EP1563103B1 - Method for making an abrasion resistant steel plate and steel plate obtained - Google Patents

Description

The present invention relates to an abrasion-resistant steel and its method of manufacture.

Abrasive steels of hardness close to 400 Brinell containing about 0.15% of carbon as well as manganese, nickel, chromium and molybdenum are known at levels of less than a few% in order to have sufficient quenchability. These steels are soaked so as to have a completely martensitic structure. They have the advantage of being relatively easy to implement by welding, cutting or folding. But they have the disadvantage of having a limited resistance to abrasion. It is certainly known to increase the resistance to abrasion by increasing the carbon content and therefore the hardness. But this procedure has the disadvantage of deteriorating the ability to implement.

The object of the present invention is to overcome these disadvantages by providing an abrasion-resistant steel sheet which, all things being equal, has a better abrasion resistance than that of known steels having a hardness of 400. Brinell, while having an applicability comparable to that of these steels.

• éventuellement au moins un élément pris parmi Nb, Ta et V en des teneurs telles que Nb/2 + Ta/4 + V ≤ 0,5%,
• éventuellement au moins un élément pris parmi Se, Te, Ca, Bi, Pb en des teneurs inférieures ou égales à 0, 1 %,

le reste étant du fer et des impuretés résultant de l'élaboration, la composition chimique satisfaisant en outre les relations suivantes : $$\mathrm{C}\mathrm{*}\mathrm{=}\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\ge }\mathrm{0}\mathrm{,}\mathrm{095}\mathrm{\%}$$

et : $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}\mathrm{2}\mathrm{-}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{2}\mathrm{\ge }\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}$$

et : $$\mathrm{1}\mathrm{,}\mathrm{05}\mathrm{\times }\mathrm{Mn}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{54}\mathrm{\times }\mathrm{Ni}+0,50\times \mathrm{Cr}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{3}\mathrm{\times }{\left(\mathrm{Mo}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}\mathrm{1}\mathrm{,}\mathrm{8}\mathrm{ou\; mieux}\mathrm{2}$$

avec : K = 1 si B ≥ 0,0005% et K = 0 si B < 0,0005%,
l'acier ayant une structure constituée de martensite ou d'un mélange de martensite et de bainite auto-revenue, ladite structure contenant en outre des carbures et de 5% à 20% d'austénite.
Selon ce procédé, on soumet la pièce ou la tôle à un traitement thermique de trempe, effectué dans la chaude de mise en forme à chaud telle que le laminage ou après austénitisation par réchauffage dans un four, qui consiste à :

• refroidir la pièce ou la tôle à une vitesse de refroidissement moyenne supérieure à 0,5°C/s entre une température supérieure à AC₃ et une température T = 800 - 270xC* - 90xMn -37xNi - 70XCr - 83x(Mo + W/2), et T-50°C environ, la température étant exprimée en °C et les teneurs en C*, Mn, Ni, Cr, Mo et W étant exprimées en % en poids,
• puis refroidir la pièce ou la tôle à une vitesse de refroidissement moyenne à coeur Vr < 1150xep^-1,7 (en °C/s) et supérieure à 0,1°C/s entre la température T et 100°C, ep étant l'épaisseur de la pièce ou la tôle exprimée en mm,
• et à refroidir la pièce ou la tôle jusqu'à la température ambiante, éventuellement, on effectue un planage.

For this purpose, the subject of the invention is a method for manufacturing a part, and in particular a sheet, made of steel for abrasion, the chemical composition of which comprises, by weight:

0.1% ≤ C <0.23%

0% ≤ If ≤ 2%

0% ≤ Al ≤ 2%

0.5% ≤ Si + Al ≤ 2%

0% ≤ Mn ≤ 2.5%

0% ≤ Ni ≤ 5%

0% ≤ Cr ≤ 5%

0% ≤ Mo ≤ 1%

0% ≤ W ≤ 2%

0.05% ≤ Mo + W / 2 ≤ 1%

0% ≤ Cu ≤ 1.5%

0% ≤ B ≤ 0.02%

0% ≤ Ti ≤ 0.67%

0% ≤ Zr ≤ 1.34%

0.05% <Ti + Zr / 2 ≤ 0.67%

0% ≤ S ≤ 0.15%

N <0.03%

• optionally at least one element selected from Nb, Ta and V in contents such that Nb / 2 + Ta / 4 + V ≤ 0.5%,
• optionally at least one element selected from Se, Te, Ca, Bi, Pb in contents of less than or equal to 0.1%,

