EP1751321B1 - Steel with high mechanical strength and wear resistance - Google Patents

Description

The present invention relates to a steel with high mechanical strength and high wear resistance.

In many industries, high wear resistance steels are used. Examples are steels intended to manufacture equipment for the mineral industries and which must resist abrasion. They are also steels intended to manufacture tools for cold or medium-hot forming of metal parts and which must withstand wear by metal-to-metal friction. For these tooling applications, at least the steels must retain good properties despite heating at temperatures up to 500 ° C or 600 ° C.

In addition to this resistance to wear, the steels considered here must have suitable properties in order to be machined or welded. They must finally be able to withstand shocks or intense efforts.

In general, in order to obtain all the desired properties, steels containing approximately between 0.3% and 1.5% of carbon, less than 2% of silicon and less than 2% of manganese are used. up to 3% nickel, between 1% and 12% chromium, between 0.5% and 5% molybdenum, with possible addition of vanadium or niobium.

In these steels, the wear resistance results mainly from the hardening caused by the secondary precipitation of molybdenum carbides. This resistance to wear can be improved, if necessary, by the presence of large ldéburitic carbides especially rich in chromium.

The presence of high levels of strong carburizing elements, such as molybdenum and vanadium, which provide a sufficiently hard and temperature-stable secondary precipitation, however, has the disadvantage of causing the formation of strongly segregated veins in these elements and carbon. and, therefore, very hard and very fragile. These segregated veins make machining or welding difficult. In addition, they constitute fragile areas which, even localized, can very significantly reduce the resistance to shocks and intense bending forces parts.

The object of the present invention is to remedy this drawback by proposing a means for obtaining a steel whose properties are equivalent to those known steels, but the harmfulness of segregated veins is significantly reduced.

• éventuellement un ou plusieurs éléments pris parmi le vanadium, le niobium et le tantale en des teneurs telle que $$\mathrm{V}\le \mathrm{1,45}\%,\mathrm{Nb}\le \mathrm{1,45}\%,\mathrm{Ta}\le \mathrm{1,45}\%,\mathrm{et\; V}+\mathrm{Nb}/2+\mathrm{Ta}/4\le \mathrm{1,45}\%,$$
  
• éventuellement jusqu'à 0,1 % de bore,
• éventuellement jusqu'à 0,19 % de soufre, jusqu'à 0,38% de sélénium et jusqu'à 0,76 % de tellure, la somme S + Se/2 + Te/4 restant inférieure ou égale à 0,19 %,
• éventuellement jusqu'à 0,01 % de calcium,
• éventuellement jusqu'à 0,5 % de terres rares
• éventuellement jusqu'à 1 % d'aluminium,
• éventuellement jusqu'à 1 % de cuivre,

le reste étant du fer et des impuretés résultant de l'élaboration. La composition satisfaisant en outre : $$800\le \mathrm{D}\le 1150$$

avec : $$\mathrm{D}=540{\left(\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}\right)}^{\mathrm{0,25}}+245{\left(\mathrm{Mo}+3\mathrm{V}+\mathrm{1,5}\mathrm{Nb}+\mathrm{0,75}\mathrm{<figure-callout\; id="0"\; label="Ta"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Ta</figure-callout>}\right)}^{\mathrm{0,30}}+125{\mathrm{<figure-callout\; id="0"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^{\mathrm{0,20}}+\mathrm{15,8}\mathrm{Mn}+\mathrm{7,4}\mathrm{Ni}+18\mathrm{Si}$$

For this purpose, is described below a way to reduce the harmfulness of segregated veins of a steel with high mechanical strength and high wear resistance, not in accordance with the invention, whose composition comprises by weight : 0.30% ≤ C ≤ 1.42% 0.05% ≤ Yes ≤ 1,5% mn ≤ 1.95% Or ≤ 2.9% 1.1% ≤ Cr ≤ 7.9% 0.61% ≤ MB ≤ 4.4%

• optionally one or more elements selected from vanadium, niobium and tantalum in contents such that $$\mathrm{V}\le 1.45\%,\mathrm{Nb}\le 1.45\%,\mathrm{Your}\le 1.45\%,\mathrm{and\; V}+\mathrm{Nb}/2+\mathrm{Your}/4\le 1.45\%,$$
  
• optionally up to 0.1% boron,
• optionally up to 0.19% sulfur, up to 0.38% selenium and up to 0.76% tellurium, the sum S + Se / 2 + Te / 4 remaining less than or equal to 0.19 %
• optionally up to 0.01% calcium,
• possibly up to 0.5% rare earths
• optionally up to 1% aluminum,
• optionally up to 1% copper,

the rest being iron and impurities resulting from the elaboration. The satisfactory composition furthermore: $$800\le \mathrm{D}\le 1150$$

with: $$\mathrm{D}=540{\left(\mathrm{C}\right)}^{0.25}+245{\left(\mathrm{MB}+3\mathrm{V}+1.5\mathrm{Nb}+0.75\mathrm{Your}\right)}^{0.30}+125{\mathrm{Cr}}^{0.20}+15.8\mathrm{mn}+7.4\mathrm{Or}+18\mathrm{Yes}$$

• on substitue tout ou partie du molybdène par une proportion double de tungstène de telle sorte que la teneur W en tungstène soit supérieure ou égale à 0,21 %,
• et on ajoute du titane et/ou du zirconium destinés à former, essentiellement en cours de solidification, des gros carbures, et un supplément de carbone δC égal à Ti/4 + Zr/8, de sorte que la teneur en carbone après ajustement sera visée égale à C' = C avant ajustement + Ti/4 + Zr/8.

