WO2022242859A1 - Method for manufacturing a high strength steel plate and high strength steel plate - Google Patents

Method for manufacturing a high strength steel plate and high strength steel plate Download PDF

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Publication number
WO2022242859A1
WO2022242859A1 PCT/EP2021/063415 EP2021063415W WO2022242859A1 WO 2022242859 A1 WO2022242859 A1 WO 2022242859A1 EP 2021063415 W EP2021063415 W EP 2021063415W WO 2022242859 A1 WO2022242859 A1 WO 2022242859A1
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WIPO (PCT)
Prior art keywords
steel plate
stage
high strength
quenching
temperature
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PCT/EP2021/063415
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French (fr)
Inventor
Philippe HERNAUT
Isabelle TOLLENEER
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Nlmk Clabecq
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Priority to PCT/EP2021/063415 priority Critical patent/WO2022242859A1/en
Priority to BR112023023984A priority patent/BR112023023984A2/en
Priority to PCT/EP2022/063622 priority patent/WO2022243461A1/en
Priority to AU2022278620A priority patent/AU2022278620A1/en
Priority to EP22730159.5A priority patent/EP4341451A1/en
Priority to CN202280035013.5A priority patent/CN117377782A/en
Publication of WO2022242859A1 publication Critical patent/WO2022242859A1/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
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • 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
    • C21D1/56—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
    • 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
    • C21D1/613—Gases; Liquefied or solidified normally gaseous material
    • 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
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
    • 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
    • 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
    • C22C38/06—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
    • C22C38/18—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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C21D2211/001—Austenite
    • 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
    • C21D2211/002—Bainite
    • 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
    • C21D2211/005—Ferrite
    • 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

Definitions

  • the annealing temperature AT is higher than the Ac3 transformation point of the steel, and the quenching temperature QT is such that the structure of the steel after the final heat treatment contains at least 20% of retained austenite and at least 65% of martensite and, preferably, the sum of the ferrite and bainite contents is less than 10%.
  • EP 3555337 describes a hot-rolled flat sheet product having a tensile strength of 800 - 1 .500 MPa, yield strength of more than 700 MPa, an elongation at break A of 7-25% and a hole expansion l of more than 20%.
  • Invention JP6237364 involves Q&P heat treatment technology to produce wear resistant cold-rolled steel quenched in the intercritical temperature range (Ac1-Ac3).
  • This method contains a step of cold-rolling which is a step intended for the production of thin steel (max. 1 ,2 mm) and cannot be used for the production of steel plate with a higher thickness, such as up to 16 mm.
  • the structure of the final product will contain an increased volume of ferrite (quenching in the intercritical temperature range), which will impede obtaining improved strength properties.
  • None of the documents proposes a method for manufacturing high strength steel plates with cross-thickness structural and strength uniformity while at the same time ensuring industrial production rates.
  • the temperature during second phase 122 decreases down to a temperature between 210°C and 550°C, preferably between 210°C and 300°C.
  • the duration of the second phase 122 is between 5 and 16 min.
  • the second phase ensures cross thickness structural and strength uniformity and, due to even cross-thickness temperature distribution, prevents geometric defects (wave, buckle, camber). This prevents heavy scale formation on the surface.
  • the first stage of the two-stage quenching step preferably ends at a temperature range above Ms and wherein the second stage of the quenching step preferably starts at a temperature range above Ms and ends at a temperature range between Ms and Mf.
  • the two-stage quenching step ends at QT, between Ms and Mf.
  • the high strength steel sheet may comprise a chemical composition including, in wt %, the following components. Indicated range values are included.
  • the following chemical compositions can be considered individually or in combination.
  • the invention is illustrated by the following examples: The method is applied to a steel plate with the alloy chemical composition according to the following table 1 :
  • references 827615 corresponds to table 1.
  • the hot rolling step applies before the hardening process described in table 3 below, according to the method of the present invention.
  • the hardening process is carried out according to the method of the invention shown in the following Table 3:

Abstract

The present invention relates to a method for manufacturing a high strength steel plate, comprising the steps of providing a hot-rolled steel plate, heating (10) the steel plate to an austenitisation temperature range, quenching (12) the steel plate in two stages (121, 122) with a different cooling speed applied to each stage, the cooling speed of the first stage (121) being higher than the cooling speed of the second stage (122). The invention also relates to a high strength steel plate manufactured with the method.