the remainder being iron and impurities resulting from the preparation, the chemical composition further satisfying the following relationships: $$\mathrm{VS}\mathrm{*}\mathrm{=}\mathrm{VS}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{8}\mathrm{\ge }\mathrm{0}\mathrm{,}\mathrm{095}\mathrm{\%}$$

and $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}\mathrm{2}\mathrm{-}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{2}\mathrm{\ge }\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}$$

and $$\mathrm{1}\mathrm{,}\mathrm{05}\mathrm{\times }\mathrm{mn}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{54}\mathrm{\times }\mathrm{Or}+0,50\times \mathrm{Cr}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{3}\mathrm{\times }{\left(\mathrm{MB}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}\mathrm{1}\mathrm{,}\mathrm{8}\mathrm{or\; better}\mathrm{2}$$

with: K = 1 if B ≥ 0.0005% and K = 0 if B <0.0005%,
the steel having a structure consisting of martensite or a mixture of martensite and self-tempering bainite, said structure further containing carbides and 5% to 20% austenite.
According to this method, the part or the sheet is subjected to a quenching heat treatment, carried out in hot hot shaping such as rolling or after austenitization by reheating in an oven, which consists in:

• cool the workpiece or sheet at an average cooling rate greater than 0.5 ° C / s between a temperature above AC ₃ and a temperature T = 800 - 270xC * - 90xMn -37xNi - 70XCr - 83x (Mo + W / 2), and T-50 ° C approximately, the temperature being expressed in ° C and the contents of C *, Mn, Ni, Cr, Mo and W being expressed in% by weight,
• then cool the workpiece or the sheet at an average cooling rate at core Vr <1150xep ^-1.7 (in ° C / s) and greater than 0.1 ° C / s between the temperature T and 100 ° C, ep being the thickness of the part or sheet metal expressed in mm,
• and to cool the workpiece or the sheet to room temperature, optionally, planing is carried out.

Optionally, quenching may be followed by tempering at a temperature below 350 ° C, and preferably below 250 ° C.

The invention also relates to a part, in particular sheet metal, according to claim 8. The sheet obtained in particular by this method has a flatness characterized by an arrow less than or equal to 12 mm / m and preferably less than 5 mm / m, the steel having a structure consisting of 5% to 20% retained austenite, the remainder of the structure being martensitic or martensite-bainitic, and contains carbides. The thickness of the sheet may be between 2 mm and 150 mm.

Preferably, the hardness is between 280 HB and 450 HB.

The invention will now be described in a more precise but nonlimiting manner and be illustrated by examples.

