EP3354764B1 - Quench hardened steel - Google Patents

Quench hardened steel Download PDF

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Publication number
EP3354764B1
EP3354764B1 EP17153320.1A EP17153320A EP3354764B1 EP 3354764 B1 EP3354764 B1 EP 3354764B1 EP 17153320 A EP17153320 A EP 17153320A EP 3354764 B1 EP3354764 B1 EP 3354764B1
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European Patent Office
Prior art keywords
steel
optionally
component
region
hardness
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EP17153320.1A
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German (de)
English (en)
French (fr)
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EP3354764A1 (en
Inventor
Tommi Liimatainen
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SSAB Technology AB
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SSAB Technology AB
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Priority to PL17153320T priority Critical patent/PL3354764T3/pl
Application filed by SSAB Technology AB filed Critical SSAB Technology AB
Priority to EP17153320.1A priority patent/EP3354764B1/en
Priority to ES17153320T priority patent/ES2793323T3/es
Priority to HUE17153320A priority patent/HUE049006T2/hu
Priority to KR1020197024443A priority patent/KR20190112021A/ko
Priority to AU2018211466A priority patent/AU2018211466A1/en
Priority to CN201880008524.1A priority patent/CN110225991A/zh
Priority to JP2019540449A priority patent/JP2020508393A/ja
Priority to PCT/EP2018/051971 priority patent/WO2018138270A1/en
Priority to US16/480,034 priority patent/US20190368014A1/en
Publication of EP3354764A1 publication Critical patent/EP3354764A1/en
Priority to ZA2019/04183A priority patent/ZA201904183B/en
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Publication of EP3354764B1 publication Critical patent/EP3354764B1/en
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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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/55—Hardenability tests, e.g. end-quench tests
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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

Definitions

  • the present disclosure relates to a quench-hardenable steel that provides excellent cold formable properties and that can be reheat-quench hardened to provide a steel having excellent impact strength and hardness.
  • the steel has a reduced tendency to hardening induced cracking even when using a very high cooling rate, such as water quenching, and without any tempering.
  • the quench hardening can be carried out over the entire component formed from the steel, or only part of the component such as only at the edge of a blade.
  • the disclosure also relates to a method of quench hardening steel.
  • Quench hardening is a process in which steel and cast iron alloys are strengthened and hardened.
  • a steel is heated to an austenization temperature (preferably around 900°C), soaked (i.e. equalized), and then rapidly cooled (quenched) preferably with a liquid such as water or oil.
  • Quenched steels are typically brittle due to an overabundance of martensite.
  • the steels can be tempered, i.e. heated to below the critical point (A c1 ) to reduce the hardness and increase the toughness. Tempering however also increases the processing steps and costs.
  • Quenching may be through the entire steel sample, or localised for example at a blade edge.
  • Targeted heating means such as induction heating may be used if only localised or even surface hardening is required.
  • the quench medium itself can also influence the tendency for crack formation to occur.
  • the cooling rate is typically much higher in comparison to oil. While this gives rise to harder steels, the likelihood of stress cracking is higher. Transformational stresses can be reduced during tempering, but this inevitably reduces the hardness of the steel.
  • Impact strength is related to the ability of the material to dissipate the energy of an impact through its structure, which is a tendency favoured by softer, more malleable materials.
  • Hardened steels may be extremely resistant to wear at their surface, but often this high hardness leads to a relatively brittle material with low impact strength. Even so, the combination of high hardness and good impact strength is highly desirable in some scenarios.
  • agricultural equipment such as ploughs are required to have high wear resistance and high hardness to allow them to withstand the abrasion they experience when in use.
  • ploughs made from hardened materials with low impact strength may be at risk of damage when they encounter stones or other hard objects in the soil.
  • a reduced risk of hardening cracks is extremely important when the cooling rate is higher than standard water quenching, as can occur when using powerful stirring and/or when using salt water (brine) as the quenching bath.
  • US 5,454,883A relates to a steel plate having a high toughness (impact strength), low yield ratio (tensile strength / yield strength) and high fatigue strength. Tempering is an indispensable step in the process of producing such a steel plate.
  • the steel plate in the as-quenched state has a metallographical microstructure substantially composed of martensite, bainite or mixture thereof.
  • the tempering step generates a metallographical microstructure substantially composed of ferrite and cementite. It is conceivable that the steel plate would not have a high hardness due to the microstructure substantially composed of ferrite and cementite.
  • quench hardenable steels that provide high hardness steels with good impact strength and with a reduced risk of hardening cracks. Furthermore, there remains a need for quench hardenable steels having good structural properties, which allow localised quench hardening particularly using water as the quench medium.
  • the present disclosure relates to a hardenable steel composition consisting of the following, in terms of weight percent: C 0.36-0.55%, Mn 0.1-0.7%, Si 0-0.7%, Mo 0.2-1.9%, Al ⁇ 0.2% (not including 0%), Cr optionally ⁇ 1%, Ni optionally ⁇ 1%, Cu optionally ⁇ 1%, Nb optionally ⁇ 0.1%, Ti optionally ⁇ 0.2%, V optionally ⁇ 0.2%, B optionally ⁇ 0.01%, W optionally ⁇ 1%, and Co optionally ⁇ 1%, wherein
  • the present disclosure relates to a hardenable steel that is capable of being reheat-quench hardened to form a steel satisfying the following equation: V J + H X * Hv 10 > A X wherein
  • the hardenable steel is capable of being reheat-quench hardened from a temperature above A c3 to a temperature below M S at an average cooling rate of ⁇ 50°C/s (more preferably ⁇ 80°C/s) to form a crack-free hardened steel.
