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 austenitisation, 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; • Partitioning the steel plate (P stage): tempering the steel plate by heating it to the PT temperature (slightly above Ms), holding the steel plate to partition carbon from martensite to retained austenite and cooling the steel plate.

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 that simultaneously supports high production rates while guaranteeing better control of the cooling of the plates.

Thus, there is a need for a method for manufacturing high strength steel plates that simultaneously supports high production rates while guaranteeing better control of the cooling of the plates. 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. 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,

Partitioning the steel plate.

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 between 12°C/s and 25°C/s, more preferably at

15°C/s, the cooling speed corresponding to the mean cooling speed obtained between the end of the heating step and the end of the quenching step, the mean cooling speed being equal to (T1 - T2) / (d / S) with:

T 1 : Temperature at the surface of the steel plate at the end of the heating step measured by a pyrometer measuring said temperature at the end of an austenitising furnace where the heating step occurs,

T2: Temperature at the surface of the steel plate at the end of the quenching step measured by a pyrometer measuring said temperature at 4 meters after a last cooling section in a quenching unit where the quenching step occurs, - d: Distance between the location of T 1 measurement and the location of

T2 measurement

S: Steel plate speed between the location of T1 measurement and the location of T2 measurement.

According to an embodiment, the temperature reached at the end of first phase is between 350°C and 650°C, the temperature being measured on the surface of the steel plate. 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 320°C, preferably between 210°C and 300°C, the temperature being measured on the surface of the steel plate.

According to an embodiment, the duration of the second phase is between 5 and 16 minutes.

According to an embodiment, the partitioning step comprises 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 partition carbon from martensite to retained austenite, during a time between 15min and 30min, 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. According to an embodiment, the high strength steel plate has a tensile strength of at least 1.300 MPa, a yield strength of at least 800 MPa and a total elongation of at least 11 %.

According to an embodiment, the high strength steel plate 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, wherein the high strength steel plate comprises manganese, in wt %, between 1 % - 2,6%.

According to an embodiment, the high strength steel plate comprises, in wt%, between 0,4% - 2,0% of Chromium, between 0,20% - 0,8% of Molybdenum, between 0,8% - 1 ,6% of Silicon.

The invention also relates to a high strength steel plate manufactured with the method described above, having an tensile strength of at least 1 .300 MPa, a yield strength of at least 800 MPa and a total elongation of at least 11%. According to an embodiment, the high strength steel plate has a loss of volume of less than 0,450mm ³ measured via a profilometer via the “pin on disk” method with the parameters of the test being A load of 20N,

A speed of 20 cm/s,

A track radius of 8mm,

A distance of 30.000 cycles,

An alumina ball with a diameter of 6mm.

According to an embodiment, the high strength steel plate has a bending performance along the transverse and longitudinal directions, corresponding to a ratio of radius to steel plate thickness of less than 3, with a plate thickness of 5mm, the bending tests being performed by bending a 2000mm x 600mm steel plate having a thickness of 5mm along the longitudinal direction corresponding to the rolling direction and along the transverse direction corresponding to the direction transverse to the rolling direction, the radius corresponding to the radius of a punch tool performing the bending.

According to an embodiment, the high strength steel plate has a bending performance along the transverse direction corresponding to a ratio of radius to steel plate thickness of less than 3 with a plate thickness of 10mm and having a bending performance along the longitudinal direction corresponding to a ratio of radius to steel plate thickness of less than 4 with a plate thickness of 10mm, the bending tests being performed by bending a 2000mm x 600mm steel plate having a thickness of 10mm along the longitudinal direction corresponding to the rolling direction and along the transverse direction corresponding to the direction transverse to the rolling direction, the radius corresponding to the radius of a punch tool performing the bending.

According to an embodiment, the high strength 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 high strength steel plate further comprises manganese, in wt %, between 1 % - 2,6%.

According to an embodiment, the high strength steel plate further comprises, in wt%, between 0,4% - 2,0% of Chromium, between 0,20% - 0,8% of Molybdenum, between 0,8% - 1 ,6% of Silicon.

