EP1563104B1 - Method for making an abrasion resistant steel plate and plate obtained - Google Patents

Description

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

Abrasion resistant steels are well known and are generally steels of high hardness (between 400 and 500 Brinell) having a martensitic structure, and containing from 0.12% to 0.3% carbon. It is generally accepted that in order to increase the wear resistance it is sufficient to increase the hardness, but this is done to the detriment of other properties such as the ability to weld or shape by folding, for example . Also, in order to obtain steels having both very good wear resistance and good processability, other means have been sought besides increasing the hardness.

This is how we proposed in EP 0527 276 and in US5,393,358 to improve the abrasion resistance of a steel containing from 0.05% to 0.45% of carbon, up to 1% of silicon, up to 2% of manganese, up to 2% of copper , up to 10% nickel, up to 3% chromium, up to 3% molybdenum, boron, niobium and vanadium, adding from 0.015% to 1.5% titanium, so that form large titanium carbides. This steel is hardened and therefore comprises a martensitic structure, the increase in abrasion resistance being obtained by the presence of large titanium carbides. But, especially when the steel is cast ingots, this improvement is limited because, under the effect of abrasive stresses, the carbides come off and no longer fulfill their role. In addition, in these steels, the presence of large titanium carbides deteriorates the ductility. As a result, the sheets made with these steels are difficult to glide and bend, which limits their possible uses.

The object of the present invention is to overcome these disadvantages by providing an abrasion-resistant steel sheet having a good flatness and which, all things being equal, has an abrasion resistance better than that of steels. known.

$$0\%\le \mathrm{Si}\le 2\%$$

$$0\%\le \mathrm{Al}\le 2\%$$

$$\mathrm{0}\mathrm{,}\mathrm{35}\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{+}\mathrm{Al}\mathrm{\le }\mathrm{2}\mathrm{\%}$$

$$0\%\le \mathrm{Mn}\le 2,5\%$$

$$0\%\le \mathrm{Ni}\le 5\%$$

$$0\%\le \mathrm{Cr}\le 5\%$$

$$\begin{array}{l}0\%\le \mathrm{Mo}\le 0,50\%\\ 0\%\le \mathrm{W}\le 1.00\%\end{array}$$

$$\mathrm{0}\mathrm{,}\mathrm{1}\mathrm{\%}\mathrm{\le }\mathrm{Mo}+\mathrm{W}/2\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{50}\mathrm{\%}$$

$$0\%\le \mathrm{Cu}\le 1,5\%$$

$$0\%\le \mathrm{B}\mathrm{\le }0,02\%$$

$$0\%\le \mathrm{Ti}\le 2\%$$

$$0\%\le \mathrm{Zr}\le 4\%$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{+}\mathrm{Zr}/2\mathrm{\le }\mathrm{2}\mathrm{\%}$$

$$0\%\le \mathrm{S}\mathrm{\le }0,15\%$$

$$\mathrm{N}\mathrm{\le }0,03\%$$

• 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, avec C* = C - Ti/4 - Zr/8 + 7xN/8: $$0,10\%\le \mathrm{C}*\mathrm{\le }0,55\%$$

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 :

• 1,05xMn + 0,54xNi +0,50xCr + 0,3x(Mo + W/2)^1/2 + K > 1,8 ou mieux 2 avec : K = 0,5 si B ≥ 0,0005% et K = 0 si B < 0,0005%,

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 comprise entre T = 800 - 270xC* - 90xMn -37xNi - 70XCr - 83x(Mo + W/2), et T-50°C, 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: $$\mathrm{0}\mathrm{,}\mathrm{35}\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{8}\mathrm{\%}$$

$$0\%\le \mathrm{Yes}\le 2\%$$

$$0\%\le \mathrm{al}\le 2\%$$

$$\mathrm{0}\mathrm{,}\mathrm{35}\mathrm{\%}\mathrm{\le }\mathrm{Yes}\mathrm{+}\mathrm{al}\mathrm{\le }\mathrm{2}\mathrm{\%}$$