  • the hardenable steel consists of the following, in terms of weight percent: C 0.36-0.55%, Mn 0.1-0.7%, Si 0-0.7%, Mo 0.2-1.9%, Al ⁇ 0.2% (not including 0%), Cr optionally ⁇ 1%, Ni optionally ⁇ 1%, Cu optionally ⁇ 1%, Nb optionally ⁇ 0.1%, Ti optionally ⁇ 0.2%, V optionally ⁇ 0.2%, B optionally ⁇ 0.01%, W optionally ⁇ 1%, and Co optionally ⁇ 1%, the balance being iron, residual contents and unavoidable impurities.
  • the hardenable steel satisfies the following relationships:
  • the present disclosure also relates to a component formed from the hardenable steel (or hardenable steel composition), wherein at least a region of the component has been reheat-quench hardened to form a hardened region.
  • the entire component has been reheat-quench hardened.
  • the microstructure of the hardened region of the component is martensitic.
  • the hardened region of the component has a Vickers hardness of ⁇ 630 Hv10.
  • the hardened region of the component has a Charpy V impact strength of ⁇ 3 J/cm 2 at 20°C.
  • the hardened region satisfies the following: V J + H X * Hv 10 > A X wherein
  • the present disclosure also relates to a method for producing hardened steel, comprising the steps of:
  • the present disclosure relates to a method for producing hardened steel, comprising the steps of:
  • step b. the entire component is heated to a temperature above A c3 , and more preferably in step c. (or step c'.) the entire component is then quenched with the quenching fluid to form a hardened component.
  • the hardenable steel in step a' consists of the following, in terms of weight percent: C 0.36-0.55%, Mn 0.1-0.7%, Si 0-0.7%, Mo 0.2-1.9%, Al ⁇ 0.2% (not including 0%), Cr optionally ⁇ 1%, Ni optionally ⁇ 1%, Cu optionally ⁇ 1%, Nb optionally ⁇ 0.1%, Ti optionally ⁇ 0.2%, V optionally ⁇ 0.2%, B optionally ⁇ 0.01%, W optionally ⁇ 1%, and Co optionally ⁇ 1%, the balance being iron, residual contents and unavoidable impurities.
  • the hardenable steel in step a' satisfies the following relationships:
  • the steel is crack-free after step c. (or step c'.).
  • the cooling rate in step c. is ⁇ 50°C/s, more preferably ⁇ 80°C/s.
  • the hardenable steel (composition) of the present disclosure provides excellent quench hardening characteristics with reduced risk for quench cracking compared to standard hardenable steels such as 51 CrV4, 38MnB5, 42MnV7, and the like.
  • the steel is able to provide hardness levels of >630 Hv10 when quench hardened using water, and can typically be used even without tempering (i.e. the quench hardened steel is crack-free without tempering).
  • the excellent formability and structural properties of the steel prior to quench hardening allows it to be hot or cold formed into a range of components, quench hardened (optionally locally) to form a martensitic steel using a simple and cost effective process, and then used without any additional tempering steps.
  • hardenable steel composition
  • hardenable steel composition set out above, as well as the hardenable steel capable of being reheat-quench hardened to form a steel satisfying the equation V J + (H X ⁇ Hv10) > A X as set out above.
  • crack-free is meant that the steel meets ISO 23278:2015, Class 2X.
  • the ISO23278:2015 standard specifies acceptance levels for indication from imperfections in ferromagnetic steel welds detected by magnetic particle testing. This tolerance level means that, when tested using magnetic particle detection according to the ISO23278:2015 standard, any linear imperfections such as cracks/defects are below 1.5 mm in length, and non-linear imperfections such as cracks/defects are below 3 mm in maximum dimension.
  • a linear imperfection is defined as having a length greater than three times its width, while a non-linear imperfection is defined as having a length less than or equal to three times its width.
  • rack-free is meant that the steel meets ISO23278:2015, Class 2X when held at room temperature for 24 hours, more preferably for 1 week, after quench hardening.
  • martensite and “martensitic” include martensite, self-tempered (or auto-tempered) martensite and tempered martensite.
  • a steel containing ⁇ 95% martensite has ⁇ 95% of is structure being martensite, self-tempered martensite and tempered martensite.
  • a c1 is the temperature at which austenite ( ⁇ ) begins to form during heating.
  • a r1 is the temperature at which austenite ( ⁇ ) to ferrite ( ⁇ ) transformation is completed during cooling.
  • a c3 is the temperature at which transformation of ferrite ( ⁇ ) to austenite ( ⁇ ) is completed during heating.
  • a r3 is the temperature at which austenite ( ⁇ ) begins to transform to ferrite ( ⁇ ) during cooling.