According to an embodiment, the high strength 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;

- Figure 7 illustrates the worn volume tests; - Figure 8 illustrates the bending tests;

- Figure 9 illustrates the bending performances of the steel plates obtained 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. In particular, the invention relates to a quenching and partitioning 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. The invention proposes a method for manufacturing high strength steel plates that simultaneously supports high production rates while guaranteeing better control of the cooling of the plates.

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 austenitisation zone. The steel plate is heated in a austenitising furnace. The steel plate is brought to a temperature greater than Ac3, i.e. a temperature between 820°C-930°C, preferably 850°C-920°C, preferably to a temperature of 900°C-920°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. 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. 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. In other words, a further advantage is that one obtains a better control of the cross-thickness structural and strength uniformity.

To perform the first phase, the steel plate is transferred to a quenching unit, provided with successive cooling sections. 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 between 12°C/s and 25°C/s, more preferably at 15°C/s (or about 15°C/s) - these speeds unsure industrial production rates.

The cooling speed corresponds to the mean cooling speed obtained between the end of the heating step and the end of the quenching step, or in other words, the cooling speed corresponds to the mean cooling speed obtained between the end of the austenitising furnace and the end of the quenching unit, the mean cooling speed being equal to (T1 - T2) / (d / S) with the following four variables:

- T1 : Temperature at the surface of the steel plate at the end of the heating step measured by a pyrometer inside the austenitisation furnace where the heating step occurs,

- T2: Temperature at the surface of the steel plate at the end of the quenching step (or in other words, at the exit of the quenching unit) measured by a pyrometer measuring said temperature at 4 meters after a last cooling section in the quenching unit where the quenching step occurs,

- d: Distance between the location of T 1 measurement and the location of T2 measurement

- S: Steel plate speed between the location of T 1 measurement and the location of T2 measurement.

To obtain T1 , the pyrometer may be inside the furnace, in the last section of the furnace, just before the exit of the furnace, where the temperature is homogeneous and corresponding to the end of the heating step. To obtain T2, the pyrometer may be outside the quenching unit with an oblique orientation to measure the temperature at 4 meters after a last cooling section in the quenching unit.

The temperature during first phase 121 decreases down to a temperature between 350°C and 650°C. The temperature is measured on the surface of the steel plate. For example, a pyrometer is used. The temperature is measured at the exit of the quenching unit. 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/s and 5°C/s, preferably between 0,5°C/s and 2,5°C/s - these speeds 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 320°C preferably between 210°C and 300°C. The temperature is measured on the surface of the steel plate. For example, a pyrometer is used. The temperature is measured at the entry of the partitioning furnace. 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 austenitisation 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 partitioning step 13. Within the partitioning step 13, the method comprises a tempering step 14, an holding step 16 and a cooling step 18. Within the partitioning step 13, the method comprises the tempering step 14 of heating the steel plate. Within the partitioning step 13, the method comprises the tempering step 14 of heating the steel plate to a partitioning temperature PT between 300°C and 500°C, preferably at 400°C in a tempering furnace. Within the partitioning step 13, the method further comprises an holding step 16. Within the partitioning step 13, the method further comprises the holding step 16 to partition carbon from martensite to retained austenite for its stabilization. The holding step 16 occurs during a time between 15min and 30min, a time of preferably 22 minutes. Then, within the partitioning step 13, the method further comprises a cooling step 18. Within the partitioning step 13, the method further comprises the cooling step 18 to cool the steel plate The cooling step 18 occurs, for example by cooling the steel plate in air. During this cooling step 18, additional martensite may result. A structure consisting of tempered martensite, retained austenite and freshly quenched martensite is formed.

