WO2013171231A1 - High strength steel with increased e-modulus and method for producing said steel - Google Patents

High strength steel with increased e-modulus and method for producing said steel Download PDF

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WO2013171231A1
WO2013171231A1 PCT/EP2013/059966 EP2013059966W WO2013171231A1 WO 2013171231 A1 WO2013171231 A1 WO 2013171231A1 EP 2013059966 W EP2013059966 W EP 2013059966W WO 2013171231 A1 WO2013171231 A1 WO 2013171231A1
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steel
melt
hot rolling
modulus
composition
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PCT/EP2013/059966
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French (fr)
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Monika KRUGLA
Urszula Alicija SACHADEL
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Tata Steel Nederland Technology Bv
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making non-ferrous alloys
    • C22C1/02—Making non-ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

  • the invention relates to a high strength steel with a increased E-modulus and to a method for producing said steel.
  • a known way to increase the modulus of elasticity and reduce the weight of steel is by incorporating ceramic particles of different natures, such as carbides, nitrides, oxides or borides. These particles have a much higher elastic modulus, ranging from about 250 to 550 GPa, than that of the steel base which' value is around 210 GPa.
  • One way of including ceramic particles uniformly distributed in a matrix of steel is by means of powder metallurgy. Despite improved mechanical properties compared to steels containing no dispersion of ceramic particles, this type of method has problems. Reactions of the metal powders are difficult to prevent because of the high surface area of the metal powders.
  • a steel is proposed based on the formation of large quantities of titaniumdiboride particles (TiB 2 ). It describes a steel comprising 2.5 to 7.5% titanium and boron in the order of 0.8 to 4%. The result of these additions is that large numbers of eutectic TiB 2 -precipitates are formed in the steel matrix and these particles act as ceramic inclusions as a result of which the E-modulus increases and the density of the steel decreases.
  • a disadvantage of this steel type is that it is very difficult to produce the TiB 2 -precipitates as fine eutectic precipitates.
  • a further problem is the control of the morphology of the particles which tend to become sharp edged and may thus serve as a stress inducing particle and an initiation spot for cracks.
  • a similar solution is chosen in JP2007-051341 where the mandatory presence of large amounts of titanium and vanadium is complemented by a mandatory presence of large amounts of chromium to produce and stabilise the formation of V 3 B 4 making the costs of the alloy very high.
  • the structure of the steel comprises at least 5 wt% of ⁇ ((Fe,Cr) 2 B and Fe 2 B). All compositional percentages are in weight percent (wt.%) unless otherwise indicated.
  • the unavoidable impurities are elements unavoidably contained in the steel due to circumstances such as raw materials, manufacturing facilities, etc.
  • the amount of carbon is limited because of the weldability of the steel.
  • the resistance to cracking and the toughness of the heat affected zone decreases if the carbon content exceeds 0.24%.
  • a preferable maximum from this point of view would be 0.20%.
  • From the point of view of weldability a preferable maximum carbon value is 0.15%, preferably 0.10% and more preferably 0.06%.
  • a suitable minimum carbon level is 0.01%.
  • a suitable minimum carbon level is 0.02%.
  • Boron is an important and mandatory element because it forms the basis for the (Fe,Cr) 2 B and Fe 2 B. Both types of the precipitates raise the E modulus of the steel. Below the lower end of the inventive range the contribution of the precipitates to the increase of the E-modulus is insufficient.
  • the boron content is at least 0.5%.
  • the maximum boron content is 2.5%.
  • a suitable minimum boron content was found to be 0.75% or even 0.9%.
  • Chromium is an important and mandatory element effective in forming the (Fe,Cr) 2 B type boride and raising the Young's modulus of steel. A minimum amount of 2% is required to achieve this effect. If the chromium content exceeds 20%, big and coarse particles will be formed, and it will have an adverse effect on the hot-workability, ductility and toughness. A suitable maximum value for chromium was found to be 15% or even 13%. In an embodiment the chromium content is at most 8%. The chromium content is preferably at least 4%.
  • Manganese contributes to the strengthening of the matrix by solid solution or by transformation strengthening. Manganese is added mainly to enlarge the austenite region during hot rolling, similar to Cu and Ni additions. However if the amount increases above a value of 1.5% the risk of segregation and band formation increases to an unacceptable level. Manganese is also effective in binding sulphur thereby reducing the risk of hot-cracking during casting.