To this end :

• all or part of the molybdenum is replaced by a double proportion of tungsten so that the tungsten content W is greater than or equal to 0.21%,
• and titanium and / or zirconium are added to form, substantially during solidification, large carbides, and addition of carbon δC equal to Ti / 4 + Zr / 8, so that the carbon content after adjustment will be equal to C '= C before adjustment + Ti / 4 + Zr / 8.

$$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)\mathrm{x\; C}\prime \ge \mathrm{0,07}$$

c'est à dire encore, compte tenu de ce que C' = (C + Ti/4 + Zr/8) (où C = teneur en carbone avant ajustement) : $$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)\ge 2\left(-\mathrm{C}+\sqrt{C^2+\mathrm{0,07}}\right)$$

et, $$\mathrm{Ti}+\mathrm{Zr}/2\le \mathrm{1,49}\%$$

The added contents of titanium and / or zirconium will be such that: $$\mathrm{Ti}+\mathrm{Zr}/2\ge 0.2\mathrm{x\; W}$$

$$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)\mathrm{x\; C}\text{'}\ge 0.07$$

that is to say again, considering that C '= (C + Ti / 4 + Zr / 8) (where C = carbon content before adjustment): $$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)\ge 2\left(-\mathrm{C}+\sqrt{{\mathrm{<figure-callout\; id="2"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}}^2+0.07}\right)$$

and, $$\mathrm{Ti}+\mathrm{Zr}/2\le 1.49\%$$

The amount of carbon added δC forming early titanium carbides and / or zirconium, is no longer available and therefore does not intervene in the secondary hardening precipitation of molybdenum carbides, tungsten, vanadium and secondarily chromium. This depends on the free carbon C * after adjustment = C '- Ti / 4 - Zr / 8. As a result, the hardening of the steel is not modified, with the dispersion closely related to the practical dispersions of achievement of steelworks targets. In this respect, it is estimated that the resulting dispersion on the invoice D does not exceed ± 5%, so that it is desired:
0.95 x D before adjustment ≤ D after adjustment ≤ 1.05 x D before adjustment, where D after adjustment = 540 (C'-Ti / 4 - Zr / 8) ^0.25 + 245 (MB after adjustment + W / 2 + 3 V + 1.5 Nb + 0.75 Ta) ^0.30 + 125 Cr ^0.20 + 15.8 Mn + 7.4 Ni + 18 Si.

Preferably, the composition is adjusted so that D after adjustment = D before adjustment.

When the content of chromium is between 2.5 and 3.5%, and if the carbon, titanium and zirconium contents are such that C ≥ 0.51% before adjustment, the contents in W are preferably limited so that, after adjustment,
W ≤ 0.85% if Mo <1.21% and W / Mo ≤ 0.7 if Mo ≥ 1.21%.

$$\mathrm{0,05}\%\le \mathrm{Si}\le \mathrm{1,5}\%,$$

$$\mathrm{Mn}\le \mathrm{1,95}\%,$$

$$\mathrm{Ni}\le \mathrm{2,9}\%,$$

$$\mathrm{1,1}\%\le \mathrm{Cr}\le \mathrm{7,9}\%,$$

$$0\%\le \mathrm{Mo}\le \mathrm{4,29}\%,$$

$$\mathrm{0,21}\%\le \mathrm{W}\le \mathrm{4,9}\%$$

$$\mathrm{0,61}\%\le \mathrm{Mo}+\mathrm{W}/2\le \mathrm{4,4}\%$$

$$0\%\le \mathrm{Ti}\le \mathrm{1,49}\%$$

$$0\%\le \mathrm{Zr}\le \mathrm{2,9}\%$$

$$\mathrm{0,2}\%\le \mathrm{Ti}+\mathrm{Zr}/2\le \mathrm{1,49}\%$$

• éventuellement un ou plusieurs éléments pris parmi le vanadium, le niobium et le tantale, en des teneurs telles que V ≤ 1,45 %, Nb ≤ 1,45 %, Ta ≤ 1,45 % et V + Nb/2 + Ta/4 ≤ 1,45 %,
• éventuellement jusqu'à 0,1 % de bore,
• moins de 0.005% de soufre,
• éventuellement jusqu'à 0,01 % de calcium,
• éventuellement jusqu'à 0,5 % de terres rares,
• éventuellement jusqu'à 1 % d'aluminium,
• éventuellement jusqu'à 1 % de cuivre,

le reste étant du fer et des impuretés résultant de l'élaboration,
la composition satisfaisant les conditions suivantes : $$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)/\mathrm{W}\ge \mathrm{0,20}$$

$$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)\mathrm{x\; C}\ge \mathrm{0,07}$$

0,3 % ≤ C* ≤ 1,42 %, et de préférence ≤ 1,1 % $$800\le \mathrm{D}\le 1150$$

avec $$\mathrm{D}=540{\left(\mathrm{C*}\right)}^{\mathrm{0,25}}+245{\left(\mathrm{Mo}+\mathrm{W}/2+3\mathrm{V}+\mathrm{1,5}\mathrm{Nb}+\mathrm{0,75}\mathrm{<figure-callout\; id="0"\; label="Ta"\; filenames="imgf0001.png"\; state="\{\{state\}\}"><figure-callout\; id="3"\; label="Ta"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Ta</figure-callout></figure-callout>}\right)}^{\mathrm{0,3}}+125{\mathrm{<figure-callout\; id="0"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^{\mathrm{0,20}}+\mathrm{15,8}\mathrm{Mn}+\mathrm{7,4}\mathrm{Ni}+18\mathrm{Si}$$

et $$\mathrm{C}*=\mathrm{C}-\mathrm{Ti}/4-\mathrm{Zr}/8,$$

en outre, si C* ≥ 0,51 %, et si 2,5 % ≤ Cr ≤ 3,5 %, alors W ≤ 0,85 % si Mo < 1,21 %, et W/Mo ≤ 0,7 si Mo ≥ 1,21 %.The invention thus relates to a steel with high mechanical strength and high resistance to wear, possibly obtainable by the process according to the invention, the chemical composition of which comprises, by weight: $$0.35\%\le \mathrm{C}\le 1.47\%,$$