$$0\%\le \mathrm{mn}\le 2,5\%$$

$$0\%\le \mathrm{Or}\le 5\%$$

$$0\%\le \mathrm{Cr}\le 5\%$$

$$\begin{array}{l}0\%\le \mathrm{MB}\le 0,50\%\\ 0\%\le \mathrm{W}\le 1.00\%\end{array}$$

$$\mathrm{0}\mathrm{,}\mathrm{1}\mathrm{\%}\mathrm{\le }\mathrm{MB}+\mathrm{W}/2\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{50}\mathrm{\%}$$

$$0\%\le \mathrm{Cu}\le 1,5\%$$

$$0\%\le \mathrm{B}\mathrm{\le }0,02\%$$

$$0\%\le \mathrm{Ti}\le 2\%$$

$$0\%\le \mathrm{Zr}\le 4\%$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{+}\mathrm{Zr}/2\mathrm{\le }\mathrm{2}\mathrm{\%}$$

$$0\%\le \mathrm{S}\mathrm{\le }0,15\%$$

$$\mathrm{NOT}\mathrm{\le }0,03\%$$

• optionally at least one element selected from Nb, Ta and V in such contents as 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, with C * = C - Ti / 4 - Zr / 8 + 7xN / 8: $$0,10\%\le \mathrm{VS}*\mathrm{\le }0,55\%$$

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

• 1.05xMn + 0.54xNi + 0.50xCr + 0.3x (Mo + W / 2) ^1/2 + K> 1.8 or better 2 with: K = 0.5 if B ≥ 0.0005% and K = 0 if B <0.0005%,

according to this method, the part or the sheet is subjected to a quenching heat treatment carried out in the hot hot forming furnace such as rolling or after austenitization by reheating in an oven, which consists in:

• cooling the workpiece or sheet at an average cooling rate greater than 0.5 ° C / s between a temperature above AC ₃ and a temperature between T = 800 - 270xC * - 90xMn -37xNi - 70XCr - 83x (Mo + W / 2), and T-50 ° C, 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, and in particular a sheet obtained in particular by this method, the steel having a structure consisting of 5% to 20% retained austenite, the remainder of the structure being martensitic or martensite-bainitic with carbides . When the piece is a sheet, its thickness may be between 2 mm and 150 mm and its flatness may be characterized by an arrow less than or equal to 12 mm / m, and preferably less than 5 mm / m.

la dureté est, de préférence, comprise entre 280 HB et 450 HB.When the carbon content is such that: $$\mathrm{0}\mathrm{,}\mathrm{1}\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{2}\mathrm{\%}$$

the hardness is preferably between 280 HB and 450 HB.

la dureté est, de préférence, comprise entre 380 HB et 550 HB.When the carbon content is such that: $$\mathrm{0}\mathrm{,}\mathrm{2}\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{3}\mathrm{\%}$$

the hardness is preferably between 380 HB and 550 HB.

la dureté est, de préférence, comprise entre 450 HB et 650 HB.When the carbon content is such that: $$\mathrm{0}\mathrm{,}\mathrm{3}\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{\%}$$

the hardness is preferably between 450 HB and 650 HB.

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

• de 0,35% à 0,8% de carbone, et de préférence, plus de 0,45%, voire plus de 0,5%, et de 0% à 2% de titane, de 0% à 4% de zirconium, ces teneurs devant êtres telles que: 0,05% ≤ Ti+Zr/2 ≤ 2%. Le carbone est destiné d'une part à obtenir une structure martensitique suffisamment dure, d'autre part à former des carbures de titane et/ou de zirconium. La somme Ti+Zr/2 doit être supérieure à 0,05%, de préférence supérieure 0,10%, et mieux encore, supérieure à 0,3%, ou même supérieure à 0,5% pour qu'il y ait un minimum de carbures formés, mais doit rester inférieure à 2%, et de préférence inférieure ou égale à 0,9%, car au-delà, la ténacité et l'aptitude à la mise en oeuvre sont détériorées.
• 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 0,35% et 2% et de préférence supérieure à 0,5%, et mieux encore, supérieure à 0,7%. 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.
• De 0% (ou des traces) à 0,50% de molybdène. Cet élément augmente la trempabilité et forme 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 0,50% 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 ceux qui sont 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 en général pas 0,03%. Cet élément 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:

• from 0.35% to 0.8% of carbon, and preferably more than 0.45% or even more than 0.5%, and from 0% to 2% of titanium, from 0% to 4% of zirconium these contents must be such that: 0.05% ≤ Ti + Zr / 2 ≤ 2%. The carbon is intended on the one hand to obtain a sufficiently hard martensitic structure, on the other hand to form titanium carbides and / or zirconium. The sum Ti + Zr / 2 must be greater than 0.05%, preferably greater than 0.10%, and more preferably greater than 0.3%, or even greater than 0.5%, so that there is minimum carbides formed, but must remain less than 2%, and preferably less than or equal to 0.9%, because beyond, the toughness and workability are deteriorated.
• From 0% (or traces) to 2% silicon and 0% (or traces) at 2% aluminum, the sum Si + Al being between 0.35% and 2% and preferably greater than 0 , 5%, and more preferably greater than 0.7%. 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.
• From 0% (or traces) to 0.50% molybdenum. This element increases the quenchability and forms in martensite or bainite thin carbides hardening including self-precipitation precipitation during cooling. It is not necessary to exceed 0.50% 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%, he has no significant effect, it causes difficulties in hot rolling 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 those 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 process but generally does not exceed 0.03%. This element can react with titanium or zirconium to form nitrides which must not be too big to not 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 must be such that: $$\mathrm{0}\mathrm{,}\mathrm{1}\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{NOT}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{55}\mathrm{\%}$$

The expression C - Ti / 4 - Zr / 8 + 7xN / 8 = 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 0.1%, and preferably greater than or equal to 0.22%, to have a martensite having a minimum hardness, but beyond 0.55% the toughness and the the processability are too deteriorated.

In addition, the chemical composition must be 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: $$\mathrm{Tremp}=1,05\times \mathrm{mn}+0,54\times \mathrm{Or}+0,50\times \mathrm{Cr}+0,3\times (\mathrm{MB}+\mathrm{W}/2{)}^{1/2}+\mathrm{K}>1,8\mathrm{or\; better}2\mathrm{with}:\mathrm{K}=0,5\mathrm{if\; B}>\mathrm{or\; equal}0,0005\%\mathrm{and\; K}=0\mathrm{if\; B}<0,0005\%,$$

Note that, especially when Tremp is between 1.8 and 2, it is preferable that the silicon content is greater than 0.5% so as to promote the formation of retained austenite.

In addition, the contents of Ti, Zr and N should preferably be such that: Ti + Zr / 2 - 7xN / 2 ≥ 0.05%, and more preferably greater than 0.1%, and more preferably, greater than 0.3% for the carbide content to be sufficient.

Finally, 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. This structure further comprising large titanium or zirconium carbides, or even carbides of niobium, tantalum or vanadium, formed at high temperature. The inventors have found that the effectiveness of large carbides for the improvement of the abrasion resistance could be obelated by premature loosening thereof and that this loosening could be avoided by the presence of metastable austenite which is transformed in fresh martensite under the effect of abrasion phenomena. The conversion of the metastable austenite to fresh martensite is by swelling, this transformation in the abraded underlayer increases the resistance to carburetion and thus improves the 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 a cooling rate Vr <1150xep ^-1.7 , (in this formula, ep is the thickness of the sheet expressed in mm, and the cooling rate is expressed in ° C / s) below a temperature T = 800 - 270xC * - 90xMn -37xNi - 70XCr - 83x (Mo + W / 2), (expressed in ° C), on the one hand it was favored to obtain a significant proportion of residual austenite, and on the other hand, the residual stresses generated by the phase changes were reduced.

To make a sheet having a good resistance to abrasion and well flat, the steel is made, it flows in the form of 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 directly in the hot rolling or carried out later, possibly after a cold planing or half-hot.

• on chauffe l'acier au-dessus du point AC₃ de façon à lui conférer une structure entièrement austénitique,
• puis on le refroidit à une vitesse de refroidissement moyenne supérieure à la vitesse critique de transformation bainitique jusqu'à une température égale ou légèrement inférieure (de moins de 50°C environ) à une température T = 800 - 270xC* - 90xMn -37xNi - 70XCr - 83x(Mo + W/2), (exprimée en °C),
• 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 comprise entre 0,1°C/s, pour obtenir une dureté suffisante, et 1150xep^-1,7, 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.