  • the manganese content is at most 1.0%. In an embodiment of the invention the manganese content is at least 0.3%. A suitable maximum is 0.8%.
  • Aluminium is added only for deoxidation. The formation of coarse alumina particles must be prevented and therefore the maximum content should not exceed 0.1%. A suitable maximum aluminium content is 0.05%. The nickel and/or the copper content is up to 1.0% respectively. These elements when added increase the austenite range during hot rolling. When added, suitable minimum values for both elements are 0.1% respectively.
  • the nitrogen content is as low as possible. It should be noted that nitrogen is not added on purpose. Its presence is a consequence of the use of master alloys containing nitrogen.
  • a suitable minimum content is 0.001 wt.% (10 ppm).
  • a preferred minimum is 0.002% (20 ppm).
  • a suitable maximum value for the nitrogen content is 0.02% (200 ppm).
  • Silicon is an element effective in deoxidation. If silicon is added for this purpose in addition or instead of aluminium, a minimum amount of 0.05% is needed. Silicon is also effective in solid-solution hardening. The formation of silicon oxides adversely affects the picklability and galvanisability of the steel plate. If the silicon content exceeds 0.4%, the risk of forming Laves phases which adversely affects toughness increases.
  • the molybdenum content is up to 0.4%. Molybdenum increases the strength of the steel, but for economic reasons the amount is limited at 0.4%. Preferably the molybdenum is at most 0.1%. If added as an alloying element the content needs to be at least 0.05%.
  • Niobium can be added in small amounts up to 0.1% to induce an added strengthening of the steel in the form of fine carbides or carbonitrides. If added as an alloying element the content needs to be at least 0.01%.
  • Sulphur and phosphorus have a detrimental effect on the steel because of the risk of formation of MnS and segregation of phosphorus on the grain boundaries which adversely affect hot and cold formability of the steel. Therefore sulphur and phosphorus levels need to be kept below 0.040 and 0.030 wt.% respectively, preferably below 0.020 and 0.015 wt.% respectively.
  • the E-modulus of the steel is at least 220 GPa, preferably at least 230 GPa, more preferably at least 235 GPa.
  • the increase in E-modulus is determined by the presence of the (Fe,Cr) 2 B and Fe 2 B precipitates. These precipitates are very stable. Precipitates present in the cast product will not dissolve during reheating and hot-rolling, and therefore if the ⁇ ((Fe,Cr) 2 B and Fe 2 B) is above a certain value, e.g. 5 wt.%, in the cast material, it will also be above that certain value in the hot-rolled material.
  • the structure of the steel comprises at least 8 wt% of ⁇ ((Fe,Cr) 2 B and Fe 2 B), more preferably at least 10 or even 15 wt%.
  • Increasing amounts of these precipitates result in an increased E-modulus.
  • the increased amount of precipitates is achieved primarily by increasing the boron and chromium content of the steel, thereby creating more potential for forming the (Fe,Cr) 2 B and Fe 2 B precipitates.
  • the precipitates have an average size of below 10 ⁇ .
  • the average size is below 3 ⁇ .
  • a method for producing the steel according to the invention comprising the steps of:
  • the composition of the melt will also be the composition of the solidified steel.
  • the steel melt is produced by using ferro-alloys.
  • other alloys could be used, in addition to or instead of ferro-alloys, such as technically pure boron or chromium.
  • the steel according to the invention can be mass produced using conventional low- carbon steel producing equipment without the need to use powder metallurgy as a production method.
  • the steel melt from which the steel according to the invention is made is produced by using ferro-alloys and/or the aforementioned other alloys, and not by powder metallurgy.
  • the precipitates form from the melt upon solidification as a result of the right choice of chemistry and further processing and there is no need to include the precipitates in the steel by adding the precipitates as prefabricated ceramic particles by means of powder metallurgy.
  • Ferroalloy refers to various alloys of iron with a high proportion of one or more other elements, manganese, chromium, boron or silicon. It is used in the production of steels and alloys as a raw material. Beside ferro-alloys other alloys can be added to the melt, such as technically pure boron or chromium, e.g. to control the level of inclusions which tend to be higher in ferro-alloys than in the technically pure alloys.