Description

Method for manufacturing a high strength steel plate and high strength steel plate
Technical field
The invention pertains to the field of metallurgy, specifically to a method for manufacturing high-strength steel plate and high-strength steel plate.
Prior art
The Quenching & Partitioning (hereinafter Q&P) heat treatment process is a technology designed for the production of the 3rd generation advanced high- strength steels (3rd GEN AHSS), characterized by a moderately low yield strength to tensile strength ratio, increased total elongation and improved wear resistance. This set of product properties makes this technology promising for the production of structural materials applicable in the field of mechanical engineering, where stricter requirements are imposed on such parameters as ductility, wear resistance and crack resistance. These materials are used, for example, in construction and in the production of strength and wear-resistant parts of heavy trucks, special equipment or agricultural machinery.
The main objective of the Q&P heat-treatment process is to create a complex- phase structure in a relatively low-cost low-alloyed steel (whose typical chemistry corresponds to that of TRIP steels) with higher quantity of retained austenite capable of strain-induced martensitic transformation. When a steel product is stressed, such austenite is transformed into strain-induced martensite, which ensures better strength with a simultaneous increase in ductility due to the TRIP effect (“TRIP” for “TRansformation Induced Plasticity”).
Figure 1 is a diagram showing the Q&P heat treatment process. The essence of the Q&P heat treatment process lies in the sequential implementation of the following process operations:
• Heating to the temperature of austenitization, above Ac1 ;
• Quenching with cooling to the QT temperature between the start and end points of martensite transformation, Ms and Mf (Q stage) and soaking for the formation of a certain quantity of quenching martensite; • Heating to the PT temperature (slightly above Ms) and soaking to redistribute carbon from martensite to retained austenite for its stabilization (P stage);
• Final cooling, where a structure consisting of tempered martensite, retained austenite and freshly quenched martensite is formed.
US 20060011274 relates to the production of high-strength steel with retained austenite and describes a heat treatment process that includes steel alloy annealing at an annealing temperature to produce austenite in said steel alloy, subsequent quenching at a temperature to transform at least a portion of said austenite into martensite, followed by carbon partitioning to transfer carbon from martensite to said austenite and subsequent cooling of the steel alloy to a desired temperature.
US 20060011274 represents a technology for one-stage quenching of a steel alloy to obtain martensitic-austenitic structure. Single-stage quenching to the temperature of martensite formation is only suitable for the production of thin cold- rolled steel with a thickness of max. 1 mm, since it does not ensure cross-thickness structural and strength uniformity in thick rolled products and the absence of geometric defects (wave, buckle, camber) due to uneven cross-thickness temperature distribution.
EP 3164513 describes a method for manufacturing a high-strength steel sheet having a tensile strength of more than 1 .100 MPa, yield strength > 700 MPa, uniform elongation UE of at least 8,0% and total elongation TE of at least 10 %. The chemistry of such a steel plate in mass percent is as follows: 0,1% < C < 0,25%, 4,5% < Mn < 10%, 1 < Si < 3%, 0,03 < Al < 2,5%, the rest is Fe and impurities, while the composition is such that CMn Index = Cx(1 + Mn / 3,5) < 0,6. The method for producing such a steel plate is also disclosed, comprising the following steps: soaking the steel of the specified composition to an annealing temperature AT higher than the Ac1 transformation point of the steel, but less than 1 .000 °C, cooling the annealed sheet to a quenching temperature QT between 190-80 °C at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, maintaining the steel sheet at an overageing temperature PT between 350-500 °C for an overaging time of Pt of more than 5s and less than 600s and cooling the sheet down to ambient temperature. In a preferred embodiment, the annealing temperature AT is higher than the Ac3 transformation point of the steel, and the quenching temperature QT is such that the structure of the steel after the final heat treatment contains at least 20% of retained austenite and at least 65% of martensite and, preferably, the sum of the ferrite and bainite contents is less than 10%.
EP 3164513 involves a Q&P heat treatment method for producing high- strength mid-manganese steels, i.e. this method covers only steels with manganese content in the range of 4,5-10%. When applying this method to steels with a different chemical composition, the target indicators of strength, ductility, wear resistance, etc. will be missed. Further, due to the high content of manganese, the thermal treatment causes a cross-thickness structural non-uniformity.