• plus de 0,1% de carbone de façon à avoir une dureté suffisante et afin de permettre la formation de carbures, mais moins de 0,23%, et de préférence moins de 0,22%, pour que l'aptitude au soudage et au découpage soit bonne.
• de 0% à 0,67% de titane et de 0% à 1,34% de zirconium, ces teneurs devant êtres telles que la somme Ti+Zr/2 soit supérieure à 0,05%, de préférence supérieure à 0,1%, et mieux encore, supérieure à 0,2%, pour que l'acier contienne des gros carbures de titane ou de zirconium qui augmentent la résistance à l'abrasion. Mais la somme Ti+Zr/2 doit rester inférieure à 0,67% car, au-delà, l'acier ne contiendrait pas assez de carbone libre pour que sa dureté soit suffisante. Par ailleurs la teneur Ti +Zr/2 sera préférentiellement inférieure à 0,50% ou mieux 0,40% voire 0,30 % si l'on a besoin de privilégier la ténacité du matériau.
• De 0% (ou des traces) à 2% de silicium et de 0% (ou des traces) à 2% d'aluminium, la somme Si+Al étant comprise entre 0,5% et 2% et de préférence supérieure à 0,7% ou mieux, supérieure à 0,8%. Ces éléments, qui sont des désoxydants, ont en outre pour effet de favoriser l'obtention d'une austénite retenue métastable fortement chargée en carbone dont la transformation en martensite s'accompagne d'un gonflement important favorisant l'ancrage des carbures de titane.
• De 0% (ou des traces) à 2% ou même 2,5% de manganèse, de 0% (ou des traces) à 4% ou même 5% de nickel et de 0% (ou des traces) à 4% ou même 5% de chrome, pour obtenir une trempabilité suffisante et ajuster les différentes caractéristiques mécaniques ou d'emploi. Le nickel a, en particulier un effet favorable sur la ténacité, mais cet élément est cher. Le chrome forme également de fins carbures dans la martensite ou la bainite favorables à la résistance à l'abrasion.
• De 0% (ou des traces) à 1% de molybdène et de 0% (ou des traces) à 2% de tungstène, la somme Mo+W/2 étant comprise entre 0,05% et 1%, et de préférence reste inférieure à 0,8%, ou mieux, inférieure à 0,5%. Ces éléments augmentent la trempabilité et, forment dans la martensite ou dans la bainite de fins carbures durcissants, notamment par précipitation par auto revenu au cours du refroidissement. Il n'est pas nécessaire de dépasser une teneur de 1% en molybdène pour obtenir l'effet désiré en particulier en ce qui concerne la précipitation de carbures durcissants. Le molybdène peut être remplacé, en tout ou partie, par un poids double de tungstène. Néanmoins cette substitution n'est pas recherchée en pratique car elle n'offre pas d'avantage par rapport au molybdène et est plus coûteuse.
• Eventuellement de 0% à 1,5% de cuivre. Cet élément peut apporter un durcissement supplémentaire sans détériorer la soudabilité. Au-delà de 1,5%, il n'a plus d'effet significatif, il engendre des difficultés de laminage à chaud et coûte inutilement cher.
• De 0% à 0,02% de bore. Cet élément peut être ajouté de façon optionnelle afin d'augmenter la trempabilité. Pour que cet effet soit obtenu, la teneur en bore doit, de préférence, être supérieure à 0,0005% ou mieux 0,001%, et n'a pas besoin de dépasser sensiblement 0,01%.
• Jusqu'à 0,15% de soufre. Cet élément est un résiduel en général limité à 0,005% ou moins, mais sa teneur peut être volontairement augmentée pour améliorer l'usinabilité. A noter qu'en présence de soufre, pour éviter des difficultés de transformation à chaud, la teneur en manganèse doit être supérieure à 7 fois la teneur en soufre.
• Eventuellement au moins un élément pris parmi le niobium, le tantale et le vanadium, en des teneurs telles que Nb/2+Ta/4+V reste inférieure à 0,5% afin de former des carbures relativement gros qui améliorent la tenue à l'abrasion. Mais les carbures formés par ces éléments sont moins efficaces que les carbures formés par le titane ou le zirconium, c'est pour cela qu'ils sont optionnels et ajoutés en quantité limitée.
• Eventuellement un ou plusieurs éléments pris parmi le sélénium, le tellure, le calcium, le bismuth et le plomb en des teneurs inférieures à 0,1% chacun. Ces éléments sont destinés à améliorer l'usinabilité. A noter que, lorsque l'acier contient du Se et/ou du Te, la teneur en manganèse doit être suffisante compte tenu de la teneur en soufre pour qu'il puisse se former des séléniures ou des tellurures de manganèse.
• Le reste étant du fer et des impuretés résultant de l'élaboration. Parmi les impuretés, il y a en particulier l'azote dont la teneur dépend du procédé d'élaboration mais ne dépasse pas 0,03%, et reste en général inférieure à 0,025%. L'azote peut réagir avec le titane ou le zirconium pour former des nitrures qui ne doivent pas être trop gros pour ne pas détériorer la ténacité. Afin d'éviter la formation de gros nitrures, le titane et le zirconium peuvent être ajoutés dans l'acier liquide de façon très progressive, par exemple en mettant au contact de l'acier liquide oxydé une phase oxydée telle qu'un laitier chargé en oxydes de titane ou de zirconium, puis en désoxydant l'acier liquide, de façon à faire diffuser lentement le titane ou le zirconium depuis la phase oxydée vers l'acier liquide.