To achieve the heat treatment:

• the steel is heated above the point AC ₃ so as to give it a completely austenitic structure,
• and then cooled to a higher average cooling rate than the critical bainitic transformation rate to a temperature equal to or slightly lower (less than about 50 ° C) at a temperature T = 800 - 270xC * - 90xMn -37xNi - 70XCr - 83x (Mo + W / 2), (expressed in ° C),
• then, between the temperature thus defined (that is to say between T and T - about 50 ° C) and 100 ° C, the sheet is cooled to an average cooling rate at heart Vr between 0.1 ° C / s, to obtain a sufficient hardness, and 1150xep ^-1.7 , to obtain the desired structure,
• and the sheet is cooled to room temperature, preferably, but not necessarily, at a slow rate.

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

This gives a sheet, whose thickness can be between 2 mm and 150 mm, having excellent flatness characterized by an arrow less than 12 mm per meter without planing or with moderate planing. The sheet has a hardness between 280HB and 650HB. This hardness depends mainly on the free carbon content C * = C - Ti / 4 - Zr / 8 + 7xN / 8.

1. a) 0,1% ≤ C* ≤ 0,2%, la dureté est comprise entre 280HB et 450HB environ,
2. b) 0,2% < C* ≤ 0,3%, la dureté est comprise entre 380HB et 550HB environ,
3. c) 0,3% < C* ≤ 0,5%, la dureté est comprise entre 450HB et 650HB environ.

Depending on the free carbon contents C *, several domains corresponding to levels of increasing hardness can be defined, and in particular:

1. a) 0.1% ≤ C * ≤ 0.2%, the hardness is between 280HB and 450HB approximately,
2. b) 0.2% <C * ≤ 0.3%, the hardness is between 380HB and 550HB,
3. c) 0.3% <C * ≤ 0.5%, the hardness is between 450HB and 650HB approximately.

The hardness being a function of the free carbon content C *, the same hardness can be obtained with very different titanium or zirconium contents. With equal hardness, the resistance to abrasion is all the higher as the titanium or zirconium content is important. Similarly, titanium content or equal zirconium, the resistance to abrasion is even better than the hardness is high. In addition, the implementation of the steel is all the easier as the free carbon content is low, but with equal free carbon content, ductility is even better than the titanium content is low. All of these considerations make it possible to choose the carbon and titanium or zirconium contents which lead to all of the properties best suited to each field of application.

• 280 à 450 HB : godets, bennes de camions et de dumpers, blindages de cyclones, trémies, moules à parpaings,
• 380 à 550 HB : blindages de broyeurs à percussion, lame d'attaque de bulldozer, lames de benne preneuse, grilles de cribles,
• 450 à 650 HB : plaques de blindage de broyeur à cylindres, renforts de godets, renforts sous lame d'attaque, bouclier de lame éperon, bord d'attaque.

Depending on the hardness levels, the uses are for example:

• 280 to 450 HB: buckets, trucks and dump trucks, cyclone shields, hoppers, block molds,
• 380 to 550 HB: hammer mill shields, bulldozer blade, clamshell blades, screen grates,
• 450 to 650 HB: roll crusher shield plates, bucket reinforcements, reinforcements under attack blade, spur blade shield, leading edge.

By way of example, steel sheets identified A to G according to the invention and H to J according to the prior art are considered. The chemical compositions of the steels, expressed in 10 ^-3 % by weight, as well as the hardness, the residual austenite content of the structure 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 % aust Rus AT 360 850 50 1300 500 700 100 500 400 2 6 460 10 1.42 B 640 850 50 400 1500 700 110 450 620 3 7 555 14 2.72 VS 590 520 570 550 320 1850 470 - 540 - 7 570 12 2.24 D 705 460 630 1090 280 2450 430 100 825 - 7 580 13 3.14 E 690 370 25 740 310 2100 460 - 795 - 6 605 10 2.83 F 350 810 30 1200 270 1350 380 160 2 6 510 8 1.32 BOY WUT 390 790 35 1,210 250 1340 390 405 3 6 495 11 1.77 H 340 380 30 1260 470 820 370 - 410 3 6 475 1 0.86 I 315 330 25 1230 180 1360 395 165 2 6 515 2 0.7 J 367 315 30 1215 210 1375 405 430 2 5 500 2 1.01

The wear resistance index Rus varies as the logarithm of the inverse of the weight loss of a prismatic specimen rotated in a tank containing calibrated quartzite granules.