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 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 one can apply rapid cooling speed during a certain time during the quenching step (first stage) and at some point before reaching the Ms temperature, to slow down the cooling speed to anticipate the entry into the Ms-Mf temperature (second phase). This guarantees better control of the cooling of the plates. A further 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 plates 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 plates are also appropriate for ballistic resistance. The steel plate thickness is preferably between 3 mm and 16 mm (included). Due to the limited thickness, the person skilled in the art will not consider the gradient of temperature across the thickness of the steel plate while applying the method of the invention. 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. The steel plate is characterised by a tensile strength of more than 1 .300 MPa. The steel plate is also characterised by a yield strength of more than 800 MPa. The effect of these technical features is that the low ratio between yield strength and tensile strength is offering a particularly good deformation capacity (or forming capability). The steel plate is characterised by a total elongation of more than 11%. The effect of this total elongation is that the high ductile steel plate makes it possible to have good deformation capacity for mechanical construction such as heavy trucks, mining equipment or agricultural machinery. 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. This results in a ductile steel plate that acquires high-strength under stress during use - such as use in heavy trucks, mining equipment or agricultural machinery. Martensite formation contributes to the high-strength of the steel plate while the retained austenite contributes to the high ductility of the steel plate. During plastic deformation and under stress, the retained austenite is converted into martensite. This increases the high-strength and hardness 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.

Carbon (C): 0,25% - 0,45%, preferably 0,29% - 0,32%. The component C is linked to final hardness of martensitic phase.

Silicium (Si): 0,8% - 1 ,6%, preferably 1 ,45% - 1 ,55%, preferably 1 ,48%-1 ,5%. The component Si prevents the formation of carbides - this is the key element to obtain retained austenite.

Manganese (Mn): 1 ,0% - 2,6%, preferably 2,4% - 2,6% and more preferably 2,49% - 1 ,3% or preferably 1 ,45% - 1 ,6%. 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 centre 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.

Phosphorus (P): 0,025% and less.

Sulphur (S): 0,01% and less.

Chromium (Cr): 0,4% - 2,0%, preferably 0,45% - 0,55% and more preferably 0,49% - 0,5%, or preferably 1 ,15% - 1 ,3%. The component Cr also improves hardenability.

Molybdenum (Mo): 0,20% - 0,8%, preferably 0,2% - 0,3% and more preferably 0,23% - 0,24%, or preferably 0,45% - 0,55%. The component Mo also improves hardenability.

Nickel (Ni): 1 ,5% and less. Aluminium (Al): 0,01% - 0,05%, preferably 0,02% - 0,04%. The component Al is used for steel killing.

Boron (B): 0,001% and less.

Copper (Cu): 0,1% and less. Niobium (Nb): 0,06% and less.

Titanium (Ti): 0,05% and less.

Vanadium (V): 0,1% and less.

Nitrogen (N): 0,01% and less.

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 steel plates with the alloy chemical compositions according to the following table 1 (the remaining components being Fe and non voluntary added alloys):

Table 1

Hot rolling is carried out according to the modes in the following table 2:

Table 2

Chemical compositions of references 827615 and 831624 correspond 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:

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.

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:

Table 5

Thus, Ms is comprised between 290°C and 345°C and Mf is comprised between 80°C and 140°C.

Figures 4 to 6 show cross sectional views of the various steel plates of samples of the example plate#827615. 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.

Further characteristics and properties of the steel plate obtained according to the invention can be demonstrated via the measure of abrasion process under the action of a local plastic deformation. The local plastic deformation of the steel plate of the invention transforms the austenite into fresh martensite and ensures surface hardening during steel plate use. This is evaluated through testing via the “pin on disk” method. The QP samples coming from plate 827615 have been tested and compared with plates from Quard 500 (well-known in the art - martensitic abrasion resistant steels that present a hardness result of 500 FIBw) containing the following composition in Table 6.

Table 6 The parameters of the test of the “pin on disk” method are A load of 20N,

A speed of 20 cm/s,

A track radius of 8mm, - A distance of 30.000 cycles,

An alumina ball 54 with a diameter of 6mm.