EP 3555337 describes a hot-rolled flat sheet product having a tensile strength of 800 - 1 .500 MPa, yield strength of more than 700 MPa, an elongation at break A of 7-25% and a hole expansion l of more than 20%. The chemistry of such a steel plate (in wt%) is as follows: C: 0,1 - 0,3%, Mn: 1 ,5 - 3,0 %, Si: 0,5 - 1 ,8 %, Al: < 1 ,5%, P: < 0,1%, S: < 0,03%, N: < 0,008%, optionally one or more elements of the group: Cr, Mo, Ni, Nb, Ti, V, B, with the following concentration: Cr: 0,1 - 0,3%, Mo: 0,05 - 0,25%, Ni: 0,05 - 2,0%, Nb: 0,01 - 0,06 %, Ti: 0,02 - 0,07%, V: 0,1 - 0,3%, B: 0,0008 - 0,0020%, the rest is Fe and unavoidable production-related impurities. In addition, the microstructure of a flat steel product consists of at least 85 area % martensite, at least half of which is tempered martensite, with the remainder of the microstructure being < 15 vol. % retained austenite, < 15 area % bainite, < 15 area% polygonal ferrite, < 5 area% cementite and/or < 5 area% non-polygonal ferrite. This document also describes a method comprising the step of heating to a temperature of 1 .000-1 .300 °C, hot rolling to an end of hot rolling temperature TE, where TET > (A3-100 °C), subsequent quenching at a rate of more than 30 K/s to a TQ temperature, where RT < TQ < ( TMS + 100 °C) where "RT" denotes the room temperature and "TMS" denotes the martensite start temperature of the steel where TMS [°C]=462-273% C-26% Mn-13% Cr-16% Ni-30% Mo (% X = X Content of the steel), subsequent holding or heating to a partitioning temperature TP to at most 500 ° C, and subsequent cooling.
EP 3555337 presents the Q&P heat treatment method for producing high- strength hot-rolled steel with quenching immediately after the end of hot rolling with an exit temperature above (A3 - 100 °C). The main disadvantage of this method is the implementation of quenching immediately after hot rolling, which will inevitably lead to heavy scale formation on the surface.
JP 6237364 describes the production of wear-resistant cold-rolled steel plate with the volume fraction of ferrite <15%, martensite with carbide sizes of 2-500 nm up to 95%, retained austenite up to 15%, the rest being bainite and martensite; besides the ratio of the area fractions of crystalline particles ND // <111 > to ND // <100> is up to 40%. The technology is designed for producing a steel plate with a tensile strength of > 980 MPa, with the chemical composition of steel in mass percent as follows: C: 0.05 to 0.40 %, Si: 0.05 to 3.0 %, Mn: 1 .5 to 3.5 %, Al: max. 1 .5%, N: max. 0.01 %, P: max. 0.1 %, S: max. 0.005 %, Nb: max. 0.04 %, Ti: max. 0.08%, the rest - Fe with unavoidable impurities.
Invention JP6237364 involves Q&P heat treatment technology to produce wear resistant cold-rolled steel quenched in the intercritical temperature range (Ac1-Ac3). This method contains a step of cold-rolling which is a step intended for the production of thin steel (max. 1 ,2 mm) and cannot be used for the production of steel plate with a higher thickness, such as up to 16 mm. Besides, the structure of the final product will contain an increased volume of ferrite (quenching in the intercritical temperature range), which will impede obtaining improved strength properties.
None of the documents proposes a method for manufacturing high strength steel plates with cross-thickness structural and strength uniformity while at the same time ensuring industrial production rates.
Disclosure of the invention
A first object of the invention is to provide a method for manufacturing a high strength steel plate, comprising the steps of Providing a hot-rolled steel plate,
Heating the steel plate to an austenitisation temperature range, Quenching the steel plate in two stages with a different cooling speed applied to each stage, the cooling speed of the first stage being higher than the cooling speed of the second stage.
According to an embodiment, the first stage of the two-stage quenching step ends at a temperature range above Ms and wherein the second stage of the quenching step starts at a temperature range above Ms and ends at a temperature range between Ms and Mf.
According to an embodiment, the first stage is a water quenching stage.
According to an embodiment, the cooling speed of the first stage is between 10°C/s and 30°C/s, preferably at about 15°C/s.
According to an embodiment, the temperature reached at the end of first phase is between 350°C and 650°C.
According to an embodiment, the second stage of the two-stage quenching step is an air or water quenching stage.