To manufacture a sheet according to the invention, a steel is produced whose chemical composition comprises, in% by weight:

• more than 0.1% carbon so as to have sufficient hardness and to permit the formation of carbides, but less than 0.23%, and preferably less than 0.22%, so that the weldability and the cutting is good.
• from 0% to 0.67% of titanium and from 0% to 1.34% of zirconium, these contents being such that the sum Ti + Zr / 2 is greater than 0.05%, preferably greater than 0.1. %, and more preferably greater than 0.2%, for the steel to contain large titanium or zirconium carbides which increase the abrasion resistance. But the sum Ti + Zr / 2 must remain below 0.67% because, beyond, the steel would not contain enough free carbon for its hardness is sufficient. Moreover, the Ti + Zr / 2 content will preferably be less than 0.50% or better still 0.40% or even 0.30% if it is necessary to favor the toughness of the material.
• From 0% (or traces) to 2% silicon and 0% (or traces) at 2% aluminum, the sum Si + Al being between 0.5% and 2% and preferably greater than 0 , 7% or better, greater than 0.8%. These elements, which are deoxidizing agents, also have the effect of favoring the obtaining of a metastable retained austenite highly loaded with carbon, the transformation of which in martensite is accompanied by a large swelling favoring the anchoring of the titanium carbides.
• From 0% (or traces) to 2% or even 2.5% manganese, from 0% (or traces) to 4% or even 5% nickel and 0% (or traces) at 4% or even 5% chromium, to obtain a sufficient quenchability and adjust the different mechanical characteristics or use. Nickel has a particularly favorable effect on toughness, but this element is expensive. Chromium also forms fine carbides in martensite or bainite favorable to abrasion resistance.
• From 0% (or traces) to 1% molybdenum and 0% (or traces) at 2% tungsten, the sum Mo + W / 2 being between 0.05% and 1%, and preferably remains less than 0.8%, or better, less than 0.5%. These elements increase the quenchability and form in martensite or bainite thin carbides hardening, including self-precipitation precipitation during cooling. It is not necessary to exceed a molybdenum content of 1% in order to obtain the desired effect, particularly as regards the precipitation of hardening carbides. Molybdenum can be replaced in whole or in part by a double weight of tungsten. However, this substitution is not sought in practice because it offers no advantage over molybdenum and is more expensive.
• Possibly from 0% to 1.5% copper. This element can provide additional hardening without damaging the weldability. Beyond 1.5%, it has no significant effect, it generates hot rolling difficulties and unnecessarily expensive.
• 0% to 0.02% boron. This element can be added optionally to increase quenchability. For this effect to be obtained, the boron content should preferably be greater than 0.0005% or better 0.001%, and need not exceed substantially 0.01%.
• Up to 0.15% sulfur. This element is a residual usually limited to 0.005% or less, but its content can be voluntarily increased to improve machinability. It should be noted that in the presence of sulfur, in order to avoid difficulties of hot transformation, the manganese content must be greater than 7 times the sulfur content.
• Optionally at least one of niobium, tantalum and vanadium in such quantities that Nb / 2 + Ta / 4 + V remains below 0.5% in order to form relatively large carbides which improve the resistance to corrosion. 'abrasion. But the carbides formed by these elements are less effective than the carbides formed by titanium or zirconium, that is why they are optional and added in limited quantities.
• Possibly one or more elements selected from selenium, tellurium, calcium, bismuth and lead in contents of less than 0.1% each. These elements are intended to improve machinability. It should be noted that when the steel contains Se and / or Te, the manganese content must be sufficient in view of the sulfur content so that selenides or tellurides of manganese can be formed.
• The rest being iron and impurities resulting from the elaboration. Among the impurities, there is in particular nitrogen, the content of which depends on the production method but does not exceed 0.03%, and remains generally less than 0.025%. Nitrogen can react with titanium or zirconium to form nitrides that should not be too big to deteriorate toughness. In order to avoid the formation of large nitrides, titanium and zirconium can be added to the liquid steel in a very gradual manner, for example by contacting the oxidized liquid steel with an oxidized phase such as a slag loaded with oxides of titanium or zirconium, then deoxidizing the liquid steel, so as to slowly diffuse titanium or zirconium from the oxidized phase to the liquid steel.

In addition, in order to obtain satisfactory properties, the carbon, titanium, zirconium and nitrogen contents are chosen such that: $$\mathrm{VS}\mathrm{*}\mathrm{=}\mathrm{VS}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{8}\mathrm{\ge }\mathrm{0}\mathrm{,}\mathrm{095}\mathrm{\%}$$

And preferably C * ≥ 0.12% to have a higher hardness and therefore a better abrasion resistance. The quantity C * represents the free carbon content after precipitation of the titanium and zirconium carbides, taking into account the formation of titanium and zirconium nitrides. This free carbon content C * must be greater than 0.095% to have a martensitic or martensite-bainitic structure having a sufficient hardness.