  • Producing the steels from a steel melt instead of by the process of powder metallurgy has large financial and technical advantages. This makes the steel much easier to produce in mass, and thereby much easier to produce economically.
  • composition of the melt comprises 0.02 ⁇ C ⁇ 0.15 and/or 0.050 ⁇ Mn ⁇ 1.0 and/or 2.0 ⁇ Cr ⁇ 8.0.
  • composition of the melt comprises a chromium content of at least 4%.
  • composition of the melt comprises wt.% Cr > 4.00 + 3.96 wt.% B.
  • the starting material is homogenised before hot rolling at a temperature between 0.8 T/Tm and 0.88 T/Tm.
  • the advised temperature range for homogenisation prior to hot rolling is ⁇ 0.8-0.88 T/Tm, wherein Tm is the absolute melting temperature (in °C) and T the temperature of the relevant process (in °C).
  • Tm is the absolute melting temperature (in °C)
  • T the temperature of the relevant process (in °C).
  • reheating for hot rolling has to be controlled to be below 0.9 T/Tm. For a steel with a melting temperature of 1200°C, this means that reheating for hot rolling has to be below about 1080°C and the advised temperature for homogenisation is between 960°C and 1056°C.
  • the hot rolling temperature is between 0.6 T/Tm and 0.85 T/Tm.
  • the steel preferably has to be hot rolled at a temperature above 800°C.
  • the hot rolling preferably takes place while the steel is still austenitic, although it could be hot rolled in the intercritical region.
  • the (Fe,Cr) 2 B and Fe 2 B precipitates that ensure the desired properties of the steel are formed on cooling from casting, i.e. at the solidification.
  • the steel according to the invention is used in static constructions such as sections, bridges or bridge parts, buildings, in vehicles such as cars, yellow goods, trucks, or aerospace applications.
  • static constructions such as sections, bridges or bridge parts, buildings, in vehicles such as cars, yellow goods, trucks, or aerospace applications.
  • vehicles such as cars, yellow goods, trucks, or aerospace applications.
  • automotive applications such as vehicles this type of steel can be applied a.o. in bearings, brakes and suspension components, shock mounts, roof bows, vehicle floors.
  • gears, bearings in construction the structural steels for sections.
  • the possible applications are e.g. boom and bucket arm structures on backhoes and excavators.
  • Two 30 kg vacuum furnace melts (under Ar atmosphere) were produced of casts 1 and 2.
  • the melts were done using ferro-alloys with addition of relevant alloying elements and are aluminium-killed.
  • the 30 kg melts were cast in ingots with 75mm x 100mm section by approx 475mm long.
  • the casts 3 and 4 were produced as 24 kg ingots in vacuum furnace using ferro-alloys with addition of relevant alloying elements.
  • the cast dimensions were approximately 100mm x 100mm in section and 230mm long.
  • the casts were slowly cooled down from the solidification temperature.
  • the chemical compositions of the casts are given in Table 1.
  • the as cast microstructures of Casts 1-4 are presented in Figure 1 ( Figure la - Cast 1, Figure lb - Cast 2, Figure lc - Cast 3 and Figure Id - Cast 4).
  • the length of the white line in the lower left corner represents a length of 100 ⁇ in figures la and lb, and of 20 ⁇ in figures lc and Id .
  • the fractions of relevant eutectic particles for Cast 1, Cast 2, Cast 3 and Cast 4 are given in Table 2.
  • the density of steel from Cast 1 and 2 in as cast condition is 7.7 g/cm 3 .
  • the Young's modulus for Cast 1-4 was measured in as cast condition being in a range of 223-239 GPa. The results are given in Table 3. The measurements of Young's modulus were performed with a dynamic (Impulse Excitation Technique (IET)) method.
  • IET Impulse Excitation Technique

Abstract

The invention relates to a high strength steel with a increased E-modulus and to a method for producing said steel wherein the steel comprises in wt.% : - 0.002 ≤ C < 0.24 - 0.050 ≤ Mn < 1.5 - 0.5 ≤ B < 4.0 - 2.0 ≤Cr < 20 - 0 ≤ Si < 0.4 - 0 ≤ Al < 0.1 - 0 ≤ S < 0.030 - 0 ≤ P < 0.040 - 0 ≤ Ni < 1.0 - 0 ≤ Cu < 1.0 - 0 ≤ N < 0.04 - 0 ≤ Mo < 0.4 - 0 ≤ Nb < 0.1 - balance iron and unavoidable impurities resulting from production, wherein the structure of the steel comprises at least 5 wt% of ∑((Fe,Cr)2B and Fe2B) as precipitates.