  • a c1 , A c3 , A r1 , A r3 are well known parameters that would be known to the skilled person. They may be estimated experimentally using standard methodology. Alternatively, the parameters may be estimated theoretically, for example using the formulae from Brandis (Rechnerische Betician der Umwandlungstemperaturen vonstoreleg striving Stählen. TEW - Technische Berichte, Band 1, Heft 1, 1975, 8-10 ) or Lutsenko (The Definition and Use of Technological Reserves - An Effective Way to Improve the Production Technology of Rolled Metal.
  • a c 1 739 ⁇ 22 * % C ⁇ 7 * % Mn + 2 * % Si + 14 * % Cr + 13 * % Mo ⁇ 13 * % Ni + 20 * % V
  • a c 3 902 ⁇ 255 * % C ⁇ 11 * % Mn + 19 * % Si ⁇ 5 * % Cr + 13 * % Mo ⁇ 20 * % Ni + 55 * % V
  • a r 1 741.7 ⁇ 7.13 * % C ⁇ 14.09 * % Mn + 16.26 * % Si + 11.54 * % Cr ⁇ 49.69 * % Ni
  • a r 3 913.7 ⁇ 207.13 * % C ⁇ 46.6 * % Mn + 110.54 * % Cr + 108.1 * % N
  • the term "hardenable” preferably means that a sample of the steel with a thickness of 3 mm, when heated to and soaked at 950°C for a 1 hour and then quenched with still water, has at least 90% martensitic microstructure at the centre of the sample.
  • the key characterising features of the hardenable steel composition of the disclosure is the relatively low Mn, Si and Cr content in combination with the relatively high Mo content.
  • the combination of these elements in these ratios provides a steel having good structural properties that displays exceptional impact strength and quench hardenability combined with the low risk of quench induced cracking.
  • composition The chemical composition of the hardenable steel (composition) is now described in more detail.
  • all percentages in the compositions are percentages by weight.
  • each of the preferred ranges for the various components mentioned below may be combined with any the preferred ranges of the remaining components.
  • Carbon is primarily present to ensure the hardenable steel (composition) is capable of quench hardening to form martensite. A higher carbon content will provide a harder steel. However, if the carbon content is too high, the resultant quenched steel can become too brittle and the risk of quench induced cracking increases. A maximum carbon content of 0.55% is therefore preferably used.
  • carbon is used at levels of 0.38-0.52 %, more preferably 0.40-0.50 %, even more preferably 0.41-0.48 %.
  • Silicon is included in steels to improve cleanliness during the smelt processing and it can have a positive effect on hardenability. However, too much silicon can reduce the amount of self-tempering that occurs during quenching, as well as have an impact on the surface quality of the finished steel. A maximum silicon content of 0.7 is therefore preferably used.
  • silicon is used at levels of 0.01-0.5 %, more preferably 0.05-0.35 %, most preferably 0.10-0.25%.
  • manganese is used at levels of 0.10-0.50 %, more preferably 0.15-0.40 %, most preferably 0.20-0.30 %.
  • Chromium is optionally used to increase hardenability. However, it also significantly reduces the martensite start temperature, which can negatively affect the amount of self-tempering that occurs during quenching.
  • the total chromium content is therefore preferably less than 1 %, preferably less than 0.75 %.
  • chromium is used at levels of 0-0.5 %, more preferably 0-0.3 %, most preferably 0-0.20 %.
  • Molybdenum is included to provide hardenability and improve tempering resistance.
  • molybdenum does not have a significant impact on the martensite start temperature, meaning that the use of molybdenum as the primary hardening agent ensures the martensite start temperature remains high and self-tempering is promoted during quenching.
  • Molybdenum also helps improve the impact strength of the hardened steel composition. It is well known that impact strength increases with increasing temperature, with lower temperature materials being more brittle and higher temperature materials being more ductile. The plot of impact strength against temperature for steels is therefore typically a sigmoidal shape, with a relatively steep transition region where the steel changes from brittle to ductile behaviour and impact strength increases significantly.
  • molybdenum and nickel frequently increase the impact strength of the steel, effectively lowering the temperature at which the steel transitions from brittle to ductile behaviour, with the effect from molybdenum being particularly significant. Molybdenum also helps retain hardness during tempering at relatively high temperatures, such as from 500-650 or more preferably from 500 to 600°C.
  • molybdenum is used at levels of 0.2-1.9 %.
  • molybdenum is used at levels of 0.2-1.5 %, more preferably 0.30-1.2 %, more preferably 0.40-1.1 %, even more preferably 0.50-1.0 %.
  • the combination of manganese, chromium and molybdenum provide the hardenability in the hardenable steel (composition).
  • molybdenum is used in favour of the other two to provide a steel with high hardenability and high impact strength, that retains a high martensite start temperature, and which is more able to retain hardness and impact strength during tempering after hardening.
  • the hardenable steel (composition) disclosed herein preferably contains a relatively higher level of molybdenum in comparison to chromium and manganese, and preferably satisfies the following conditions:
  • %Mo ⁇ %Cr+2 ⁇ %Mn Preferably: %Mo ⁇ %Cr+2 ⁇ %Mn.
  • the steel preferably contains enough manganese, chromium and molybdenum to ensure a good level of hardenability.