Given the possible formation of titanium nitrides or zirconium, so that the amount of titanium or zirconium carbides is sufficient, the contents of Ti, Zr and N must be such that: $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}\mathrm{2}\mathrm{-}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{2}\mathrm{\ge }\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}$$

avec : K = 1 si B ≥ 0,0005% et K = 0 si B < 0,0005%,In addition, the chemical composition is chosen so that the quenchability of the steel is sufficient, given the thickness of the sheet that is to be manufactured. For this, the chemical composition must satisfy the relation: $$\mathrm{Tremp}=\mathrm{1}\mathrm{,}\mathrm{05}\mathrm{\times }\mathrm{mn}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{54}\mathrm{\times }\mathrm{Or}+0,50\times \mathrm{Cr}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{3}\mathrm{\times }{\left(\mathrm{MB}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}\mathrm{1}\mathrm{,}\mathrm{8}\mathrm{or\; better}\mathrm{2}$$

with: K = 1 if B ≥ 0.0005% and K = 0 if B <0.0005%,

In addition, and to obtain a good resistance to abrasion, the micrographic structure of the steel consists of martensite or bainite or a mixture of these two structures, and from 5% to 20% retained austenite. In addition, this structure comprises large titanium or zirconium carbides formed at high temperature, and optionally carbides of niobium, tantalum or vanadium. Due to the manufacturing process that will be described later, this structure is returned, so that it also includes molybdenum carbides or tungsten and possibly chromium carbides.

The inventors have found that the effectiveness of large carbides for the improvement of the abrasion resistance could be obelated by the premature loosening thereof and that this loosening could be avoided by the presence of metastable austenite which is transformed under the effect of abrasion phenomena. The transformation of the metastable austenite is by swelling, this transformation in the abraded undercoat increases the resistance to carburetion and thus improves abrasion resistance.

On the other hand, the high hardness of the steel and the presence of embrittling titanium carbides make it necessary to limit the leveling operations as much as possible. From this point of view, the inventors have found that by slowing down cooling sufficiently in the bainitomensitic transformation domain, the residual deformations of the products are reduced, which makes it possible to limit the leveling operations. The inventors have found that cooling the workpiece or the sheet at an average cooling rate at core Vr <1150 × ep ^-1.7 (in this formula, ep is the thickness of the sheet expressed in mm, and the speed of cooling is expressed in ° C / s) below a temperature T = 800 - 270xC * - 90xMn -37xNi - 70XCr - 83x (Mo + W / 2), (expressed in ° C), the residual stresses generated were reduced by phase changes. This cooling slowed in the bainito-martensitic field has, in addition, the advantage of causing a self-income which generates the formation of carbides of molybdenum, tungsten or chromium and improves the wear resistance of the matrix surrounding the big carbides.

In order to manufacture a flat sheet having good abrasion resistance and good processability, the steel is made, cast in the form of a slab or ingot. The slab or slug is hot-rolled to obtain a sheet which is subjected to a heat treatment which makes it possible at the same time to obtain the desired structure and a good flatness without subsequent planing or with limited planing. The heat treatment can be carried out in the hot rolling or later, possibly after a cold or mid-heat planing.

• on chauffe l'acier au-dessus du point AC₃ de façon à lui conférer une structure entièrement austénitique, dans laquelle cependant subsistent des carbures de titane ou de zirconium,
• puis on le refroidit à une vitesse de refroidissement moyenne à coeur supérieure à la vitesse critique de transformation bainitique jusqu'à une température comprise entre T = 800 - 270xC* - 90xMn -37xNi - 70XCr - 83x(Mo + W/2), et T-50°C, environ, de façon à éviter la formation de constituants ferrito-perlitiques, pour cela, il suffit en général de refroidir à une vitesse supérieure à 0,5°C/s,
• puis, entre la température ainsi définie (c'est à dire comprise entre T et T-50°C environ) et 100°C environ, on refroidit la tôle à une vitesse de refroidissement moyenne à coeur Vr inférieure à 1150xep^-1,7, et supérieure à 0,1°C/s, pour obtenir la structure souhaitée,
• et on refroidit la tôle jusqu'à la température ambiante, de préférence, sans que ce soit obligatoire, à une vitesse lente.