$$0.05\%\le \mathrm{Yes}\le 1.5\%,$$

$$\mathrm{mn}\le 1.95\%,$$

$$\mathrm{Or}\le 2.9\%,$$

$$1.1\%\le \mathrm{Cr}\le 7.9\%,$$

$$0\%\le \mathrm{MB}\le 4.29\%,$$

$$0.21\%\le \mathrm{W}\le 4.9\%$$

$$0.61\%\le \mathrm{MB}+\mathrm{W}/2\le 4.4\%$$

$$0\%\le \mathrm{Ti}\le 1.49\%$$

$$0\%\le \mathrm{Zr}\le 2.9\%$$

$$0.2\%\le \mathrm{Ti}+\mathrm{Zr}/2\le 1.49\%$$

• optionally one or more elements selected from vanadium, niobium and tantalum, in such quantities as V ≤ 1.45%, Nb ≤ 1.45%, Ta ≤ 1.45% and V + Nb / 2 + Ta / 4 ≤ 1.45%,
• optionally up to 0.1% boron,
• less than 0.005% sulfur,
• optionally up to 0.01% calcium,
• possibly up to 0.5% rare earths,
• optionally up to 1% aluminum,
• optionally up to 1% copper,

the rest being iron and impurities resulting from the elaboration,
the composition satisfying the following conditions: $$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)/\mathrm{W}\ge 0.20$$

$$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)\mathrm{x\; C}\ge 0.07$$

0.3% ≤ C * ≤ 1.42%, and preferably ≤ 1.1% $$800\le \mathrm{D}\le 1150$$

with $$\mathrm{D}=540{\left(\mathrm{C\; *}\right)}^{0.25}+245{\left(\mathrm{MB}+\mathrm{W}/2+3\mathrm{V}+1.5\mathrm{Nb}+0.75\mathrm{Your}\right)}^{0.3}+125{\mathrm{Cr}}^{0.20}+15.8\mathrm{mn}+7.4\mathrm{Or}+18\mathrm{Yes}$$

and $$\mathrm{C}*=\mathrm{C}-\mathrm{Ti}/4-\mathrm{Zr}/8,$$

in addition, if C * ≥ 0.51%, and if 2.5% ≤ Cr ≤ 3.5%, then W ≤ 0.85% if Mo <1.21%, and W / Mo ≤ 0.7 if Mo ≥ 1.21%.

• Si < 0,45 %, si l'on souhaite privilégier la conductivité thermique, ou
• Si ≥ 0,45 % si on souhaite privilégier l'aptitude au travail à chaud, ou encore :
  • Mo + W/2 ≥ 2,2 % pour augmenter la résistance à l'adoucissement de l'acier et lui conférer une résistance élevée ;
  • Cr ≥ 3,5 % pour contribuer à la fois à la trempabilité et au durcissement ;
  • C ≤ 0,85 % si on veut privilégier la ténacité,
    ou
  • C > 0,85 % si on veut obtenir une tenue à l'usure la plus élevée possible.

Preferably, the steel may further satisfy one or more of the following conditions:

• If <0.45%, if one wishes to favor thermal conductivity, or
• If ≥ 0.45% if one wishes to privilege the ability to work when hot, or:
  • Mo + W / 2 ≥ 2.2% to increase the softening resistance of steel and give it high strength;
  • Cr ≥ 3.5% to contribute to both hardenability and hardening;
  • C ≤ 0.85% if we want to favor toughness,
    or
  • C> 0.85% if we want to obtain the highest wear resistance possible.

afin de privilégier la ténacité,
ou tel que : $$\mathrm{Ti}+\mathrm{Zr}/2\ge \mathrm{0,7}\%$$

afin de privilégier la résistance à l'usure.In addition, the steel may be such that: $$\mathrm{Ti}+\mathrm{Zr}/2<0.7\%$$

to favor tenacity,
or as: $$\mathrm{Ti}+\mathrm{Zr}/2\ge 0.7\%$$

in order to favor the resistance to wear.

• on élabore un acier liquide ayant la composition souhaitée en ajustant les teneurs en titane et/ou en zirconium dans le bain d'acier fondu, préférentiellement en évitant à tout instant les sur-concentrations locales en titane et/ou zirconium dans le bain d'acier fondu,
• on coule ledit acier pour obtenir un demi-produit ;
• puis on soumet ledit demi-produit à un traitement de mise en forme par déformation plastique à chaud et, éventuellement, à un traitement thermique, pour obtenir ladite pièce.

The invention also relates to a method for manufacturing a steel part according to the invention, according to which:

• a liquid steel having the desired composition is produced by adjusting the contents of titanium and / or zirconium in the molten steel bath, preferably by avoiding at any time the local over-concentrations of titanium and / or zirconium in the bath of molten steel,
• said steel is cast to obtain a semi-finished product;
• then subjecting said semi-product to a forming treatment by hot plastic deformation and, optionally, a heat treatment, to obtain said part.

Preferably, in order to limit the transient overconcentrations in the liquid bath, the addition of titanium and / or zirconium is made by gradually adding titanium and / or zirconium to a slag covering the liquid steel bath and allowing the titanium and / or zirconium to slowly diffuse into the liquid steel bath.

The addition of titanium and / or zirconium can also be carried out by introducing a wire comprising titanium and / or zirconium into the bath of liquid steel, while stirring the bath.

The invention finally relates to a steel piece according to the invention that can be obtained by the manufacturing method according to the invention.

The invention will now be described in more detail but in a nonlimiting manner and illustrated by examples and the single figure which represents the rate of segregation of tungsten as a function of the ratio (Ti + Zr / 2) / W for different steels.

It is known that tungsten is an alloying element whose effects on the properties of steel are comparable to those of molybdenum. In particular, it is known that tungsten has effects of hardening and resistance to thermal softening comparable to those of molybdenum in the proportion of two parts of tungsten to one part molybdenum. However, tungsten is little used, except in some high-alloy steels not concerned by the present invention, and this, in particular, because it is much more expensive than molybdenum. In addition, tungsten, like molybdenum, has the disadvantage of segregating very strongly and giving rise to segregated veins very hard and very fragile.

However, the inventors have found, in a new and surprising way, that in the presence of sufficient quantities of titanium or zirconium, the segregation of tungsten is very substantially attenuated; effect particularly interesting to exploit when, in addition, the molybdenum content is already also relatively high.