Description

HIGH STRENGTH STEEL WITH INCREASED E-MODULUS AND METHOD FOR PRODUCING SAID STEEL
The invention relates to a high strength steel with a increased E-modulus and to a method for producing said steel.
In the automotive industry, where lighter vehicles and safety are of constant concern, a steel with a higher E-modulus allows the use of thinner gauge plate which still has the stiffness and strength of a thicker, and thus heavier plate. Also in other types of applications such as in yellow goods, aerospace or construction the combination of high E-modulus and high strength is an interesting combination of properties.
A known way to increase the modulus of elasticity and reduce the weight of steel is by incorporating ceramic particles of different natures, such as carbides, nitrides, oxides or borides. These particles have a much higher elastic modulus, ranging from about 250 to 550 GPa, than that of the steel base which' value is around 210 GPa. One way of including ceramic particles uniformly distributed in a matrix of steel is by means of powder metallurgy. Despite improved mechanical properties compared to steels containing no dispersion of ceramic particles, this type of method has problems. Reactions of the metal powders are difficult to prevent because of the high surface area of the metal powders. Even compaction and sintering, there may be residual porosity that may play a role in inducing fracture during cyclic loading. Uniform distribution of the particles in the matrix is difficult to achieve. Moreover the chemical composition of interfaces matrix/particle, and therefore their cohesion is difficult to control because of the surface contamination of the powders before sintering.
This type of process therefore potentially suitable for production in small quantities but not for economic production on the scale required for the automotive industry. Manufacturing costs associated with this type of manufacturing process are high.
In EP1897963 a steel is proposed based on the formation of large quantities of titaniumdiboride particles (TiB2). It describes a steel comprising 2.5 to 7.5% titanium and boron in the order of 0.8 to 4%. The result of these additions is that large numbers of eutectic TiB2-precipitates are formed in the steel matrix and these particles act as ceramic inclusions as a result of which the E-modulus increases and the density of the steel decreases. A disadvantage of this steel type is that it is very difficult to produce the TiB2-precipitates as fine eutectic precipitates. A further problem is the control of the morphology of the particles which tend to become sharp edged and may thus serve as a stress inducing particle and an initiation spot for cracks. A similar solution is chosen in JP2007-051341 where the mandatory presence of large amounts of titanium and vanadium is complemented by a mandatory presence of large amounts of chromium to produce and stabilise the formation of V3B4 making the costs of the alloy very high.
It is an object of this invention to provide a steel product with a higher E-modulus than the E-modulus of conventional steels.
It is also an object of this invention to provide a steel product without the presence of TiB2 or vanadium containing borides
It is also an object of this invention to provide a method of mass producing the steel product of the invention in an economical way.
One or more of these objects are reached by a steel comprising in wt.%
- 0.002 < C < 0.24
- 0.050 < Mn < 1.5
- 0.5 < B < 4.0
- 2.0 < Cr < 20.0
- 0 < Si < 0.4
- 0 < Al < 0.1
- 0 < S < 0.030
- 0 < P < 0.040
- 0 < Ni < 1.0
- 0 < Cu < 1.0
- 0 < N < 0.04
- 0 < Mo < 0.4
- 0 < Nb < 0.1
- balance iron and unavoidable impurities resulting from production,
wherein the structure of the steel comprises at least 5 wt% of ∑((Fe,Cr)2B and Fe2B). All compositional percentages are in weight percent (wt.%) unless otherwise indicated. The unavoidable impurities are elements unavoidably contained in the steel due to circumstances such as raw materials, manufacturing facilities, etc.
The amount of carbon is limited because of the weldability of the steel. The resistance to cracking and the toughness of the heat affected zone decreases if the carbon content exceeds 0.24%. A preferable maximum from this point of view would be 0.20%. From the point of view of weldability a preferable maximum carbon value is 0.15%, preferably 0.10% and more preferably 0.06%. A suitable minimum carbon level is 0.01%.