In all cases, to achieve the heat treatment:

• the steel is heated above the point AC ₃ so as to give it a completely austenitic structure, in which, however, titanium or zirconium carbides remain,
• then cooled to an average core cooling rate above the critical bainitic transformation rate to a temperature of between T = 800 - 270xC * - 90xMn -37xNi - 70XCr - 83x (Mo + W / 2), and T-50 ° C, approximately, so as to avoid the formation of ferrito-pearlitic constituents, for this, it is generally sufficient to cool at a speed greater than 0.5 ° C / s,
• then, between the temperature thus defined (that is to say between about T and T-50 ° C) and about 100 ° C, the sheet is cooled to an average cooling rate Vr heart less than 1150xep ^-1.7 , and greater than 0.1 ° C / s, to obtain the desired structure,
• and the sheet is cooled to room temperature, preferably, but not required, at a slow rate.

In addition, an expansion treatment, such as a tempering at a temperature of less than or equal to 350 ° C, and preferably less than 250 ° C, can be carried out.

By average cooling rate is meant the cooling rate equal to the difference between the start and end temperatures of cooling divided by the cooling time between these two temperatures.

This gives a sheet, whose thickness can be between 2 mm and 150 mm, having excellent flatness characterized by an arrow less than 3 mm per meter without planing or with moderate planing. The sheet has a hardness between 280HB and 450HB. This hardness depends mainly on the free carbon content C * = C - Ti / 4 - Zr / 8 + 7xN / 8. The higher the free carbon content, the greater the hardness. The lower the free carbon content, the more work is easy. At equal free carbon content, the higher the titanium content, the better the abrasion resistance.

As an example, consider sheets of 30mm thick steel, labeled A, B, C and D according to the invention, E and F according to the prior art and G and H given for comparison. The chemical compositions of the steels, expressed in 10 ^-3 % by weight, as well as the hardness and a wear resistance index Rus, are reported in Table 1. Table 1 VS Yes al mn Or Cr MB W Ti B NOT HB Rus AT 180 550 30 1750 200 1700 150 - 150 2 6 360 1.51 B 140 210 610 1450 650 1720 230 120 160 3 7 345 1.42 VS 220 830 25 1250 220 1350 275 350 2 5 360 2.03 D 158 780 35 1250 250 1340 260 110 3 5 363 1.3 E 175 360 25 1720 200 1200 250 - 20 3 5 420 1.08 F 150 320 30 1730 250 1260 310 - - 2 6 380 1 BOY WUT 210 340 25 1230 260 1350 280 350 2 5 360 1.11 H 150 320 25 1255 250 1360 260 105 3 6 366 0.81

The wear resistance of the steels is measured by the weight loss of a prismatic specimen rotated in a tank containing calibrated granules of quartzite for a period of 5 hours.

The wear resistance index Rus of a steel is the ratio of the wear resistance of the steel F, taken as a reference, to the wear resistance of the steel under consideration.

The sheets A to H are austenitized at 900 ° C.

• la tôle en acier A est refroidie à une vitesse moyenne de 0,7°C/s au dessus de la température T définie plus haut (environ 460°C), et à une vitesse moyenne de 0,13°C/s en dessous, conformément à l'invention;
• les tôles en aciers B, C, D, sont refroidie à une vitesse moyenne de 6°C/s au dessus de la température T définie plus haut (environ 470°C), et à une vitesse moyenne de 1,4°C/s en dessous, conformément à l'invention ;
• les tôles en acier E, F, G et H, données à titre de comparaison, ont été refroidies à une vitesse moyenne de 20°C/s au dessus de la température T définie plus haut, et à une vitesse moyenne de 12°C/s en dessous.

After austenitization:

• the steel sheet A is cooled at an average speed of 0.7 ° C / s above the temperature T defined above (about 460 ° C), and at an average speed of 0.13 ° C / s below according to the invention;
• the steel sheets B, C, D are cooled at an average speed of 6 ° C / s above the temperature T defined above (about 470 ° C), and at an average speed of 1.4 ° C / s below, according to the invention;
• the steel sheets E, F, G and H, given by way of comparison, were cooled at an average speed of 20 ° C./s above the temperature T defined above, and at an average speed of 12 ° C. / s below.

The sheets A to D have a martensite-bainitic self-regenerating structure containing about 10% retained austenite, as well as titanium carbides, while the plates E to G have a completely martensitic structure, the sheets G and H also containing large titanium carbides.

It can be seen that, although having hardnesses less than those of the E and F sheets, the sheets A, B, C and D have substantially better abrasion resistance. The lower hardnesses, which essentially correspond to lower free carbon contents, lead to better processability.