All the sheets have a thickness of 30 mm, and the sheets corresponding to the steels A to G have been quenched according to the invention, after austenitization at 900 ° C.

• pour les tôles en acier B et D: refroidissement à une vitesse moyenne de 0,7°C/s au-dessus de la température T définie plus haut, et à une vitesse moyenne de 0,13°C/s en dessous, conformément à l'invention;
• pour les tôles en acier A, C, E, F, G : refroidissement à une vitesse moyenne de 6°C/s au-dessus de la température T définie plus haut, et à une vitesse moyenne de 1,4°C/s en dessous, conformément à l'invention ;
• pour les tôles en acier H, I, J, données à titre de comparaison : austénitisation 900°C suivie de refroidissement 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.

At near austenitization, the cooling conditions are:

• for steel sheets B and D: cooling at an average speed of 0.7 ° C / s above the temperature T defined above, and at an average speed of 0.13 ° C / s below, in accordance with to the invention;
• for steel sheets A, C, E, F, G: cooling at an average speed of 6 ° C / s above the temperature T defined above, and at an average speed of 1.4 ° C / sec below, according to the invention;
• for the steel sheets H, I, J, data for comparison: austenitization 900 ° C followed by cooling 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 according to the invention have a martensite-bainitic structure containing from 5% to 20% retained austenite, whereas the sheets given for comparison have a completely martensitic structure, that is to say, martensitic and not containing more than 2 or 3% retained austenite. All plates contain carbides.

The comparison of the wear resistances, shows that, with similar hardness and titanium content, the sheets in accordance with the invention have an average coefficient Rus which is greater by 0.5 than that of the sheets according to the prior art. In particular, comparison of Examples A and H which differ essentially in structure (residual austenite content of 10% for A, fully martensitic structure for H) shows the incidence of the presence of residual austenite in the structure. It should be noted that the difference in residual austenite content results both from the difference between the heat treatments and the difference between the silicon contents.

It may further be observed that, all else being substantially equal, the contribution to wear resistance attributable to titanium carbides is significantly greater when their presence is combined with that of residual austenite, according to the invention, only when these carbides are precipitated within a matrix essentially devoid of residual austenite. Thus, for similar differences in titanium content (and therefore in TiC, the carbon being always in excess), the pair of steels F, G (according to the invention) is clearly different from the pair of steels I, J, term of gain of holding brought by the titanium. For F, G, the resistance gain Rus supplied by 0.245% of Ti is 0.46, whereas it is only 0.31 for a difference of 0.265% of Ti in the case of the pair I, J. .

This observation is attributable to the increased crimping effect of the titanium carbides by the surrounding matrix, when it contains residual austenite capable of being transformed into hard martensite with swelling under the effect of abrasive stresses.

Furthermore, the deformation after cooling, without planing, for the steel sheets according to the invention is less than 10 mm / m, and is about 15 mm / m for the steel sheet H.

This results in practice, either the possibility of delivering the products without planing, or the execution of a planing to satisfy a requirement of more severe flatness (for example 5mm / m) but achieved more easily and by introducing less constraints of the makes the original deformation less of the products according to the invention.