In an embodiment a suitable minimum carbon level is 0.02%.
Boron is an important and mandatory element because it forms the basis for the (Fe,Cr)2B and Fe2B. Both types of the precipitates raise the E modulus of the steel. Below the lower end of the inventive range the contribution of the precipitates to the increase of the E-modulus is insufficient. In an embodiment of the invention the boron content is at least 0.5%. On the other hand, if the content exceeds 3.5% the borides will become big and rough and will have an adverse effect on the hot-workability, ductility and toughness. Preferably the maximum boron content is 2.5%. A suitable minimum boron content was found to be 0.75% or even 0.9%. It should be noted that at least 5 wt% of ∑((Fe,Cr)2B and Fe2B) must be interpreted such that the sum of (Fe,Cr)2B and Fe2B must be above 5wt.%, but that it is possible that the situation wherein there is no (Fe,Cr)2B and more than 5% Fe2B or the other way around is also embodied by this criterium. Also both (Fe,Cr)2B and Fe2B can be present.
Chromium is an important and mandatory element effective in forming the (Fe,Cr)2B type boride and raising the Young's modulus of steel. A minimum amount of 2% is required to achieve this effect. If the chromium content exceeds 20%, big and coarse particles will be formed, and it will have an adverse effect on the hot-workability, ductility and toughness. A suitable maximum value for chromium was found to be 15% or even 13%. In an embodiment the chromium content is at most 8%. The chromium content is preferably at least 4%.
Manganese contributes to the strengthening of the matrix by solid solution or by transformation strengthening. Manganese is added mainly to enlarge the austenite region during hot rolling, similar to Cu and Ni additions. However if the amount increases above a value of 1.5% the risk of segregation and band formation increases to an unacceptable level. Manganese is also effective in binding sulphur thereby reducing the risk of hot-cracking during casting. Preferably the manganese content is at most 1.0%. In an embodiment of the invention the manganese content is at least 0.3%. A suitable maximum is 0.8%.
Aluminium is added only for deoxidation. The formation of coarse alumina particles must be prevented and therefore the maximum content should not exceed 0.1%. A suitable maximum aluminium content is 0.05%. The nickel and/or the copper content is up to 1.0% respectively. These elements when added increase the austenite range during hot rolling. When added, suitable minimum values for both elements are 0.1% respectively.
Preferably the nitrogen content is as low as possible. It should be noted that nitrogen is not added on purpose. Its presence is a consequence of the use of master alloys containing nitrogen. A suitable minimum content is 0.001 wt.% (10 ppm). A preferred minimum is 0.002% (20 ppm). A suitable maximum value for the nitrogen content is 0.02% (200 ppm).
Silicon is an element effective in deoxidation. If silicon is added for this purpose in addition or instead of aluminium, a minimum amount of 0.05% is needed. Silicon is also effective in solid-solution hardening. The formation of silicon oxides adversely affects the picklability and galvanisability of the steel plate. If the silicon content exceeds 0.4%, the risk of forming Laves phases which adversely affects toughness increases.
The molybdenum content is up to 0.4%. Molybdenum increases the strength of the steel, but for economic reasons the amount is limited at 0.4%. Preferably the molybdenum is at most 0.1%. If added as an alloying element the content needs to be at least 0.05%.
Niobium can be added in small amounts up to 0.1% to induce an added strengthening of the steel in the form of fine carbides or carbonitrides. If added as an alloying element the content needs to be at least 0.01%.
Sulphur and phosphorus have a detrimental effect on the steel because of the risk of formation of MnS and segregation of phosphorus on the grain boundaries which adversely affect hot and cold formability of the steel. Therefore sulphur and phosphorus levels need to be kept below 0.040 and 0.030 wt.% respectively, preferably below 0.020 and 0.015 wt.% respectively.
In an embodiment of the invention the boron and chromium content satisfy the following relationship:
wt.% Cr > 4.00 + 3.96 wt.% B.
The inventors found that when the boron and chromium content satisfy this relationship and the amount of the Cr- and B-based precipitates is above certain value, e.g. 5wt%, in the steel, then the E-modulus of the steel is increased significantly.