Comparison of Examples C, D, F, G and H shows that the increase in abrasion resistance does not result simply from the addition of titanium, but from the combination of titanium addition and structure. containing residual austenite. In fact, it can be seen that the steels F, G and H, the structure of which does not contain residual austenite, have fairly comparable abrasion resistance, whereas the steels C and D which contain residual austenite have significantly better abrasion.

In addition, the comparison of the pairs G and H on the one hand and C and D on the other hand, show that the presence of residual austenite significantly increases the efficiency of the titanium. For Examples C and D, the change from 0.110% to 0.350% titanium results in an increase in abrasion resistance of 56%, whereas for G and H steels, the increase is only 37%.

This observation is attributable to the increased crimping effect of titanium carbides by the surrounding matrix, when it contains residual austenite that can turn into hard, swelling martensite in service.

In addition, the deformation after cooling, without planing, for steel sheets A or B are 6 mm / m and 17 mm / m for steel sheets E and F. These results show the reduction of deformation of the products obtained. thanks to the invention.

• soit, on peut livrer les produits sans planage (gain sur le coût et sur les contraintes résiduelles),
• soit, on peut réaliser un planage pour satisfaire une exigence de planéité plus sévère' (par exemple 5mm/m) mais plus facilement et en introduisant moins de contraintes du fait de la déformation originelle moindre sur les produits selon l'invention.

As a result, in practice, depending on the level of requirement of flatness of the users,

• either, one can deliver the products without planing (gain on the cost and on the residual stresses),
• either, one can carry out planing to satisfy a requirement of flatness more severe '(for example 5mm / m) but more easily and by introducing less constraints due to the original deformation less on the products according to the invention.

Claims (13)

1. Process for manufacturing a part, and especially a plate, made of an abrasion-resistant steel, the chemical composition of which comprises, by weight:
   
           0.1% ≤ C < 0.23%
   
           0% ≤ Si ≤ 2%
   
           0% ≤ Al ≤ 2%
   
           0.5% ≤ Si + Al ≤ 2%
   
           0% ≤ Mn ≤ 2.5%
   
           0% ≤ Ni ≤ 5%
   
           0% ≤ Cr ≤ 5%
   
           0% ≤ Mo ≤ 1%
   
           0% ≤ W ≤ 2%
   
           0.05% ≤ Mo +W/2 ≤ 1%
   
           0% ≤ B ≤ 0.02%
   
           0% ≤ Ti ≤ 0.67%
   
           0% ≤ Zr ≤ 1.34%
   
           0.05% ≤ Ti +Zr/2 ≤ 0.67%
   
           0% ≤ S ≤ 0.15%
   
           N < 0.03%
   
   

   - optionally, 0% to 1.5% of copper;

   - optionally, at least one element taken from Nb, Ta and V in contents such that Nb/2 + Ta/4 + V ≤ 0.5%;

   - optionally, at least one element taken from Se, Te, Ca, Bi and Pb in contents of 0.1% or less,

   the balance being iron and impurities resulting from the smelting, the chemical composition furthermore satisfying the following relationships: $$\mathrm{C}\mathrm{*}\mathrm{=}\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\ge }\mathrm{0.095}\mathrm{\%}$$

   

   
   and $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}\mathrm{2}\mathrm{-}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{2}\mathrm{>}\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}$$

   

   
   and $$\mathrm{1.05}\mathrm{\times }\mathrm{Mn}\mathrm{+}\mathrm{0.54}\mathrm{\times }\mathrm{Ni}\mathrm{+}0.50\times \mathrm{Cr}+\mathrm{0.3}\mathrm{\times }{\left(\mathrm{Mo}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}\mathrm{1.8}$$

   

   
   where K = 1 if B ≥ 0.0005% and K = 0 if B < 0.0005%,
   in which the part or the plate undergoes a hardening heat treatment carried out in the hot-forming heat, for example rolling heat, or after austenitization by reheating in a furnace, in order to carry out the hardening, in which:

   - the part or the plate is cooled at an average cooling rate of greater than 0.5°C/s between a temperature above AC₃ and a temperature between T = 800 - 270xC* - 90xMn - 37xNi - 70xCr - 83x(Mo+W/2) and about T-50°C;

   - then the part or the plate is cooled at an average core cooling rate V_c < 1150 × th^-1.7 and greater than or equal to 0.1°C/s between the temperature T and 100°C, th being the thickness of the part or plate expressed in mm; and