  • the hardenable steel (composition) preferably displays sufficient hardenability if the following relationship is satisfied:
  • Aluminium is used as a deoxidation (killing) agent. However, too high levels of aluminium should be avoided, as it can impact on the hardenability/self-tempering properties of the steel. Consequently, the preferable maximum aluminium levels are 0.2 %.
  • aluminium is used in the range 0.01-0.1 %, more preferably from 0.01-0.06 %, most preferably from 0.015-0.045 %. These low aluminium levels are particularly preferred when good electric resistance welding properties are desired.
  • Niobium and titanium control the texture of the steel after hot-rolling, and prevent grain growth during the heating step prior to quenching. They are therefore helpful in controlling the retention of properties of the hot-rolled steel during quench hardening. However, excessive amounts can lead to precipitates forming, such as large titanium nitrides which can negatively affect impact strength.
  • hot press-forming the steel can cool down rapidly during transfer to the die and the hot forming die, for instance at rates of around 20°C/s. Due to this, it is usually necessary to use a very high start temperature, to ensure the steel remains at a suitable temperature (e.g. above the austenite temperature A r3 ) until quenching.
  • a suitable temperature e.g. above the austenite temperature A r3
  • titanium is present in the range of 0-0.1%, more preferably 0.001-0.05%, more preferably 0.005-0.02%.
  • Ti/N ⁇ 3.42.
  • niobium is present in the range of 0.001-0.05%, more preferably 0.005-0.02%, most preferably 0.008-0.015%.
  • the hardenable steel (composition) contains at least one of titanium or niobium.
  • the hardenable steel (composition) contains niobium.
  • Niobium can affect the ductility of the steel, reducing its cold-formability. If very demanding cold forming is needed prior to reheat-quenching, the hardenable steel (composition) preferably has a niobium content of ⁇ 0.005%.
  • Vanadium increases hardenability, but vanadium carbides can be difficult to dissolve during austenisation prior to quench hardening. These carbides can act as nuclei for ferrite crystals to grow during quenching. Consequently, vanadium content should preferably be limited to ⁇ 0.2 %.
  • vanadium is optionally present at levels of ⁇ 0.15%, more preferably ⁇ 0.10%, more preferably ⁇ 0.06%, most preferably ⁇ 0.04%.
  • nickel has a positive influence on the impact strength of the martensitic steel.
  • Copper is preferably optionally present at ⁇ 0.4%, more preferably ⁇ 0.1%.
  • Nickel is preferably optionally present at ⁇ 0.4%, more preferably ⁇ 0.1%.
  • Cu+Ni ⁇ 0.4%, more preferably ⁇ 0.1%.
  • Nickel has a tendency to phase separate and migrate to the surface (under the scale) during processing above the melt temperature of copper, which can be harmful to the surface quality when the material is hot formed.
  • Nickel mitigates this effect, and as such the Ni content is desirably at least 0.33*%Cu, preferably at least 0.50*%Cu, when the %Cu ⁇ 0.2.
  • Boron and tungsten can improve hardenability. However, they are typically not needed as the hardenability of the steel is primarily provided by other elements. Moreover, boron has a smaller effect on the hardenability of steels with higher carbon levels, such as those disclosed herein. In order for boron to have an effect on higher carbon steels, typically the nitrogen level must be low and/or the titanium content must be high enough, which increases the likelihood of TiN precipitates forming. Boron is therefore not harmful to the steel, it is simply not essential to provide the desired balance of properties if there are not any coarse TiN precipitates present. Cobalt is very expensive and typically unnecessary.
  • boron is present at levels of 0-0.003%, more preferably 0-0.0005%, even more preferably 0-0.0002%.
  • tungsten is present a ⁇ 0.2%, more preferably ⁇ 0.1%.
  • cobalt is present a ⁇ 0.2%, more preferably ⁇ 0.1%.
  • Co+W ⁇ 0.5%, more preferably ⁇ 0.3%, more preferably ⁇ 0.1%.
  • Residual contents include contents that may unavoidably exist in the steel, i.e. alloying elements having residual contents are not purposefully added.
  • Unavoidable impurities can be phosphorus (P), sulphur (S), nitrogen (N), hydrogen (H), oxygen (O), calcium (Ca), and rare earth metals (REM) or the like. Their contents are preferably limited as follows in order to ensure the properties of the hardenable steel (composition):
  • residual contents are controlled quantities of alloying elements, which are not considered to be impurities.
  • a residual content is normally controlled by an industrial process does not have an essential effect upon the alloy.
  • the levels of residual contents of hardenable steels are typically low.
  • the martensitic start temperature (M S ) of the hardenable steel (composition) is preferably ⁇ 340°C, more preferably ⁇ 350°C, more preferably ⁇ 360°C, more preferably ⁇ 370°C, most preferably ⁇ 375°C.
  • the hardenable steel (composition) disclosed herein is formed by hot-rolling (e.g. the above composition) and cooling to form a hardenable steel product.
  • the actual process steps used to form the steel product can vary, as can the microstructure of the resultant steel prior to quench hardening.
  • the steel product is preferably hot rolled, though cold rolling (for instance skin rolling) can be carried out prior to hardening.