In an embodiment the E-modulus of the steel is at least 220 GPa, preferably at least 230 GPa, more preferably at least 235 GPa. The increase in E-modulus is determined by the presence of the (Fe,Cr)2B and Fe2B precipitates. These precipitates are very stable. Precipitates present in the cast product will not dissolve during reheating and hot-rolling, and therefore if the∑((Fe,Cr)2B and Fe2B) is above a certain value, e.g. 5 wt.%, in the cast material, it will also be above that certain value in the hot-rolled material. Proper choice of the rolling conditions will keep decohesion of the precipitates from the matrix to an absolute minimum, thereby also keeping the reduction in E- modulus as a result of the decohesion to an absolute minimum. Conventional steels, such as dual-phase steels, usually have an E-modulus of about 200 GPa, so an improvement of 10% or more allows for a significant decrease in thickness whilst retaining the same stiffness.
In an embodiment the structure of the steel comprises at least 8 wt% of ∑((Fe,Cr)2B and Fe2B), more preferably at least 10 or even 15 wt%. Increasing amounts of these precipitates result in an increased E-modulus. The increased amount of precipitates is achieved primarily by increasing the boron and chromium content of the steel, thereby creating more potential for forming the (Fe,Cr)2B and Fe2B precipitates.
In an embodiment the precipitates have an average size of below 10 μιτι. Preferably the average size is below 3 μιτι.
According to a second aspect, a method is provided for producing the steel according to the invention comprising the steps of:
a. providing a steel melt comprising in wt.% :
- 0.002 < C < 0.24
- 0.050 < Mn < 1.5
- 0.5 < B < 4.0
- 2.0 < Cr < 20
- 0 < Si < 0.4
- 0 < Al < 0.1
- 0 < S < 0.030
- 0 < P < 0.040
- 0 < Ni < 1.0
- 0 < Cu < 1.0
- 0 < N < 0.04
- 0.0 < Mo < 0.4 - 0.0 < Nb < 0.1
- balance iron and unavoidable impurities resulting from production b. casting the melt into a suitable starting material for hot rolling by casting an ingot or bloom or by casting a slab, rod or thin slab, or by casting a strip; c. hot rolling the starting material to the finish hot rolling dimensions.
It is assumed that the composition of the melt will also be the composition of the solidified steel. In an embodiment the steel melt is produced by using ferro-alloys. Optionally other alloys could be used, in addition to or instead of ferro-alloys, such as technically pure boron or chromium. A big advantage is that according to this method the steel according to the invention can be mass produced using conventional low- carbon steel producing equipment without the need to use powder metallurgy as a production method. Preferably the steel melt from which the steel according to the invention is made is produced by using ferro-alloys and/or the aforementioned other alloys, and not by powder metallurgy. The precipitates form from the melt upon solidification as a result of the right choice of chemistry and further processing and there is no need to include the precipitates in the steel by adding the precipitates as prefabricated ceramic particles by means of powder metallurgy. Ferroalloy refers to various alloys of iron with a high proportion of one or more other elements, manganese, chromium, boron or silicon. It is used in the production of steels and alloys as a raw material. Beside ferro-alloys other alloys can be added to the melt, such as technically pure boron or chromium, e.g. to control the level of inclusions which tend to be higher in ferro-alloys than in the technically pure alloys. Producing the steels from a steel melt instead of by the process of powder metallurgy has large financial and technical advantages. This makes the steel much easier to produce in mass, and thereby much easier to produce economically.
In an embodiment the composition of the melt comprises 0.02 < C < 0.15 and/or 0.050 < Mn < 1.0 and/or 2.0 < Cr < 8.0.
In an embodiment the composition of the melt comprises a chromium content of at least 4%.
In an embodiment the composition of the melt comprises wt.% Cr > 4.00 + 3.96 wt.% B.
In an embodiment the starting material is homogenised before hot rolling at a temperature between 0.8 T/Tm and 0.88 T/Tm. The advised temperature range for homogenisation prior to hot rolling is ~0.8-0.88 T/Tm, wherein Tm is the absolute melting temperature (in °C) and T the temperature of the relevant process (in °C). In view of the low melting point of the steel, which may be below 1200°C, depending on the composition, reheating for hot rolling has to be controlled to be below 0.9 T/Tm. For a steel with a melting temperature of 1200°C, this means that reheating for hot rolling has to be below about 1080°C and the advised temperature for homogenisation is between 960°C and 1056°C. In an embodiment of the invention the hot rolling temperature is between 0.6 T/Tm and 0.85 T/Tm. The steel preferably has to be hot rolled at a temperature above 800°C. The hot rolling preferably takes place while the steel is still austenitic, although it could be hot rolled in the intercritical region. The (Fe,Cr)2B and Fe2B precipitates that ensure the desired properties of the steel are formed on cooling from casting, i.e. at the solidification.