• les éléments tels que le molybdène et le tungstène forment des carbures sous forme de fins précipités qui durcissent la matrice et ainsi permettent d'obtenir la dureté souhaitée pour l'acier. Les veines ségrégées, qui se caractérisent notamment par des sur-concentrations en molybdène ou en tungstène, présentent donc une forte augmentation de la densité de précipités durcissant et donc une forte augmentation locale de dureté et de fragilité.
• Le titane ou le zirconium forment également des carbures. Mais ces carbures sont relativement gros, et par conséquent, comparativement peu nombreux et n'ont pas d'effet durcissant notable sur la matrice métallique elle-même.
• les inventeurs ont constaté de façon nouvelle et inattendue que, lorsque l'acier contient simultanément du titane et/ou du zirconium d'une part, et du tungstène d'autre part, le tungstène a tendance à précipiter conjointement avec le titane et/ou le zirconium pour former les gros précipités non durcissant.

An hypothesis likely to shed a posteriori light on this unexpected result could be the following:

• elements such as molybdenum and tungsten form carbides in the form of fine precipitates which harden the matrix and thus make it possible to obtain the desired hardness for the steel. The segregated veins, which are particularly characterized by over-concentrations of molybdenum or tungsten, therefore have a sharp increase in the density of hardening precipitates and therefore a strong local increase in hardness and brittleness.
• Titanium or zirconium also form carbides. But these carbides are relatively large, and therefore comparatively few in number and have no noticeable hardening effect on the metal matrix itself.
• the inventors have found in a new and unexpected way that, when the steel simultaneously contains titanium and / or zirconium on the one hand, and tungsten on the other hand, tungsten tends to precipitate together with titanium and / or zirconium to form large non-hardening precipitates.

Thus, in view of these observations, it may be thought that, in the presence of titanium and / or zirconium, the tungsten content and thus the density of fine carbide-hardening precipitates is decreased, and more particularly in the segregated veins. where large carbides of titanium or zirconium are much more numerous, by the very fact of segregation. It follows that the difference in hardness between the segregated veins and the non-segregated zones would thus be substantially attenuated, and the harmfulness of the segregated veins (in particular presence of zones of increased fragility, machining difficulties, heterogeneous response to polishing and grinding, reloading by welding ...) would be reduced.

On the basis of these observations and the hypothesis which has just been formulated, the inventors have devised a way of substantially reducing the disadvantages of segregated veins of steels containing a significant proportion of molybdenum, while retaining all the properties of essential use of the steel in question.

This reduction applies to a steel, not according to the invention, which, before adjustment, contains mainly from 0.30% to 1.42% carbon, from 0.05% to 1.5% silicon, less than 1.95% manganese, less than 2.9% nickel, 1.1% to 7.9% chromium, 0.61% to 4.4% molybdenum, optionally up to 1.45% vanadium, up to 1.45% niobium, less than 1.45% tantalum with V + Nb / 2 + Ta / 4 ≤ 1.45%. This steel has a hardness index D, which will be explained later, between 800 and 1150. It may contain, in addition, up to 0.1% of boron, up to 0.19% of sulfur, up to to 0.38% selenium, up to 0.79% tellurium, the sum S + Se / 2 + Te / 4 remaining less than 0.19%, optionally up to 0.01% calcium, up to at 0.5% rare earths, up to 1% aluminum and up to 1% copper.

All or part of the molybdenum is replaced by a substantially double proportion of tungsten, titanium and / or zirconium are added so as to obtain sufficient quantities of titanium and / or zirconium taking into account the amounts of tungsten introduced into the steel, and adjusting the carbon content so that, in particular, the hardness of the steel remains substantially unchanged.

For this, for example by using the formula for calculating the hardness index D which will be explained later or by any other means known to those skilled in the art, we choose the target composition for steel without tungsten so to obtain the desired job characteristics, in particular the level of hardness. The target composition is then modified by choosing a tungsten content, thereby adjusting the molybdenum content and the titanium or zirconium and carbon contents, so that at least one of the main use characteristics, in particular the hardness, remains substantially unchanged. Then, a steel is developed corresponding to the modified analysis. By "substantially unchanged" is meant, for example, that the hardness of the steel after adjustment of the composition is equal to the hardness of the steel before adjustment of the composition to within 5%. This tolerance is introduced to take into account the practical difficulties of producing a steel with precisely defined properties. However, it is desirable that the characteristics obtained are as close as possible to the characteristics targeted for the steel before adjustment of the composition. Also, it is preferable that the tolerance is only 2%, and, insofar as one is interested only in the targeted characteristics, it is still more preferable that the characteristic of hardness aimed after adjustment of the composition is equal to the characteristic of target hardness before adjustment of the composition.

The amount of tungsten added must be greater than or equal to 0.21%, preferably greater than 0.4%, more preferably greater than 0.7%, and more preferably greater than 1.05%. Indeed, the greater the substitution of molybdenum by tungsten, the greater the effect on segregations. However, this effect depends on the titanium or zirconium contents, which generally leads to further limiting the maximum addition of tungsten.

To obtain the desired effect on the segregations, the titanium and zirconium contents must be such that the sum Ti + Zr / 2 is greater than or equal to 0.2 × W, preferably greater than or equal to 0.4 × W, more preferably greater than or equal to 0.6 x W. However, for reasons which will be explained later, it is undesirable to excessively increase the titanium or zirconium contents. This leads indirectly to limiting tungsten additions to 4.9% maximum. In general, the tungsten content remains below 2.9%, better still 1.9% or even less than or equal to 0.85%, or even 0.49%.

ou mieux : $$\mathrm{0,98}\mathrm{x\; D\; avant\; ajustement}\le \mathrm{D\; après\; ajustement}\le \mathrm{1,02}\mathrm{x\; D\; avant\; ajustement}$$

ou mieux encore : $$\mathrm{D\; après\; ajustement}=\mathrm{D\; avant\; ajustement}.$$

In addition, depending on the contents of titanium and / or zirconium, the carbon content must be adjusted so that the free carbon content C * = C '- Ti / 4 - Zr / 8 remains substantially constant, that is to say that is, so that the free carbon content C * after adjustment of the composition is substantially equal to the carbon content C before adjustment of the composition (in this formula, C 'represents the carbon content of the steel after adjusting the the composition). This condition is necessary to maintain substantially constant hardness and resistance to thermal softening of the steel. D being the index of hardness that will be defined later, we aim to have: $$0.95\mathrm{x\; D\; before\; adjustment}\le \mathrm{D\; after\; adjustment}\le 1.05\mathrm{x\; D\; before\; adjustment}$$

or better : $$0.98\mathrm{x\; D\; before\; adjustment}\le \mathrm{D\; after\; adjustment}\le 1.02\mathrm{x\; D\; before\; adjustment}$$

or better yet: $$\mathrm{D\; after\; adjustment}=\mathrm{D\; before\; adjustment}.$$