Figure 7 illustrates the way the tests are conducted. A force sensor 50 measures a load 52 applied to a sample 58 via the ball 54. The sample 58 is driven in rotation at the aforementioned speed along the aforementioned distance. A track 60 is obtained (with aforementioned track radius). The loss of material (volume, resulting from abrasion) in the track 60 is valuated via a profilometer. The final result can be presented as a 2D profile (average of 500 measured profiles) and the loss of volume is evaluated in mm ³ . The results are compared in Table 7 with Quard 500. Table 7 illustrates a property of the steel plates obtained according to the method of the invention. Table 7 illustrates results of abrasion tests, showing the intrinsic characteristics of the steel plate obtained by the method of the invention in comparison to Quard 500. Table 7 illustrates the wear property of a steel plate having the aforementioned chemical composition and obtained by the method of the invention.

Table 7

According to Table 7, tests are performed on the steel plate of the invention (sample QP827615) and the results are compared to the result of tests performed on a sample Quard 500. Table 7 shows the worn volume (in mm ³ ) after the test of figure 7. The tests on the QP sample of the invention reach a loss (or worn) volume of less than 0,450 mm ³ whereas the Quard 500 reaches a loss (or worn) volume of approx. 1 mm ³ . Thus, in comparison to Quard 500, the QP sample has a lower volume of material removed by abrasion. The resistance to abrasion of the steel plate according to the invention is improved.

The advantage of the steel plate obtained according to the invention is to succeed in improved abrasion performances combined with an improved forming capability (bending performance). This is achieved thanks to the retained austenite as a ductile phase in the initial microstructure.

Figures 8 and 9 illustrates the bending performances of the steel plates, showing further characteristics of the steel plates obtained by the method of the invention. Figure 8 illustrates the way the bending tests are performed. Figure 9 illustrates the bending performances of a steel plate having the aforementioned chemical composition and obtained by the method of the invention. The bending performances of QP samples coming from plate QP827615 with a thickness of 5mm and 10mm have been tested and compared with plates from Quard 500.

On figure 8, the bending tests are performed by bending a 2000mm x 600mm steel plate 62 having a thickness of 5mm and 10mm. The steel plates 62 is bent along a direction 64 parallel to the 2000mm dimension. The bending tests are performed along said direction 64 that may be the longitudinal direction corresponding to the rolling direction. The bending tests are also performed along said direction 64 that may be the transverse direction corresponding to the direction transverse to the rolling direction. A 90°-bending is performed by a punch tool 66 having a certain radius R. The ratio of radius of the punch tool / plate thickness is measured via different bending tentative and is defining the optimal performance of the steel in terms of forming capability.

Figure 9 illustrates the bending performance obtained with the ratio R/th corresponding to Radius (of the punch tool) / thickness (of the steel plate). For a thickness of 5mm of the plates, along both transverse and longitudinal directions, the plate of the invention (QP827615) has a R/th ratio of less than 3 (more precisely, 2,5), meaning a bending radius of less than 15mm (more precisely 12,5mm). In comparison, the Quard 500 has a R/th ratio of 3,5 (transverse direction) and 4,5 (longitudinal direction) meaning a bending radius of 17,5mm and 22,5mm respectively. For a thickness of 10mm of the plates, along the transverse direction, the plate of the invention (QP827615) has a R/th ratio of less than 3 (more precisely 2), meaning a bending radius of less than 30mm (more precisely 20mm). Along the longitudinal direction, the plate of the invention (QP827615) has a R/th ratio of less than 4 (more precisely 3,5), meaning a bending radius of less than 40mm (more precisely 35mm). In comparison, the Quard 500 has a R/th ratio of 4,5 (transverse direction) and 5 (longitudinal direction) meaning a bending radius of 45mm and 50mm respectively.

Thus, in comparison to Quard 500, the QP sample reaches smaller bending radius while the plates are bent. The bending performances of the steel plate according to the invention are improved.

The steel plate obtained according to the invention has an improved combination of abrasion and bending performances in comparison to Quard 500.