Method according to any of claims 5 to 8 wherein the hot rolled steel is subsequently subjected to cold rolling to further reduce the gauge of the material and annealed at the temperature suitable to recrystallize the grain structure.
According to a third aspect the steel according to the invention is used in static constructions such as sections, bridges or bridge parts, buildings, in vehicles such as cars, yellow goods, trucks, or aerospace applications. In all these application it is desirable to construct lightly, and the combination of high strength and high E-modulus allows to construct with a thinner gauge steel. In automotive applications such as vehicles this type of steel can be applied a.o. in bearings, brakes and suspension components, shock mounts, roof bows, vehicle floors. In aerospace applications one of the possible applications are gears, bearings, in construction the structural steels for sections. For yellow goods (lifting and excavating) the possible applications are e.g. boom and bucket arm structures on backhoes and excavators.
The invention is now further explained by means of the following, non-limitative examples.
Two 30 kg vacuum furnace melts (under Ar atmosphere) were produced of casts 1 and 2. The melts were done using ferro-alloys with addition of relevant alloying elements and are aluminium-killed. The 30 kg melts were cast in ingots with 75mm x 100mm section by approx 475mm long. The casts 3 and 4 were produced as 24 kg ingots in vacuum furnace using ferro-alloys with addition of relevant alloying elements. The cast dimensions were approximately 100mm x 100mm in section and 230mm long. The casts were slowly cooled down from the solidification temperature. The chemical compositions of the casts are given in Table 1. The as cast microstructures of Casts 1-4 are presented in Figure 1 (Figure la - Cast 1, Figure lb - Cast 2, Figure lc - Cast 3 and Figure Id - Cast 4). The length of the white line in the lower left corner represents a length of 100 μιτι in figures la and lb, and of 20 μιτι in figures lc and Id . The fractions of relevant eutectic particles for Cast 1, Cast 2, Cast 3 and Cast 4 are given in Table 2. The density of steel from Cast 1 and 2 in as cast condition is 7.7 g/cm3.
Table 2 - Fractions of eutectic particles in Cast 1 and Cast 2 (wt.% was determined by XRD and vol% by image analysis of SEM micrographs).
Figure imgf000009_0002
The Young's modulus for Cast 1-4 was measured in as cast condition being in a range of 223-239 GPa. The results are given in Table 3. The measurements of Young's modulus were performed with a dynamic (Impulse Excitation Technique (IET)) method.
Table 3 - Results of Young's Modulus measurements of Cast 1 - 4 in as cast condition by IET method.
Figure imgf000009_0001
Table 1 - Chemical composition of experimental casts, wt. %, (imp = impurity level)
Figure imgf000010_0001

Claims

1. Steel having a chemical composition comprising in wt.% :
- 0.002 < C < 0.24
- 0.050 < Mn < 1.5
- 0.5 < B < 4.0
- 2.0 < Cr < 20
- 0 < Si < 0.4
- 0 < Al < 0.1
- 0 < S < 0.030
- 0 < P < 0.040
- 0 < Ni < 1.0
- 0 < Cu < 1.0
- 0 < N < 0.04
- 0 < Mo < 0.4
- 0 < Nb < 0.1
- balance iron and unavoidable impurities resulting from production
wherein the structure of the steel comprises at least 5 wt% of ∑((Fe,Cr)2B and Fe2B) as precipitates.
2. Steel according to claim 1 wherein the steel composition comprises 0.02 < C < 0.15 and/or 0.050 < Mn < 1.0 and/or 2.0 < Cr < 8.0.
3. Steel according to claim 1 or 2 wherein the chromium content is at least 4%.
4. Steel according to any one of claim 1 to 3 wherein the E-modulus is at least 220 GPa, preferably at least 230 GPa, more preferably at least 235 GPa.