• le choix de la teneur en tungstène à substituer à une part moitié de molybdène, en fonction du degré minimal de réduction souhaité de la ségrégation (les tableaux 2, 3, 4 ou la figure peuvent constituer des guides à cet égard) ;
• le choix de la teneur en Ti et/ou Zr, plus ou moins élevée selon que l'on privilégie respectivement la tenue à l'usure ou la ténacité et qui doit par ailleurs être suffisante par rapport à l'addition de tungstène puisqu'il faut que (Ti + Zr/2) ≥ 0,2 W .
• la détermination de l'accroissement de carbone à viser en fonction des teneurs précédentes, à savoir, δC = Ti/4 + Zr/8.

In practice, the procedure for choosing the contents to be adjusted comprises:

• the choice of the tungsten content to substitute for a half of molybdenum, depending on the desired minimum degree of reduction of segregation (Tables 2, 3, 4 or Figure may provide guidance in this regard);
• the choice of the content of Ti and / or Zr, more or less high, depending on whether the wear resistance or the tenacity is preferred, and which must also be sufficient in relation to the addition of tungsten, since must (Ti + Zr / 2) ≥ 0.2 W.
• the determination of the increase of carbon to be targeted according to the preceding contents, namely, δC = Ti / 4 + Zr / 8.

We will now describe the steel according to the invention, obtainable by the process according to the invention and which has the advantage of having segregated veins less harmful than those of the same hardness steel, in accordance with the invention. prior art.

The steel according to the invention contains at least 0.35% carbon, preferably more than 0.51%, and better still more than 0.65%, in order to form enough carbides and reach the level of hardness that it is desired to obtain, but up to 1.47% and preferably less than 1.1% and more preferably less than 0.98% in order to avoid too much embrittlement of the steel. As noted above, the steel contains titanium and zirconium, and these elements combine at high temperatures with carbon to form primary carbides. Thus, after formation of the primary carbides of titanium and zirconium, the carbon called "free" which remains available to act on the properties of the matrix is free carbon, not combined with titanium and zirconium. This amount of carbon not combined with titanium and zirconium, designated C *, is such that: C * = C - Ti / 4 - Zr / 8 (C, Ti and Zr being the contents of carbon steel, titanium and zinconium, respectively, in the following C will also be called "total carbon content"). This quantity of available carbon must be sufficient to allow the precipitation of secondary carbides and in particular carbides of tungsten, molybdenum or other elements which are added to the steel, and from this point of view, this carbon content free C * must be greater than or equal to 0.3%. However, this content should not exceed 1.42%, and preferably 1.1% or better 0.98%, or more preferably 0.79%, not to excessively impair the toughness of the matrix itself.

In addition, it may be desirable to limit the content maximum total carbon C at 0.85%, or better still 0.79%, to facilitate manufacturing operations, in particular to reduce the precautions to be taken for the cooling of ingots or slabs; it is then preferable that the free carbon content C * remains less than 0.60% or even 0.50%. On the contrary, it may be desirable to choose a total carbon content C greater than 0.85% in order to improve the mechanical strength and the wear resistance of the steel. This choice is made on a case by case basis, depending on the use that is envisaged for steel.

Steel contains at least 0.05% silicon, as this element is a deoxidant. In addition, it contributes a bit to the hardening of steel. However, the silicon content must remain less than or equal to 1.5% and preferably less than or equal to 1.1%, better still 0.9%, and better still, less than or equal to 0.6%, in order to avoid excessively weaken the steel and too much reduce its ability to plastic deformation hot, for example by rolling. In addition, it may be desirable to impose a minimum silicon content of 0.45%, and better still 0.6%, in order to improve the machinability of the steel and also to improve the strength of the steel. oxidation. The improvement in oxidation resistance is particularly desirable when the steel is used to manufacture workpieces intended to work at relatively high temperatures of the order of 450 ° C to 600 ° C, which requires resistance to oxidation. sufficient softening. However, when it is desired to obtain sufficient softening resistance for such working conditions, it is desirable for the content of Mo + W / 2 to be greater than or equal to 2.2%. As a result, the minimum values of silicon content, 0.45% or better still 0.6%, are more particularly advantageous when the contents of molybdenum and tungsten are such that the sum Mo + W / 2 is greater than or equal to 2 , 2%, without this nevertheless having an exclusive character. However, for some applications, it is desirable that the thermal conductivity of the steel be as great as possible. In this case, it is desirable that the silicon content remains below 0.45%, and preferably, as low as possible.

The steel contains manganese up to 1.95% by weight in order to improve the hardenability of the steel, but this content should preferably remain less than or equal to 1.5% and better still less than or equal to 0, 9% to limit segregations that would lead to bad forgeability and insufficient toughness. It should be noted that the steel always contains a small amount of manganese, a few tenths of a percent, in particular to fix the sulfur and it is preferable that the Mn content is at least 0.4%.

Steel contains up to 2.9% nickel to adjust quenchability and improve toughness. But this element, is very expensive. Also, one does not generally seek a nickel content exceeding 0.9% or even 0.7%. Steel may not contain nickel but when nickel is not added voluntarily, it is interesting that steel contains up to 0.2% or even up to 0.4% in the form of residuals resulting from development.

The steel contains at least 1.1% chromium and better still more than 2.1%, and more preferably more than 3.1% and even more than 3.5%, to obtain sufficient quenchability and increase hardening to income, but up to 7.9%, and better still less than 5.9% or better still, less than 4.9% in order not to hinder the formation of secondary carbides, in particular containing Mo and / or W and, as such, more effective than the hardening chromium carbides.