Claims (20)

1. Process for manufacturing a part, and for example a plate, made of an abrasion-resistant steel, the chemical composition of which comprises, by weight: $$\mathrm{0.35}\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{\le }\mathrm{0.8}\mathrm{\%}$$

   

   $$\begin{array}{l}0\%\le \mathrm{Si}\le 2\%\\ 0\%\le \mathrm{Al}\le 2\%\end{array}$$

   

   $$\mathrm{0.05}\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{+}\mathrm{Al}\mathrm{\le }\mathrm{2}\mathrm{\%}$$

   

   $$0\%\le \mathrm{Mn}\le 2.5\%$$

   

   $$0\%\le \mathrm{Ni}\le 5\%$$

   

   $$0\%\le \mathrm{Cr}\le 5\%$$

   

   $$0\%\le \mathrm{Mo}\le 0.50\%$$

   

   $$0\%\le \mathrm{W}\le 1.00\%$$

   

   $$\mathrm{0.1}\mathrm{\%}\mathrm{\le }\mathrm{Mo}+\mathrm{W}/2\mathrm{\le }\mathrm{0.50}\mathrm{\%}$$

   

   $$0\%\le \mathrm{B}\le 0.02\%$$

   

   $$0\%\le \mathrm{Ti}\le 2\%$$

   

   $$0\%\le \mathrm{Zr}\le 4\%$$

   

   $$\mathrm{0.05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{+}\mathrm{Zr}/2\mathrm{\le }\mathrm{2}\mathrm{\%}$$

   

   $$0\%\le \mathrm{S}\mathrm{\le }0.15\%$$

   

   $$\mathrm{N}<0.03\%$$

   

   - 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: $$\mathrm{0.1}\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0.55}\mathrm{\%}$$

   

   and $$\mathrm{Ti}+\mathrm{Zr}\mathrm{/}\mathrm{2}\mathrm{-}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{2}\ge \mathrm{0.05}\mathrm{\%}$$

   

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

   

   where K = 0.5 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 - 270×C* - 90×Mn - 37×Ni - 70×Cr - 83×(Mo+W/2), with C* = C - Ti/4 - Zr/8 + 7×N/8, and T-50°C;

   - then the part or the plate is cooled at a core cooling rate V_c < 1150 × th^-1.7 and greater than 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, characterized in that: $$\mathrm{1.05}\mathrm{\times }\mathrm{Mn}\mathrm{+}\mathrm{0.54}\mathrm{\times }\mathrm{Ni}\mathrm{+}\mathrm{0.50}\mathrm{\times }\mathrm{Cr}\mathrm{+}\mathrm{0.3}\mathrm{\times }\mathrm{(}\mathrm{Mo}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}{\mathrm{)}}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}\mathrm{2.}$$

   
3. Process according to Claim 1 or Claim 2, characterized in that: $$\mathrm{C}\mathrm{>}\mathrm{0.45}\mathrm{\%}\mathrm{.}$$

   
4. Process according to any one of Claims 1 to 3, characterized in that: $$\mathrm{Si}\mathrm{+}\mathrm{Al}\mathrm{>}0.5\%\mathrm{.}$$

   
5. Process according to any one of Claims 1 to 4, characterized in that: $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}2\mathrm{>}0.10\%\mathrm{.}$$

   
6. Process according to any one of Claims 1 to 5, characterized in that: $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}2\mathrm{>}0.30\%\mathrm{.}$$

   
7. Process according to any one of Claims 1 to 6, characterized in that, $$\mathrm{C}\mathrm{*}\mathrm{\ge }\mathrm{0.22}\mathrm{\%}\mathrm{.}$$

   
8. Process according to any one of Claims 1 to 7, characterized in that a tempering operation is also carried out at a temperature of 350°C or below.
9. Process according to any one of Claims 1 to 8, 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.
10. Part, and especially a plate, made of an abrasion-resistant steel, the chemical composition of which comprises, by weight: $$\mathrm{0.35}\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{\le }\mathrm{0.8}\mathrm{\%}$$

    

    $$0\%\le \mathrm{Si}\mathrm{\le }2\%$$

    

    $$0\%\le \mathrm{Al}\mathrm{\le }2\%$$

    

    $$\mathrm{0.35}\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{+}\mathrm{Al}\mathrm{\le }\mathrm{2}\mathrm{\%}$$

    

    $$0\%\le \mathrm{Mn}\le 2.5\%$$

    

    $$0\%\le \mathrm{Ni}\le 5\%$$

    

    $$0\%\le \mathrm{Cr}\le 5\%$$

    

    $$0\%\le \mathrm{Mo}\le 0.50\%$$

    

    $$0\%\le \mathrm{W}\le 1.00\%$$

    

    $$\mathrm{0.1}\mathrm{\%}\mathrm{\le }\mathrm{Mo}+\mathrm{W}/2\mathrm{\le }\mathrm{0.50}\mathrm{\%}$$