   - the part or the plate is cooled down to ambient temperature and, optionally, undergoes skin pass rolling.
2. Process according to Claim 1, further characterized in that: $$\mathrm{1.05}\mathrm{\times }\mathrm{Mn}\mathrm{+}\mathrm{0.54}\mathrm{\times }\mathrm{Ni}\mathrm{+}0.50\times \mathrm{Cr}+\mathrm{0.3}\mathrm{\times }{\left(\mathrm{Mo}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}2.$$

   
3. Process according to Claim 1 or Claim 2, further
   characterized in that:
   
           C ≤ 0.22%
   
   and
   
           C* ≥ 0.12%.
   
   
4. Process according to any one of Claims 1 to 3, further characterized in that:
   
           Ti + Zr/2 ≥ 0.10%.
   
   
5. Process according to any one of Claims 1 to 4, further characterized in that:
   
           Si + Al ≥ 0.7%.
   
   
6. Process according to any one of Claims 1 to 5, characterized in that a tempering operation is also carried out at a temperature of 350°C or below.
7. Process according to any one of Claims 1 to 6, characterized in that, to add titanium to the steel, a liquid steel is brought into contact with a titanium-containing slag and the titanium is made to diffuse slowly from the slag into the liquid steel.
8. Part, and especially a plate, made of an abrasion-resistant steel, the chemical composition of which comprises, by weight:
   
           0.1% ≤ C < 0.23%
   
           0 % ≤ Si ≤ 2%
   
           0% ≤ Al ≤ 2%
   
           0.5% ≤ Si + Al ≤ 2%
   
           0% ≤ Mn ≤ 2.5%
   
           0% ≤ Ni ≤ 5%
   
           0 % ≤ Cr ≤ 5%
   
           0% ≤ Mo ≤ 1%
   
           0% ≤ W ≤ 2%
   
           0.05% ≤ Mo +W/2 ≤ 1%
   
           0% ≤ B ≤ 0.02%
   
           0% ≤ Ti ≤ 0.67%
   
           0% ≤ Zr ≤ 1.34%
   
           0.05% ≤ Ti +Zr/2 ≤ 0.67%
   
           0% ≤ S ≤ 0.15%
   
           N < 0.03%
   
   

   - optionally, 0% to 1.5% of copper;

   - optionally, at least one element taken from Nb, Ta and V in contents such that Nb/2 + Ta/4 + V ≤ 0.5%;

   - optionally, at least one element taken from Se, Te, Ca, Bi and Pb in contents of 0.1% or less,

   the balance being iron and impurities resulting from the smelting, the chemical composition furthermore satisfying the following relationships: $$\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\ge }\mathrm{0.095}\mathrm{\%}$$

   

   
   and $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}\mathrm{2}\mathrm{-}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{2}\mathrm{>}\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}$$

   

   
   and $$\mathrm{1.05}\mathrm{\times }\mathrm{Mn}\mathrm{+}\mathrm{0.54}\mathrm{\times }\mathrm{Ni}\mathrm{+}0.50\times \mathrm{Cr}+\mathrm{0.3}\mathrm{\times }{\left(\mathrm{Mo}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}\mathrm{1.8}$$

   

   
   where K = 1 if B ≥ 0.0005% and K = 0 if B < 0.0005%,
   the steel having a martensitic or martensitic-bainitic structure, said structure containing carbides and 5% to 20% residual austenite.
9. Part according to Claim 8, characterized in that: $$\mathrm{1.05}\mathrm{\times }\mathrm{Mn}\mathrm{+}\mathrm{0.54}\mathrm{\times }\mathrm{Ni}\mathrm{+}0.50\times \mathrm{Cr}+\mathrm{0.3}\mathrm{\times }{\left(\mathrm{Mo}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}2.$$

   
10. Part according to Claim 8 or Claim 9, characterized in that:
    
            C ≤ 0.22%
    
    and $$\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\ge }\mathrm{0.12}\mathrm{\%}\mathrm{.}$$

    
11. Part according to any one of Claims 8 to 10, characterized in that:
    
            Ti + Zr/2 ≥ 0.10%.
    
    
12. Part according to any one of Claims 8 to 11, characterized in that:
    
            Si + Al ≥ 0.7%.
    
    
13. Part according to any one of Claims 8 to 12, characterized in that the thickness of the plate is between 2 mm and 150 mm.