According to an embodiment, the temperature reached at the end of the second phase is between 210°C and 550°C, preferably between 210°C and 300°C.
According to an embodiment, the duration of the second phase is between 5 and 16 minutes.
According to an embodiment, the method further comprises, after the two- stage quenching step, a tempering step of heating the steel plate to a temperature between 300°C and 500 °C, preferably at 400°C, followed by a holding step to redistribute carbon from martensite to retained austenite, during a tempering time of preferably 22 minutes, and then a cooling step, by cooling the steel plate in air.
According to an embodiment, the high strength steel plate has a thickness between 3 mm and 16 mm.
The invention also relates to a high strength steel plate manufactured with the method previously described, having an tensile strength of at least 1 .500 MPa, a yield strength of at least 800 MPa and a total elongation of at least 11%. According to an embodiment, the steel plate further comprises a structure containing retained austenite from 5 to 20% and minimum 65% martensite, at least half of which is tempered martensite, while the sum of ferrite and bainite contents is below 10%.
According to an embodiment, the steel plate further comprises manganese, in wt %, between 1 ,2% - 2,6%, preferably 2,4% - 2,6%, preferably 2,5%.
According to an embodiment, the steel plate further comprises, in wt%, between 0,4% - 1 ,0%, preferably 0,45% - 0,55%, preferably 0,5% of Chrome, between 0,20% - 0,8%, preferably 0,2% - 0,4%, preferably 0,25% of Molybdenum, between 1 ,45% - 1 ,55%, preferably 1 ,5% of Silicon.
According to an embodiment, the steel plate has a thickness between 3 mm and 16 mm.
In the framework of this document, the use of the indefinite article “a”, “an” or the definite article “the” to introduce an element does not exclude the presence of a plurality of these elements. In this document, the terms “first”, “second” and the like are solely used to differentiate elements and do not imply any order in these elements.
In the framework of the present document, the use of the verbs “comprise”, “include”, “involve” or any other similar variant, as well as their conjugational forms, cannot exclude the presence of elements other than those mentioned. When the verb “comprise” is used for defining an interval by the terms “comprised between” two values, these two values should not be interpreted as excluded from the interval.
All the embodiments of the method according to the invention and the advantages of these embodiments apply mutatis mutandis to the steel plate according to the invention.
Brief description of the figures
Other characteristics and advantages of the present invention will appear on reading the following detailed description, for the understanding of which, it is referred to the attached figures where: - Figure 1 is a diagram showing a temperature pattern in a thermal treatment according to a conventional Q&P process;
- Figure 2 is a diagram showing a temperature pattern in a thermal treatment according to the invention;
- Figure 3 illustrates CCT diagrams according to a preferred embodiment of the invention;
- Figure 4 illustrates a cross sectional view of a steel plate manufactured with the method according to the invention;
- Figure 5 illustrates a cross sectional view of a steel plate manufactured with the method according to the invention;
- Figure 6 illustrates a cross sectional view of a steel plate manufactured with the method according to the invention.
The drawings in the figures are not scaled. Similar elements can be assigned by similar references in the figures. In the framework of the present document, identical or analogous elements may have the same references. The presence of reference numbers in the drawings cannot be considered to be limiting, in particular if these numbers are indicated in the claims.
Description of specific embodiments of the invention
Description of preferred embodiments of the present invention are hereafter described with references to figures, but the invention is not limited by these references. In particular, the drawings or figures described below are only schematic and are not limiting in any way.
The invention relates to a method for manufacturing a high strength steel plate, comprising the steps of providing a hot-rolled steel plate, heating the steel plate to an austenite temperature range, and quenching the steel plate in two stages with a different cooling speed applied to each stage, the cooling speed of the first stage being higher than the cooling speed of the second stage. This makes it possible to manufacture high strength steel plates ensuring cross-thickness structural and strength uniformity in the plates thanks to the second stage while at the same time ensuring industrial production rates thanks to the first stage. Figure 2 shows the different steps of the process according to the invention. The method comprises a step 10 of heating the steel plate in order to obtain the formation of austenite in the austenitization zone. The steel plate is brought to a temperature greater than Ac3, ie a temperature between 820°C-930°C, preferably 850°C-920°C, preferably to a temperature of 900°C. The steel plate is maintained at this temperature for a time between 6 min and 24 min. Then the method comprises a quenching step 12 which comprises two phases 121 and 122. The quenching speed for each phase 121 and 122 is different, the quenching speed of the first phase 121 is greater than the quenching speed of the second phase 122. The advantage is to simultaneously support high production rates thanks to a high quenching speed in the first phase while guaranteeing better control of the cooling of the plate in the second phase - and thus obtaining a better control of the cross thickness structural and strength uniformity. Also, such a two-stage quenching step makes it possible to make the cooling speed less critical than in methods where the cooling speed is the same during the whole quenching step.
