / T + 10

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

Martensite start temperature (° C.):
In order to avoid an increase in the proportion of martensitic microstructures which degrade the mechanical properties, the martensitic start temperature has to be determined as follows.
525-(350 x C)-(45 x Mn)-(16 x Mo)-(5 x Si) + (15 x Al) << 400

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

In order to avoid harmful primary deposition of AlN, the following conditions have to be fulfilled:
Al × N <5 × 10 ⁻³
This relationship is again shown schematically in FIG.

Metamorphosis 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 has to 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 formation of ferrite and austenite must be satisfied.
C + Si / 6 + Mn / 4 + (Cr + Mo) / 3> 1.

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

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

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

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

  Because the microstructures are so fine that the components of the microstructure can hardly distinguish each other by microscopy, a combination of electron microscopy and X-ray diffraction has to be used depending on the case.

  Microstructural components can be distinguished by scanning electron microscopy. In this way, an average lamellar 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 structure of the components of the microstructure of the invention and the proportions of their phases can be determined.

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

Claims (5)

Low-alloy high-tensile carbide-free alloy steel for bainitic steels for the production of strip steels, sheets and pipes, with the following chemical composition in weight%:
0.18 to 0.50 C,
0.75 to 1.5 Si,
0.09 to 0.75 Al,
Mn of 1.70 to 2.50,
0.10 to 1.5 Cr,
0.001 to 0.05 Nb,
0.002 to 0.010 N,
Up to 0.15 P,
Up to 0.05 S
Have
The balance is a tramp element of iron and steel, optionally comprising one or more elements of Mo, Ni, Co, W, Ti, or V and Zr and a rare earth,
Composed of carbide-free bainitic ferrite and retained austenite, having a microstructure with a proportion of at least 75% bainitic ferrite, at least 10% retained austenite and at most 5% martensite,
The average distance of lamellae of said retained austenite is less than 750 nm, and the average size of austenite particles is not more than 100 μm,
Kinetics of transformation, martensitic start temperature and formation of microstructures under the following conditions:
Kinetics of ferrite transformation (where C, Mn, Si and Al are elemental content in wt%, T corresponds to the cooling rate in ° C / s, T is greater than 10 ° C / s, 210 ° C / s) s) or less):
(35 × C) + (10 × Mn) -Si- (5 × Al) + Cr> 13 / T + 10,
Kinetics of the bainite transformation (where C, Mn and Al are element contents in wt%, T corresponds to the cooling rate in ° C / s, T is greater than 10 ° C / s and less than 210 ° C / s Is):
400 × exp [(−7 × C) − (4 × Mn) +8 Al + 3] / T> 1,
Martensite start temperature (° C., where C, Mn, Si, Al and Mo correspond to elemental content in wt%):
525-(350 x C)-(45 x Mn)-(16 x Mo)-(5 x Si) + (15 x Al) << 400,
To suppress cementite formation and stabilize retained austenite (where C, Si and Al correspond to elemental content in wt%):
Si + Al> 4 × C,
In order to avoid primary precipitation of AlN (where Al and N correspond to elemental content in wt%):
Al × N <5 × 10 ⁻³ ,
To meet the required combination of mechanical properties:
C + Si / 6 + Mn / 4 + (Cr + Mo) / 3> 1
Comply with,
Alloy steel. The said elements optionally alloyed have the following contents in weight percent:
Up to 5.00 Ni,
Up to 1.00 Mo,
Co up to 2.00,
Up to 1.50 W,
Up to 0.10 Ti,
Up to 0.20 V
Have
The alloy steel according to claim 1, 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. . The steel alloy according to claim 1 or 2 , wherein the average distance of the lamellae of the retained austenite is less than 500 nm. 4. Hot or cold rolled steel strip, sheet metal, pipe, molded part or forged part for the automotive industry, construction industry and construction machinery of the alloy steel according to at least one of claims 1 to 3 And use for rods and wires. Use of the alloy steel according to any one of claims 1 to 3 for wear parts and parts of protective clothing.

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