  • the steel product is hot rolled (i.e. no cold rolling is carried out).
  • the hardenable steel (composition) is hot rolled.
  • Typical process steps for forming the hardenable steel product comprise the following steps in the given sequence:
  • the step of providing a steel slab may comprise forming a melt (e.g. from suitable components that combine to make the hardenable steel composition), and extruding the melt directly into the hot rolling.
  • the step may comprise providing a preformed slab (or billet) of the hardenable steel composition and heating it to the required temperature prior to hot rolling.
  • the conditions used in the hot rolling step may be adjusted accordingly to ensure that the resultant steel has the desired balance of strength and flexibility.
  • the cooling steps following hot rolling are not critical, and may be suitably adjusted to provide the desired microstructure of the hot-rolled product.
  • the hardenability properties are influenced by the grain size of the hardenable steel.
  • Typical average austenite grain size for the hardened steel is ⁇ 25 ⁇ m, preferably around 5-20 ⁇ m, more preferably around 5-15 ⁇ m.
  • a suitable hot rolling protocol to form a product with this grain size is as follows:
  • the purpose of the rough rolling is to compress the slab and remove any porosity that may still be present following the slab formation.
  • the finish rolling refines the grain sizes in the steel.
  • the coiling temperature influences the predominant phase in the final steel. Coiling at 650-750°C (e.g. about 720°C) will promote ferrite and pearlite and provide a softer, more malleable material. However, these higher coiling temperatures will typically lead to more scale at the surface of the steel. Coiling at 580-650°C (e.g. about 630°C) reduces the likelihood of scale formation and promotes some bainite growth giving a favourable balance of properties.
  • Coiling at temperatures below 580°C (particularly below 500°C) promotes growth of high levels of bainite, which will make the resultant product less formable and potentially more difficult to process into a component prior to quench hardening.
  • a high cooling rate and coiling at a temperature below M S promotes high levels of martensite, which increases the risk of cracks forming in the coiled strip.
  • the hot-rolling comprises hot-rolling to form a steel strip having a thickness of 2-15 mm, preferably 2-12 mm.
  • Cold rolling may optionally be carried out, particularly if a strip with thickness ⁇ 2 mm is desired.
  • the hot rolling may form a plate having thickness of from 3-80 mm, preferably from 4-50 mm, more preferably 5-15 mm.
  • the thickness of the steel is at least 5 mm, more preferably at least 6 mm, more preferably at least 6.5 mm, and even more preferably at least 7 mm.
  • Preferred thicknesses therefore include 5-15 mm, more preferably 6-12 mm.
  • the thickness may vary across the component, for instance if part of the component has been machined to form an edge.
  • the component has a maximum thickness of at least 5 mm, more preferably at least 6 mm, more preferably at least 6.5 mm, and even more preferably at least 7 mm.
  • the component has a minimum thickness of at least 5 mm, more preferably at least 6 mm, more preferably at least 6.5 mm, and even more preferably at least 7 mm.
  • maximum thickness is meant the thickness of the thickest part of the component.
  • minimum thickness is meant the thickness of the thinnest part of the component.
  • coiling the strip at around 700°C will typically form a two-phase composition containing ferrite and pearlite.
  • Coiling the strip at lower temperatures, such as around 600°C will typically promote bainite to form in addition to ferrite and pearlite.
  • Lower coiling temperatures also promote a finer grain size.
  • the microstructure of the hot-rolled product may therefore vary depending on the process conditions used.
  • the microstructure is typically tailored to provide the right balance of properties to enable the material to be formed into the desired product prior to quench hardening.
  • the resultant steel is usually relatively soft (typical Vickers hardness levels range from 200-300 Hv10, preferably from 200-250 Hv10), typically with good flexibility (such as a bend radius of 2t when edges of the steel are machined), and medium-high strength (tensile strength 600-1000 MPa) so as to allow easy processing before quench hardening.
  • the properties of the hot rolled product may also be tailored by adjusting the microalloying elements. For instance, if very good fold forming properties are required, then niobium levels should be minimised, or preferably niobium should be avoided.
  • the optional processing steps can vary and may include machining, cutting (e.g. by oxyfuel, plasma, waterjet or laser cutting), grinding (e.g. sharpening of a tool edge to form a blade), and cold forming (e.g. bending, flanging or the like).
  • the material may be (reheat) quench hardened to form the final product.
  • Quench hardening is carried out using a process comprising:
  • An alternative process comprises the steps of:
  • the step a. (or a'.) of providing a component can comprise any or all of the steps i.-iv. set out above, and particularly step iv. set out above.
  • step b. the component or a region thereof is then heated to a temperature (T S ) above A c3 , i.e. the austenisation temperature.
  • T S a temperature above A c3
  • the microstructure of the steel becomes 100% austenite, although some carbides and/or nitrides such as VC, NbC, MoC, and TiN may remain undissolved.
  • the process preferably includes a soaking step (c. or c'.) after heating and before quenching, in which the component is held at a temperature above A r3 for a soak time t S .
  • T S The heat temperature (T S ) will depend primarily on the composition of the hardenable steel. Generally speaking, a lower Mo content allows a lower T S to be used. Preferably, T S is above 850°C, preferably above 900°C, more preferably above 950°C.