The cooling at a faster speed during the first phase 121 can be achieved with water as a cooling media. The cooling speed is for example between 10°C/s and 30°C/s, preferably at about 15°C/s - these speeds unsure industrial production rates. The temperature during first phase 121 decreases down to a temperature between 350°C and 650°C. Cooling at a slower speed during the second phase 122 can be done with water or air as cooling media. The cooling speed is for example between 0,5°C and 5°C, preferably between 0,5°C and 2,5°C - these speed ensure cross-thickness structural and strength uniformity in the plates. The temperature during second phase 122 decreases down to a temperature between 210°C and 550°C, preferably between 210°C and 300°C. The duration of the second phase 122 is between 5 and 16 min. The second phase ensures cross thickness structural and strength uniformity and, due to even cross-thickness temperature distribution, prevents geometric defects (wave, buckle, camber). This prevents heavy scale formation on the surface.
The quenching rate (or cooling speed during the two-stage quenching step), the temperature at the end of the water quenching (first phase) for subsequent cooling in air or water (second phase) is determined from the condition that pearlite does not form. Figure 3 shows Continuous Cooling Transformation (CCT) diagrams for the proposed method for determining critical temperatures at the end of each step of phase. Under such cooling conditions, the cross-thickness structural and strength heterogeneity is prevented and geometric defects of rolled products (wave, buckle, camber) are eliminated due to the cross-thickness homogenization of structure and temperature. Quenching step 12 ends at a temperature QT in the martensite formation zone, between the start and end points of martensite transformation, Ms and Mf.
Figure 3 shows two CCT diagrams for setting the cooling speed parameters to control the composition chemistry of the steel plate and prevent the appearance of pearlite. Figure 3 shows the decrease in temperature as a function of time (logarithmic scale). In the first diagram, the composition of the steel plate (wt%) is: Fe: 95,245; Al: 0,015; Cr: 0,45; Mn: 2,4; Mo: 0,2; Si: 1 ,4; C: 0,29. In the second diagram, the composition of the steel plate (wt%) is: Fe: 94,4765; Al: 0,04; Cr: 0,55; Cu: 0,08; Mn: 2,6; Mo: 0,3; Nb: 0,01 ; Ni: 0,08; Si: 1 ,5; Ti: 0,01 ; V: 0,01 ; B: 5,0E-4; C: 0,32; N: 0,006; P: 0,015; S: 0,002. In both diagrams, curve 20 shows the transformation of 1% of austenite into pearlite, curve 22 shows the transformation of 1% of austenite into bainite, curve 24 shows retained austenite, curve 26 shows the start of the transformation of austenite into martensite, curve 28 shows the transformation of 50% of austenite into martensite and curve 30 shows the transformation of 90% of austenite into martensite. According the composition of the steel plate of each diagram, the start of austenitization is at 920°C. Figure 3 shows that the more the alloy share in the composition of the steel plate increases, the broader the quenching speed range is available while limiting the appearance of perlite.
The method further comprises a tempering step 14 of heating the steel plate to a partioning temperature PT between 300°C and 500°C, preferably at 400°C in a tempering furnace followed by an holding step 16 to redistribute carbon from martensite to retained austenite, during a tempering time of preferably 22 minutes. Then, a cooling step 18 occurs, for example by cooling the steel plate in air. During this cooling step 18, additional martensite may result.
The first stage of the two-stage quenching step preferably ends at a temperature range above Ms and wherein the second stage of the quenching step preferably starts at a temperature range above Ms and ends at a temperature range between Ms and Mf. The two-stage quenching step ends at QT, between Ms and Mf. The advantage is that the martensite formation is better controlled because the entry into the martensite formation zone is achieved at a slower cooling speed than at the start of the quenching step. This enhances the structural uniformity of the steel plate.