These secondary carbides (ie formed during cooling after re-austenitization and especially at the time of income) are much thinner and more numerous than ldeturitic carbides (possibly obtained at the end of solidification). They thus contribute strongly to the hardening of the metal matrix after income. They are also useful for reinforcing the wear resistance of the matrix, thus limiting the risk of loosening of the very hard carbides of titanium and / or zirconium, which themselves make a strong complementary contribution to the wear resistance. steel.

Within this chromium content domain, it is desirable to distinguish two preferred subdomains. Indeed, when the chromium content is sufficiently high, this element tends to form, especially in the segregated veins, type ledeburitic carbides that are coarse and more or less arranged in inter-dendritic networks. These carbides, despite a certain favorable effect on the wear resistance, contribute mainly to an embrittlement at least local matrix. So that when one wishes to privilege the hardness and the resistance to wear at the expense of toughness, it is desirable to choose a chromium content greater than or equal to 3.5%, favoring the presence of thedeburitic type carbides. On the other hand, when seeking to promote the toughness of steel by accepting a slight reduction in wear resistance, it is preferable to choose a chromium content of less than or equal to 2.5%. However, in the intermediate range of 2.5 to 3.5% chromium, it is still possible to favor the toughness either by limiting the free carbon content to less than 0.51%, or by limiting the tungsten content or the tungsten molybdenum ratio, because tungsten, because of its propensity to form carbides more stable in temperature than those of molybdenum, tends to favor the formation of chromium ledeburitic carbides preferentially allying with them.

The molybdenum and tungsten contents of the steel must be such that the sum Mo + W / 2 is greater than or equal to 0.61%, preferably greater than or equal to 1.1%, and better still greater than or equal to 1, 6%. It is even desirable for this content to be greater than 2.2% in order to obtain a high degree of hardening and a better resistance to thermal softening, in particular when the use of the steel causes it to be heated up. at temperatures that may exceed about 450 ° C. This is the case, for example, of the steels used to produce mid-hot work tools for steel. In this case, the sum Mo + W / 2 can go up to 2.9%, even 3.4%, or even 3.9%, depending on the desired hardness and the temperature of income that one wishes to achieve on rooms. To achieve a very high level of resistance to wear of the matrix and minimize the effect of sap and thus delay the maximum removal of large carbides Ti and Zr, Mo + W / 2 can even go up to at 4.4%.

The interest attached to increasing the content of (Mo + W / 2), that is to say still to the molybdenum content before application of the process, makes it all the more interesting to take into account. Consideration since the segregation of Mo carburigenes, excluding the application of the process, will increase with the contents of these elements.

In the context defined above for the combined contents in Mo + W / 2, the tungsten content will be at least 0.21%, preferably at least 0.41%, better still at least 0.61%, in order to draw the best of the specific effect of tungsten.

The tungsten content depends on the desired degree of segregation deleteriousness reduction, as discussed above, and can also incorporate the cost of the alloy. This content may be up to 4.9% but will not usually exceed 1.9%; In general, content is lower than or equal to 0.90% or even 0.79%.

The molybdenum content may be at trace level, but preferably at least 0.51% and more preferably at least 1.4%; better still, at least 2.05%. On the other hand, depending on the target level of resistance, it will not be necessary to exceed the limiting levels of 4.29%, preferably 3.4% or, better, 2.9%, limitations which also allow to reduce accordingly the contributions of molybdenum to hardening segregation.

However, when the chromium content is between about 2.5% and 3.5%, and when the free carbon content, C * = C - Ti / 4 - Zr / 8, is greater than or equal to 0.51 %, too high tungsten content can lead to the formation of chromium carbides more or less alloyed with tungsten. These carbides, ldeburitic type, coarse and more or less arranged in interdental networks, contribute to an embrittlement at least local matrix. In order to avoid this disadvantage, when the chromium content is between 2.5% and 3.5%, and the free carbon content C * greater than or equal to 0.51%, then the tungsten content is limited to not more than 0.85% when the molybdenum content is less than 1.21%, and the tungsten / molybdenum ratio is limited to not more than 0.7 when the molybdenum content is greater than or equal to 1.21%.

The contents of titanium and zirconium must be adjusted so that the sum Ti + Zr / 2 is at least 0.21% and preferably greater than or equal to 0.41% or better, greater than or equal to 0, 61%, to obtain the desired effect of reducing the harmfulness of segregated veins. In addition, these elements contribute to the formation of large carbides that improve wear resistance. However, this sum must remain less than 1.49% and preferably less than 1.19% or even less than 0.99% or even less than 0.79% so as not to deteriorate the toughness too much. In addition, the titanium and zirconium contents must be adjusted according to whether it is desired to favor the tenacity of the steel or its resistance to wear. From this point of view, when we wish to privilege toughness of the steel, the sum Ti + Zr / 2 should preferably remain below 0.7%. When it is desired to favor the wear resistance of the steel, the sum Ti + Zr / 2 must preferably be greater than or equal to 0.7%. Finally, to be effective, that is to say lead to the formation of large carbides, the titanium and zirconium contents must be sufficient with respect to the total carbon content C. For this, the product (Ti + Zr / 2) x C must be greater than or equal to 0.07, preferably greater than or equal to 0.12, and better still greater than or equal to 0.2.

In order to contribute to compliance with the content ranges indicated for Ti + Zr / 2, the minimum titanium content may be 0%, or traces, but it is preferable that it be at least 0.21%, and better 0.41%, better still, 0.61%; the minimum zirconium content may be 0%, or traces, but it is preferable that it be at least 0.06%, or better still at least 0.11%. The maximum content of titanium is 1.49% but can be reduced to 1.19%, or even 0.99%, better to 0.79% or even 0.7%, while the maximum content of Zirconium is 2.9%, preferably 0.9%, more preferably 0.49%.

The steel optionally contains up to 1.45% of vanadium, up to 1.45% of niobium, up to 1.45% of tantalum, the sum V + Nb / 2 + Ta / 4 being less than 1, 45%, better below 0.95% and even below 0.45%. The minimum content is 0% or traces, but it is preferable that it be at least 0.11%, and more preferably at least 0.21%. The level of addition of V + Nb / 2 + Ta / 4 helps to determine resistance and income response as indicated in the D index formulation.