5. Steel according to any one of claim 1 to 4 wherein wt.% Cr > 4.00 + 3.96 wt.% B.
6. Steel according to any one of claims 1 to 2 wherein the structure of the steel comprises at least 8 wt% of ∑((Fe,Cr)2B and Fe2B), more preferably at least 10 or even 15 wt%.
7. Steel according to any one of claims 1 to 3 wherein the precipitates have an average size of below 10 μιτι, preferably below 3 μιτι.
8. Method for producing the steel of any one of claims 1 to 7 comprising the steps of:
a. providing a steel melt comprising in wt.% :
- 0.002 < C < 0.24
- 0.050 < Mn < 1.5
- 0.5 < B < 4.0
- 2.0 < Cr < 20
- 0 < Si < 0.4
- 0 < Al < 0.1
- 0 < S < 0.030
- 0 < P < 0.040
- 0 < Ni < 1.0
- 0 < Cu < 1.0
- 0 < N < 0.04
- 0.0 < Mo < 0.4
- 0.0 < Nb < 0.1
- balance iron and unavoidable impurities resulting from production b. casting the melt into a suitable starting material for hot rolling by casting an ingot or bloom or by casting a slab, rod or thin slab, or by casting a strip; c. hot rolling the starting material to the finish hot rolling dimensions.
9. Method according to claim 8 wherein the composition of the melt comprises 0.02 < C < 0.15 and/or 0.050 < Mn < 1.0 and/or 2.0 < Cr < 8.0.
10. Method according to claim 8 or 9 wherein the composition of the melt comprises a chromium content of at least 4%.
11. Method according to any one of claims 8 to 10 wherein the composition of the melt comprises wt.% Cr > 4.00 + 3.96 wt.% B.
12. Method according to any one of claims 8 to 11 wherein the hot rolling temperature is between 0.6 T/Tm and 0.85 T/Tm.
13. Method according to any one of claims 8 to 12 wherein the steel melt is produced by using ferro-alloys and optionally by using also other alloys, such as technically pure boron or chromium.
14. Method according to any one of claims 8 to 13 wherein the starting material is homogenised before hot rolling at a temperature between 0.8 T/Tm and 0.88 T/Tm.
15. Method according to any of claims 5 to 8 wherein the hot rolled steel is subsequently subjected to cold rolling to further reduce the gauge of the material and annealed at the temperature suitable to recrystallize the grain structure.
16. Steel part produced from the steel according to any one of claim 1 to 7 for use in static constructions, vehicles such as cars, yellow goods, trucks, or aerospace applications.
PCT/EP2013/059966 2012-05-14 2013-05-14 High strength steel with increased e-modulus and method for producing said steel WO2013171231A1 (en)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1046723A1 (en) * 1999-04-19 2000-10-25 Sumitomo Metal Industries, Ltd. Stainless steel product for producing polymer electrode fuel cell
JP2004218069A (en) * 2002-12-26 2004-08-05 Aichi Steel Works Ltd High rigidity steel producible by melting method, and production method therefor
JP2007051341A (en) 2005-08-18 2007-03-01 Sumitomo Metal Ind Ltd Steel with high young's modulus
EP1897963A1 (en) 2006-09-06 2008-03-12 ARCELOR France Steel sheet for the manufacture of light structures and manufacturing process of this sheet
CN101624678A (en) * 2009-08-14 2010-01-13 江苏新亚特钢锻造有限公司 High-toughness boride abrasion resisting metal material and manufacture technology thereof

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1046723A1 (en) * 1999-04-19 2000-10-25 Sumitomo Metal Industries, Ltd. Stainless steel product for producing polymer electrode fuel cell
JP2004218069A (en) * 2002-12-26 2004-08-05 Aichi Steel Works Ltd High rigidity steel producible by melting method, and production method therefor
JP2007051341A (en) 2005-08-18 2007-03-01 Sumitomo Metal Ind Ltd Steel with high young's modulus
EP1897963A1 (en) 2006-09-06 2008-03-12 ARCELOR France Steel sheet for the manufacture of light structures and manufacturing process of this sheet
CN101624678A (en) * 2009-08-14 2010-01-13 江苏新亚特钢锻造有限公司 High-toughness boride abrasion resisting metal material and manufacture technology thereof

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