The invention also relates to a high strength steel plate manufactured with the method of the invention. The steel plates are especially characterized by advanced strength, ductility, formability and wear resistance properties. These steel plate are used, for example, in construction and in the production of wear-resistant parts such as for example heavy trucks, mining equipment or agricultural machinery. The steel plate are also appropriate for ballistic resistance. The steel plate thickness is preferably between 3 mm and 16 mm (included). The method according to the invention is particularly suitable for this size of thickness. Indeed, the two-stage quenching step ensures industrial production rates and the cross-thickness uniformity of structural and strength properties of steel plates. Also, due to even cross-thickness temperature distribution, the method prevents geometric defects (wave, buckle, camber). In particular, in the embodiment with the start of the second phase 122 before the temperature Ms and then ending between Ms and Mf, the invention makes it possible to better control the formation of martensite.
The steel plate produced with the method of the invention is capable of reaching high strength values: tensile strength of more than 1.500 MPa, yield strength of more than 800 MPa and total elongation of more than 11%. Also the two-stage quenching step makes it possible to better control the fractions of each component (martensite and austenite) of the steel plate. The steel plate produced with the method of the invention has a structure containing retained austenite from 5 to 20% and minimum 65% martensite, at least half of which is tempered martensite, while the sum of ferrite and bainite contents is below 10%. A controlled volume fraction of a high-strength phase (martensite), and a highly ductile phase (austenite) is obtained.
Martensite formation contributes to the high-strength of the steel plate while the retained austenite contributes to the high ductility of the steel plate.
As an example, the high strength steel sheet may comprise a chemical composition including, in wt %, the following components. Indicated range values are included. The following chemical compositions can be considered individually or in combination.
C: 0,29% - 0,32%, preferably 0,3%. The component C is linked to final hardness of martensitic phase.
Si: 1 ,45% - 1 ,55%, preferably 1 ,5%. The component Si prevents the formation of carbides - this is the key element to obtain retained austenite.
Mn: 1 ,2% - 2,6%, preferably 2,4% - 2,6%, preferably 2,5%. Manganese improves hardenability, i.e. the possibility for the material transformation to occur at low cooling rate (thus notably beneficial for the quality of the second phase of the two-stages quenching step) but causes segregation in the thickness of the steel plate. During the quenching step, the manganese tends to migrate towards the center of the plate and be of greater concentration in the middle of the thickness. The indicated proportion is balanced between improving hardenability and limiting segregation.
P: 0,015% and less.
S: 20ppm and less.
Cr: 0,4% - 1 ,0%, preferably 0,45% - 0,55%, preferably 0,5%. The component Cr also improves hardenability.
Mo: 0,20% - 0,8%, preferably 0,2% - 0,4%, preferably 0,25%. The component Mo also improves hardenability.
Al: 0,015% - 0,04%, preferably 0,025%. The component Al is used for steel killing.
The remaining components being Fe and non-voluntary added alloys. The steel plate according to the invention is particularly suitable for the construction and agricultural machinery, which have additional stricter requirements for ductility and crack resistance.
The invention is illustrated by the following examples: The method is applied to a steel plate with the alloy chemical composition according to the following table 1 :
Figure imgf000013_0001
Table 1
Hot rolling is carried out according to the modes in the following table 2:
Figure imgf000013_0002
Table 2
Chemical composition of references 827615 corresponds to table 1. The hot rolling step applies before the hardening process described in table 3 below, according to the method of the present invention. The hardening process is carried out according to the method of the invention shown in the following Table 3:
Figure imgf000014_0001
Table 3
The Water Quenching Stop Temperature corresponds to the temperature reached at the end of the first phase of the two-phase quenching step. Air cooling before tempering corresponds to the duration of the second phase of the two-phase quenching step. Entry tempering furnace temperature is the temperature reached at the end of the second phase of the two-phase quenching step.
Mechanical properties are shown in the following Table 4. A decrease in the temperature of the end of hardening increases the strength characteristics of rolled products and reduces ductility.
Figure imgf000014_0002
Table 4
TD stands for Tranversal Direction (perpendicular to rolling); YS stands for Yield Strength; TS stands for Tensile strength; Total El: Total Elongation; RA: retained austenite.
In addition, one has performed simulation to verify microstructure type for a 16 mm thickness plate and the retained austenite is evaluated at 10,9%.