These elements have the advantage of greatly improving the resistance to softening by the precipitation of carbides type MC. Among these elements, it is preferable to choose vanadium and to add it in contents of between 0.11% and 0.95%. Niobium, although it can be used, has the disadvantage of precipitating at a higher temperature than vanadium, which greatly reduces the forgeability of the steel. Therefore the presence of niobium is not recommended and, in any case, it is desirable that the niobium content remains less than 1% or even 0.5% or, better still, less than 0.05%.

A sulfur content of less than 0.005% is preferable when looking for good toughness.

The steel optionally contains up to 0.5% rare earths to facilitate the germination of the carbides and refine the structure, and possibly up to 0.1% boron to improve the quenchability.

Steel can also contain up to 1% copper. This element is not desired but can be provided by raw materials that would be too expensive to sort. Nevertheless, the copper content must be limited because this element has an adverse effect on hot ductility. In this respect, the presence of Ni in a content at least equal to that of copper is desired, at least when the copper content exceeds about 0.5%. Indeed, a sufficient content of nickel mitigates the harmfulness of copper.

In the same way, the steel may contain aluminum which, like silicon, can contribute to the deoxidation of the liquid metal. The aluminum content will be at trace level or better, at least equal to 0.006%, more preferably at least 0.020%. On the other hand, the content of this element must remain less than or equal to 1% to ensure sufficient cleanliness, and preferably will not exceed 0.100%, better still less than 0.050%.

The rest of the composition consists of iron and impurities resulting from the elaboration. Note that, when an element is not added voluntarily during the elaboration, its content is 0% or traces, that is to say corresponding, depending on the element, either to the limits of detection by the methods of analysis are the quantities brought by the raw materials without there being a significant effect on the properties.

The hardening obtained during the income of this steel depends on the elements dissolved in the matrix, such as manganese, nickel and silicon but above all elements likely to form carbides such as molybdenum, tungsten, vanadium and niobium. and, to a lesser extent, chromium as well as free carbon in the matrix, i.e. carbon which has not been fixed by titanium and zirconium. As indicated above, the free carbon content is C * = C - Ti / 4 - Zr / 8.

The inventors have found that the hardening of this steel can be evaluated according to the chemical composition by means of the formula: $$\mathrm{D}=540{\left(\mathrm{C\; *}\right)}^{0.25}+245{\left(\mathrm{MB}+\mathrm{W}/2+3\mathrm{V}+1.5\mathrm{Nb}+0.75\mathrm{Your}\right)}^{0.30}+125{\mathrm{x\; Cr}}^{0.20}+15.8\mathrm{x\; Mn}+7.4\mathrm{x\; Ni}+18\mathrm{x\; Si.}$$

D is a hardness index which represents the hardening resulting from income for standard income conditions (550 ° C for 1 hour). The higher the value of D is, the higher the hardness after the temperature is determined at high temperature, or the higher the temperature that makes it possible to reach a given hardness level.

Moreover, with a given value of D, the hardness varies as a function of temperature and tempering time as is known to those skilled in the art.

It will be noted that this formula applies equally well to the steel according to the invention or the steel obtained by the process according to the invention. In all cases, the grades to be taken into account are the actual grades of the steel for which the calculation is made.

• entre 800 et 900
• entre 901 et 950
• entre 951 et 1000
• entre 1001 et 1075
• entre 1076 et 1150

In general, the coefficient D is between 800 and 1150. However, this interval can be decomposed into sub-intervals according to the level of hardness desired by the user and the expected tempering temperature. In particular the value of D will be in the following intervals:

• between 800 and 900
• between 901 and 950
• between 951 and 1000
• between 1001 and 1075
• between 1076 and 1150

In these ranges, the typical hardness levels obtained after returning to 550 ° C for one hour are, as an indication, respectively of the order of: 45HRC, 52 HRC, 57 HRC, 60 HRC and 63 HRC.

$$\mathrm{0,21}\%\le \mathrm{Ti}\le \mathrm{1,19}\%$$

Zr : 0 % ou traces $$\mathrm{0,05}\%\le \mathrm{Si}\le \mathrm{0,9}\%$$

$$\mathrm{Mn}\le \mathrm{0,9}\%$$

$$\mathrm{Ni}\le \mathrm{0,9}\%$$

$$\mathrm{2,1}\%\le \mathrm{Cr}\le \mathrm{4,9}\%$$

$$\mathrm{2,05}\%\le \mathrm{Mo}\le \mathrm{2,9}\%$$

$$\mathrm{0,21}\%\le \mathrm{W}\le \mathrm{0,79}\%$$

$$\mathrm{0,21}\%\le \mathrm{V}\le \mathrm{0,45}\%$$

Nb : 0% ou tracesTaking into account all the conditions indicated previously, one can choose a preferred domain of composition defined as follows, for the steel according to the invention: $$0.55\le \mathrm{C}\le 1.1\%$$

$$0.21\%\le \mathrm{Ti}\le 1.19\%$$

Zr: 0% or traces $$0.05\%\le \mathrm{Yes}\le 0.9\%$$

$$\mathrm{mn}\le 0.9\%$$

$$\mathrm{Or}\le 0.9\%$$

$$2.1\%\le \mathrm{Cr}\le 4.9\%$$

$$2.05\%\le \mathrm{MB}\le 2.9\%$$

$$0.21\%\le \mathrm{W}\le 0.79\%$$

$$0.21\%\le \mathrm{V}\le 0.45\%$$

Nb: 0% or traces

Within this area, it is possible to identify sub-domains, or groups, defined by the ranges of carbon and titanium content and which correspond to the fact that the toughness or wear resistance is more or less preferred. .