  • the hardenable steel (composition) preferably contains Ti and/or Nb.
  • T S is too high, the rate of decarburisation at the surface of the component can become problematic, particularly if the soaking atmosphere is air. Likewise, crystal grain growth can also occur, so excessively high T S levels are to be avoided for steels that do not contain Nb and/or Ti.
  • an upper value for T S is 1050°C, preferably 1000°C. If higher temperatures are used, the soaking time should be kept as short as possible and optionally a protective atmosphere (i.e. an inert gas or vacuum) should be used.
  • a protective atmosphere i.e. an inert gas or vacuum
  • the optional soak time t S will vary depending on the shape size of the component.
  • the soak time will typically be long enough to ensure complete austenisation, but not too long so as to avoid excessive grain growth. Excessive soak times beyond those needed to ensure complete austenisation moreover increase the costs unnecessarily.
  • a soak time for a component having a maximum thickness of 6mm would typically be around 12 minutes when the temperature of the soak furnace is about 950°C. Most components are therefore completely austenised when the soak time is 30 minutes or less, preferably 20 minutes or less.
  • the soak media is not critical, and air, inert gas or a vacuum may be used, preferably air.
  • air inert gas or a vacuum
  • an inert gas or vacuum should be used as the soak media, to mitigate the risk of decarburisation.
  • the soak time will be correspondingly less, such as 2 minutes or less, preferably 1 minute or less. This ensures that only the area of interest reaches the austenising temperature, and excessive heat transfer to other regions is avoided.
  • the sample may be heated by any suitable means.
  • a furnace is typically used.
  • localised heating may be used, such as induction heating or a flame.
  • the furnace temperature and the temperature of the soak medium will be above T S .
  • a component being heated to around 930°C will typically be heated/soaked in a furnace set to 950°C.
  • the steel product may be in the form of a blank which is heated then processed (e.g. by hot stamping) to form the final component, which is then quench cooled. Since it is typically not possible to carry out these processing steps in a heating furnace, higher soak temperatures are required to ensure that the residual heat in the product keeps the temperature of the final component above A r3 throughout the processing steps prior to quenching. If multiple process steps are to be carried out, the steel product may be placed in the furnace between processing steps to ensure its temperature is maintained suitable high. In any event, the temperature should be maintained above A r3 prior to quenching.
  • the steel is 100% austenite after step b. (or b'.) and optional step c. (or c'.).
  • any molybdenum carbide in the austenite dissolves to form free molybdenum atoms.
  • the heated region of the steel does not contain any molybdenum carbide.
  • undissolved carbides can be desirable as they prevent grain ground and lower the risk of hardening cracks forming.
  • the heated region is then quenched using a quenching fluid.
  • the purpose of quenching is to rapidly cool the austenised steel down to below M S , the martensite start temperature.
  • the quenching i.e. step d. or d'.
  • steps b. and c. or b'. and c'.
  • step d. the steel is quenched to below M S .
  • the steel is quenched to below 100°C, more preferably the steel is quenched to room temperature.
  • the improved resistance to quench cracking is believed to arise in part due to the influence the molybdenum has in promoting self-tempering during the quenching.
  • a higher M S temperature will promote self-tempering at an earlier stage as the steel cools.
  • Mn, Cr and Ni have a bigger impact on M S than Mo has, so the steel of the disclosure provides good hardenability in combination with a relatively high M S . Due to the relatively high M S temperature in the steel of the disclosure, the steel undergoes more extensive self-tempering during quenching. Moreover, the lower carbon levels at the surface due to decarburisation raise the M S in these regions, which further helps to lengthen the time that the surface undergoes self-tempering.
  • molybdenum helps relieve internal stresses as the steel is cooled. Moreover, molybdenum also helps improve the impact strength of the quenched steel, decreasing the brittle to ductile transition temperature in a Charpy-V impact test.
  • the quench media may be any suitable media to ensure rapid cooling of the component, i.e. a cooling rate of > 20°C/s, preferably > 50°C/s, more preferably >80°C/s.
  • Suitable quench media include oil or water, with water being preferred.
  • Suitable means for agitating the quench water include a propeller.
  • water as the quench media is meant water or salt water (i.e. brine).
  • the region of the component subjected to quench hardening has a martensitic microstructure.
  • the microstructure of the region comprises ⁇ 90 martensite, preferably ⁇ 95% martensite, more preferably ⁇ 98% martensite, more preferably ⁇ 99% martensite, more preferably ⁇ 99.5% martensite, most preferably 100% martensite.
  • the martensitic region can contain precipitates such as carbides including (for example) MoC, NbC, TiC, NbTiC, and VC, and nitrides including (for example) TiN and VN, as well as carbonitrides.
  • precipitates such as carbides including (for example) MoC, NbC, TiC, NbTiC, and VC, and nitrides including (for example) TiN and VN, as well as carbonitrides.
  • the hardened region is preferably crack-free, and advantageously crack-free without the need for any tempering steps.
  • the Vickers hardness of the region is preferably ⁇ 630 Hv10, preferably ⁇ 650 Hv10, more preferably ⁇ 670 Hv10, even more preferably ⁇ 700 Hv10.