The following table 5 shows temperatures of Ac3, Ms and Mf for the material ratio mentioned above:
Figure imgf000015_0001
Table 5
Figures 4 to 6 show cross sectional views of the various steel plates of samples of the example. Figure 4 is a Lepera etching micrograph in the longitudinal direction, quarter thickness (example N1 ). A x50 lense is used. The scale of 50pm is visible on the micrograph. Figure 5 is a Lepera etching mircrograph in the transverse direction, quarter thickness (example N2). A x20 lense is used. The scale of 100pm is visible on the micrograph. Figure 6 is a Lepera etching micrograph in the longitudinal direction, quarter thickness (example N3). A x50 lense is used. The scale of 50pm is visible on the micrograph. On the figures, reference 40 shows the retained austenite white microstructures. Figure 4-6 show that, thanks to the invention, the retained austenite is well distributed in the steel plates. This means that the retained austenite performs its function at all locations in the steel plate, namely, to provide high ductility to the steel plate. As a result, abrasion resistance is consistent throughout the thickness of the steel plate. Under constraints, the retained austenite turns into martensite in a homogeneous manner which increases the hardness of the steel plate in the context of its use, in an homogeneous manner as well. Thus, thanks to the method according to the invention, and especially the two-stage quenching step, one obtains a better control of the cross-thickness structural and strength uniformity while ensuring industrial production rates.
The present invention has been described in relation to the specific embodiments which have a value that is purely illustrative and should not be considered to be limiting. The skilled person will notice that the invention is not limited to the examples that are illustrated and/or described here above. The invention comprises each of the new technical characteristics described in the present document, and their combinations.

Claims

Claims
1 . A method for manufacturing a high strength steel plate, comprising the steps of - Providing a hot-rolled steel plate,
- Heating (10) the steel plate to an austenitisation temperature range,
- Quenching (12) the steel plate in two stages (121 , 122) with a different cooling speed applied to each stage, the cooling speed of the first stage (121 ) being higher than the cooling speed of the second stage (122).
2. The method of claim 1 , wherein the first stage (121 ) of the two-stage quenching step (12) ends at a temperature range above Ms and wherein the second stage (122) of the quenching step (12) starts at a temperature range above Ms and ends at a temperature range between Ms and Mf.
3. The method of claim 1 or 2, wherein the first stage (121 ) is a water quenching stage.
4. The method of any one of claims 1 to 3, wherein the cooling speed of the first stage (121 ) is between 10°C/s and 30°C/s, preferably at about 15°C/s.
5. The method of any one of claims 1 to 4, wherein the temperature reached at the end of first phase (121 ) is between 350°C and 650°C.
6. The method of any one of claims 1 to 5, wherein the second stage (122) of the two-stage quenching step (12) is an air or water quenching stage.
7. The method of any one of claims 1 to 6, wherein the temperature reached at the end of the second phase (122) is between 210°C and 550°C, preferably between 210°C and 300°C.
8. The method of any one of claims 1 to 7, wherein the duration of the second phase (122) is between 5 and 16 minutes.
9. The method of any one of claims 1 to 8, further comprising, after the two- stage quenching step (12), a tempering step (14) of heating the steel plate to a temperature between 300°C and 500 °C, preferably at 400°C, followed by a holding step (16) to redistribute carbon from martensite to retained austenite, during a tempering time of preferably 22 minutes, and then a cooling step (18), by cooling the steel plate in air.
10. The method of any one of the preceding claims, wherein the high strength steel plate has a thickness between 3 mm and 16 mm.
11. A high strength steel plate manufactured with the method of any one of the preceding claims, having an tensile strength of at least 1 .500 MPa, a yield strength of at least 800 MPa and a total elongation of at least 11%.
12. The high strength steel plate of claim 11 , further comprising a structure containing retained austenite from 5 to 20% and minimum 65% martensite, at least half of which is tempered martensite, while the sum of ferrite and bainite contents is below 10%.
13. The high strength steel plate of claim 11 or 12, further comprising manganese, in wt %, between 1 ,2% - 2,6%, preferably 2,4% - 2,6%, preferably 2,5%.
14. The high strength steel plate of any one of claims 11 to 13, further comprising, in wt%, between 0,4% - 1 ,0%, preferably 0,45% - 0,55%, preferably 0,5% of Chrome, between 0,20% - 0,8%, preferably 0,2% - 0,4%, preferably 0,25% of Molybdenum, between 1 ,45% - 1 ,55%, preferably 1 ,5% of Silicon.
15. The high strength steel plate of the one of claims 11 to 14, having a thickness between 3 mm and 16 mm.
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