    

    $$0\%\le \mathrm{B}\le 0.02\%$$

    

    $$0\%\le \mathrm{Ti}\le 2\%$$

    

    $$0\%\le \mathrm{Zr}\le 4\%$$

    

    $$\mathrm{0.05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{+}\mathrm{Zr}/2\mathrm{\le }\mathrm{2}\mathrm{\%}$$

    

    $$0\%\le \mathrm{S}\mathrm{\le }0.15\%$$

    

    $$\mathrm{N}<0.03\%$$

    

    - 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: $$\mathrm{0.1}\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0.55}\mathrm{\%}$$

    

    and $$\mathrm{Ti}+\mathrm{Zr}\mathrm{/}\mathrm{2}\mathrm{-}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{2}\ge \mathrm{0.05}\mathrm{\%}$$

    

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

    

    where K = 0.5 if B ≥ 0.0005% and K = 0 if B < 0.0005%,
    the flatness of which is characterized by a bow of less than 12 mm/m, the steel having a martensitic or martensitic-bainitic structure, said structure also containing 5% to 20% residual austenite and carbides.
11. Part according to Claim 10, characterized in that: $$\mathrm{1.05}\mathrm{\times }\mathrm{Mn}\mathrm{+}\mathrm{0.54}\mathrm{\times }\mathrm{Ni}\mathrm{+}\mathrm{0.50}\mathrm{\times }\mathrm{Cr}\mathrm{+}\mathrm{0.3}\mathrm{\times }\mathrm{(}\mathrm{Mo}\mathrm{+}\mathrm{W}\mathrm{/}\mathrm{2}{\mathrm{)}}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{+}\mathrm{K}\mathrm{>}\mathrm{2.}$$

    
12. Part according to Claim 10 or Claim 11, characterized in that: $$\mathrm{C}\mathrm{>}\mathrm{0.45}\mathrm{\%}\mathrm{.}$$

    
13. Part according to any one of Claims 10 to 12, characterized in that: $$\mathrm{Si}\mathrm{+}\mathrm{Al}\mathrm{>}0.5\%\mathrm{.}$$

    
14. Part according to any one of Claims 10 to 13, characterized in that: $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}2\mathrm{>}0.10\%\mathrm{.}$$

    
15. Part according to any one of Claims 10 to 14, characterized in that: $$\mathrm{Ti}\mathrm{+}\mathrm{Zr}\mathrm{/}2\mathrm{>}0.30\%\mathrm{.}$$

    
16. Part according to any one of Claims 10 to 15, characterized in that: $$\mathrm{C}\mathrm{*}\mathrm{\ge }\mathrm{0.22}\mathrm{\%}\mathrm{.}$$

    
17. Part according to any one of Claims 10 to 16, characterized in that it is a plate with a thickness between 2 mm and 150 mm and the flatness of which is characterized by a bow of less than 12 mm/m.
18. Part according to any one of Claims 10 to 17, characterized in that the Brinell hardness is between 280 and 450 and: $$\mathrm{0.1}\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0.2}\mathrm{\%}\mathrm{.}$$

    
19. Part according to any one of Claims 10 to 17, characterized in that the Brinell hardness is between 380 and 550 and: $$\mathrm{0.2}\mathrm{\%}<\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0.3}\mathrm{\%}\mathrm{.}$$

    
20. Part according to any one of Claims 10 to 17, characterized in that the Brinell hardness is between 450 and 650 and: $$\mathrm{0.3}\mathrm{\%}<\mathrm{C}\mathrm{-}\mathrm{Ti}\mathrm{/}\mathrm{4}\mathrm{-}\mathrm{Zr}\mathrm{/}\mathrm{8}\mathrm{+}\mathrm{7}\mathrm{\times }\mathrm{N}\mathrm{/}\mathrm{8}\mathrm{\le }\mathrm{0.5}\mathrm{\%}\mathrm{.}$$