  • Vickers hardness may be measured using standard SFS EN ISO 6507-1:2006, for instance using a DuraScan 80 as a hardness meter.
  • the hardness of a steel can vary according to the depth from the surface. Typically, the surface hardness is slightly lower, as the surface may decarburize during austenisation. Likewise, the centre of the steel cools more slowly, so the hardness at the centre is typically lower.
  • the hardness of the steel is preferably the hardness at 1 ⁇ 4 the sample thickness or 4 mm from the surface, whichever is less.
  • the average hardness of the sample is preferably ⁇ 630 Hv10, preferably ⁇ 650 Hv10, more preferably ⁇ 670 Hv10, even more preferably ⁇ 700 Hv10.
  • the "average hardness” corresponds to the mean of several hardness measurements taken at points evenly spaced through the samples thickness, for instance every 1 mm starting 0.5 mm from the surface, preferably starting 1 mm from the surface.
  • the entire thickness of the sample is preferably ⁇ 630 Hv10, preferably ⁇ 650 Hv10, more preferably ⁇ 670 Hv10, even more preferably ⁇ 700 Hv10.
  • the steel sample is preferably cut in two across its thickness (i.e. perpendicular to a surface), then the hardness is measured on the exposed face which transverses the steel.
  • the hardened region e.g. a ⁇ 5mm sample quenched with water
  • the hardened region preferably has a median Charpy V impact strength of ⁇ 3 J/cm 2 at 20°C, preferably ⁇ 6 J/cm 2 at 20°C, preferably ⁇ 12 J/cm 2 at 20°C and more preferably ⁇ 18 J/cm 2 at 20°C.
  • the Charpy V impact strength may be measured using standard ISO 148 at 20°C (e.g. ISO148:2010). Any suitable sample size in accordance with the standard can in principle be used (e.g. 5 mm, 7.5 mm or 10 mm), although a 5 mm sample is typical. Typically at least three (preferably at least five) measurements are taken, with the test specimen taken being longitudinal to the main hot rolling direction.
  • Tempering of the quenched steel reduces the hardness, but increases the impact strength. Likewise, quenching with oil will produce a less hard steel with a higher impact strength.
  • the advantage of the steel of the disclosure is not merely the high hardness or high impact strength, but the combination of both properties which may be optimised relative to one another by tempering.
  • the hardened steels according to the disclosure have a combination of impact strength and hardness that falls to the right (or above) the various plotted lines. It should be noted that the higher values of A X are typically only achievable when the steel is quenched with water and tempered following hardening.
  • the entire component is quenched in step d. (or d'.).
  • the entire component is tempered in step e. (or e'.).
  • the entire component is reheat-quench hardened.
  • the entire component has a hardness of ⁇ 630 Hv10, preferably ⁇ 650 Hv10, more preferably ⁇ 670 Hv10, even more preferably ⁇ 700 Hv10.
  • the product may need to be processed differently during the hot rolling stages to ensure the final product has the desired properties.
  • the hot rolled product is air cooled following hot rolling, the overall properties vary depending on the balance of ferrite, pearlite and bainite.
  • the steel does not possess suitable structural properties for end use, as the resultant steel is generally quite brittle.
  • One possible option is therefore to quench the steel following finish rolling to provide a hardened product which may be further processed then reheat-quench hardened to form the final product.
  • the resultant quenched hot rolled steel is not coiled, since the coiling process would likely fracture the quenched steel.
  • the quench hardening following hot rolling is preferably carried out with a cooling rate of 20-50°C/s, for example by quenching with oil.
  • a cooling rate of 20-50°C/s
  • This will form a product having a hardness preferably in the region of 475-560 Hv10 with good impact strength.
  • the material is therefore hard enough for most final uses, but not so hard that it cannot be processed (for example cut or sharpened) into a final component.
  • Tempering may optionally be carried out at this stage to lower the hardness and improve impact strength.
  • a region (such as a blade edge) may then be reheat-quench hardened with a cooling rate of > 50°C/s (for example quenching with water) to form a region having higher hardness.
  • the component may be tempered following quench hardening. Tempering may be carried out by heating the component to a tempering temperature T Q (preferably at a temperature of from 150°C to 700°C), holding the component for a tempering time t Q , then cooling the component down to room temperature. Preferably, the cooling following tempering is done in air, preferably still air.
  • Tempering of the component reduces the hardness, but increases tensile strength, ductility and toughness of the component.
  • the molybdenum provides tempering resistance (i.e. it helps to retain the hardness during tempering), especially at temperatures well above M S .
  • tempering is not essential to prevent hardening induced cracking.
  • the impact strength is typically sufficiently high even without tempering.
  • the only tempering done may be the heat treatment to fuse any powder paint coatings on the component following quench hardening. Such treatments are preferably carried out at around 175-225°C (such as 175-200°C or 200-225°C), which is typically well below M S .
  • the steel retains its martensitic structure and typically has a hardness of ⁇ 575 Hv10 (more preferably ⁇ 600 Hv10, more preferably ⁇ 625 Hv10 and with a high carbon content even ⁇ 650 Hv10) together with acceptable other properties like high impact strength, low brittleness and good tensile strength.
  • the disclosure consequently provides a method comprising the following steps:
  • steps 1)-3) may form part of the hot rolling process used to form the steel.
  • step 4) preferably comprises the mechanical processing steps such as stamping, cutting etc. to form a final component, followed by optional tempering.
  • steps 1)-3) may be carried out on a preformed component, in which case the method is essentially a double reheat-quench method. Even so, optional step 4) may still include tempering, as well as some mechanical processing such as sharpening of an edge.
  • the cooling rate in the first quenching step is preferably > 20°C/s, though higher rates such as > 50°C or even > 80 °C/s are possible. If higher rates are used (> 50°C/s), preferably step 4) will involve tempering, preferably tempering at > 300°C.
  • the hardness of the component after step 4) will be from 350 to 600 Hv10, preferably from 475 to 560 Hv10.
  • Step 5) involves heating a localised region of the component, such as by induction heating or flame heating. This localised region is then quench cooled in a second quenching step with a very fast quenching rate (> 50°C/s, preferably > 80°C/s).
  • the hardness of the region is preferably >630 Hv10, preferably ⁇ 650 Hv10, more preferably ⁇ 670 Hv10, even more preferably ⁇ 700 Hv10.
  • the first and second quenching fluid may be the same. However, in this case, step 4) will preferably comprise tempering so as to reduce the hardness of the overall component before the second quenching step.
  • the second quench fluid is water.
  • the first quench fluid is oil.
  • Hardness tests to determine Vickers hardness were carried out according to standard SFS EN ISO 6507-1:2006 using a DuraScan 80 as a hardness meter.
  • Figure 2 shows the plot of hardness vs. depth for an 8mm thick sample of steel 1-A, with the hardness plotted as Vickers (Hv10) hardness, with the corresponding Rockwell C hardness levels being shown. The measurements were taken by cutting the steel into two and measuring the hardness on the exposed cut face which transverses the steel.
  • Figure 3 shows a similar plot for steel 1-B, while Figure 4 shows a similar plot for steel 1-C. These plots show the steels have relatively low hardness when quench hardened at 910°C. Steel 1-B is able to obtain reasonable hardness at the surface when quench hardened at 980°C, although the hardness is not retained through the entire thickness of the sample. Steel 1-C obtains a similar hardness throughout the entire thickness of the sample, but the hardness obtained is approximately 600 Hv10 (55 HRC).
  • Figure 5 shows the equivalent plot for steel 1-D according to the disclosure. The plot clearly shows significantly higher hardness values which are retained through the entire sample thickness.
  • Figure 6 shows an equivalent result is obtained for a 12 mm thick sample.
  • Figure 7 shows the Vickers hardness at 2mm depth for samples of steel 1-D quenched from T S 980°C after tempering at temperatures of 150-600°C.
  • the plot shows the steel shows good hardness retention during tempering, retaining Rockwell hardness values of > 55 HRC when tempering at 200°C.
  • steel 1-D has excellent impact toughness at both quench temperatures, whereas the remaining steels only achieve good impact toughness in the notched test at the higher quench temperature.
  • Steel 1-D according to the disclosure achieves A X values in line with the preferred ranges above for each value of H X .
  • the impact strength of steel 1-D is relatively low, but it is still very good considering the hardness. Tempering the steel reduces the hardness and increases the impact strength
  • the steel retains the high A X value for all tempering temperatures, with a particularly good combination of values being obtained for tempering at 175°C.
  • the 51CrV4 and occasionally 38MnB5 steels could not be successfully water quenched due to quench cracks forming. When oil quenched, both steels provided good hardness, but relatively poor impact strengths.
  • 42MnV7 can be quenched with either water or oil, but the resultant steel has relatively poor impact strength. Also, the water quenched steel has a high tendency to form cracks.
  • Steel 2-D can be quenched with either water or oil to give a steel with a better combination of hardness and impact strength. Tempering the quenched steel reduces the hardness, but increases the impact strength. No hardening induced cracks were observed.

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EP17153320.1A EP3354764B1 (en) 2017-01-26 2017-01-26 Quench hardened steel
ES17153320T ES2793323T3 (es) 2017-01-26 2017-01-26 Acero endurecido por temple
HUE17153320A HUE049006T2 (hu) 2017-01-26 2017-01-26 Edzett acél
PL17153320T PL3354764T3 (pl) 2017-01-26 2017-01-26 Hartowana kąpielowo stal
AU2018211466A AU2018211466A1 (en) 2017-01-26 2018-01-26 Quench hardened steel
CN201880008524.1A CN110225991A (zh) 2017-01-26 2018-01-26 淬火硬化钢
KR1020197024443A KR20190112021A (ko) 2017-01-26 2018-01-26 경화강의 ?칭
JP2019540449A JP2020508393A (ja) 2017-01-26 2018-01-26 焼入硬化鋼
PCT/EP2018/051971 WO2018138270A1 (en) 2017-01-26 2018-01-26 Quench hardened steel
US16/480,034 US20190368014A1 (en) 2017-01-26 2018-01-26 Quench hardened steel
ZA2019/04183A ZA201904183B (en) 2017-01-26 2019-06-26 Quench hardened steel

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