These groups are:

Group A :

$$0.85\%\le \mathrm{C}\le 1.1\%$$

$$0.70\%\le \mathrm{Ti}\le 1.19\%$$

$$\mathrm{0,61}\%\le \mathrm{Ti}\le \mathrm{0,99}\%$$

Group B : $$0.65\%\le \mathrm{C}\le 1.1\%$$

$$0.61\%\le \mathrm{Ti}\le 0.99\%$$

$$\mathrm{0,41}\%\le \mathrm{Ti}\le \mathrm{0,79}\%$$

Group C : $$0.65\%\le \mathrm{C}\le 0.98\%$$

$$0.41\%\le \mathrm{Ti}\le 0.79\%$$

$$\mathrm{0,21}\%\le \mathrm{Ti}\le \mathrm{0,70}\%$$

Group D : $$0.51\%\le \mathrm{C}\le 0.85\%$$

$$0.21\%\le \mathrm{Ti}\le 0.70\%$$

Within each of these groups, the hardness level can be adjusted by taking into account the influences of the various alloying elements indicated by the expression of the hardness index D.

At a given hardness level, the different groups, in the order A, B, C, and D, are in the direction of a reinforcement of the level of toughness at the cost of a reduction in wear resistance.

A particularly interesting embodiment, corresponding to a preferred choice in favor of toughness, consists of adjusting the composition in order to obtain:
W = 0.2 to 0.9% and (Ti + Zr / 2) at least equal to 0.35% but less than 0.49%, with (Mo + W / 2 + 3 V + 1.5 Nb + 0.75 Ta) between 2.5%, better 3.0% in minimum values, and 4.5%, better 3.5% in values maximum, the free carbon C * being also between 0.51% and 1%, better, between 0.6% and 0.9%.

Another particularly interesting embodiment, corresponding to a preferential choice in favor of wear resistance, consists in adjusting the composition so as to obtain:
W = 0.2 to 0.9% and (Ti + Zr / 2) at least equal to 0.49% but less than 0.95%, with (Mo + W / 2 + 3 V + 1.5 Nb + 0,75 Ta) between 2,5%, better 3,0% in minimum values, and 4,5%, better 3,5% in maximum values, the free carbon C * being between 0,51% and 1%, better, between 0.6% and 0.9%.

According to the present invention, it is desirable for titanium and zirconium to be in the form of primary carbides and not in the form of nitrides which are likely to form in the molten steel, especially when the transient overconcentrations of titanium and zirconium in the liquid just after the addition are too high given the levels of dissolved nitrogen that still exist in the liquid steel.

Also, in order to develop the steel according to the invention, it is possible to introduce titanium and zirconium in such a way that these two elements react little with nitrogen and react essentially with carbon. This is achieved by avoiding, in the liquid phase of the steel, the transient over-concentrations of Ti or Zr during the additions of Ti and Zr.

• tout d'abord, on élabore un acier liquide par fusion de l'ensemble des éléments de la nuance selon l'invention, à l'exception du titane et/ou du zirconium,
• puis on ajoute au bain d'acier fondu le titane et le zirconium en évitant à tout instant les surconcentrations locales en titane et/ou en zirconium dans le bain d'acier fondu.

To manufacture a steel part according to the invention, one can then proceed as follows:

• firstly, a liquid steel is produced by melting all the elements of the grade according to the invention, with the exception of titanium and / or zirconium,
• then titanium and zirconium are added to the molten steel bath, avoiding at any time the local overconcentrations of titanium and / or zirconium in the molten steel bath.

Then a steel is poured in the form of a semi-finished product such as an ingot or a slab, it is shaped by hot plastic deformation and for example by rolling the semi-finished product, and then the product obtained is subjected to a possible heat treatment. .

• soit ajouter du titane et/ou du zirconium dans le laitier couvrant le bain d'acier liquide, en laissant le titane et le zirconium diffuser lentement dans le bain d'acier.
• soit ajouter du titane et/ou du zirconium de façon continue par l'intermédiaire d'un fil composé de cet ou de ces éléments tout en agitant le bain d'acier liquide par du gaz ou par tout autre procédé adapté.
• soit ajouter le titane et/ou le zirconium en soufflant une poudre contenant cet ou ces éléments dans le bain d'acide liquide tout en agitant le bain par du gaz ou par tout autre procédé.

In order to introduce titanium and zirconium into the liquid steel, avoiding any local overconcentration, it is possible to proceed in various ways, and in particular:

• either add titanium and / or zirconium to the slag covering the molten steel bath, allowing titanium and zirconium to slowly diffuse into the steel bath.
• either adding titanium and / or zirconium continuously via a wire composed of this or these elements while stirring the liquid steel bath with gas or by any other suitable method.
• either adding titanium and / or zirconium by blowing a powder containing this or these elements into the liquid acid bath while stirring the bath with gas or by any other method.

In the context of the present invention, it is preferred to use the various embodiments which have just been described. But it is understood that any method to avoid local overconcentration of titanium and / or zirconium may be used.

This particular addition procedure of Ti and Zr is however not necessary for the preparation of the steel considered here but is an option.

The heat treatments to which the manufactured part can be subjected are of conventional type for tool steels. Such a heat treatment may optionally comprise one or more anneals to facilitate cutting and machining, then austenitization followed by cooling in a mode adapted to the thickness, such as air or oil cooling. possibly followed by one or more income depending on the level of hardness you want to achieve.

By the method just described, we obtain steel parts having the same main features of use as the steel parts according to the prior art. But these pieces have veins segregated very attenuated compared to those observed on the parts according to the prior art. As a result, these parts are easier to machine or weld and tougher than the parts according to the prior art.

By way of example, and to illustrate the synergistic effect between tungsten and titanium or zirconium, it is possible to produce parts in steels whose nominal compositions are given in Table 1. This table indicates the chemical compositions, the value of the index D of hardness and a segregation index Γ s accounting for the cumulative segregation and embrittlement segregation of Molybdenum and Tungsten in the segregated veins, which may create the secondary hardening. For this purpose, the contents of molybdenum and tungsten in (Mos and Ws) and out (Moh and Wh) segregated veins were measured by means of a microprobe by masking the large titanium carbides in order to take into account the levels of Molybdenum and Tungsten in the matrix, apart from what can be fixed in these large carbides of titanium and zinconium (which are themselves likely to contain molybdenum or tungsten, forming in fact mixed carbides (Ti Zr Mo W) C ). In this way we appreciate the hardening and weakening part of Mo and W with respect to the metal matrix.

This defines the segregation rate, Γs MW, of the cumulative concentrations in (Mo + W / 2), equal to: $$\mathrm{\Gamma s}\mathit{MW}=\left(\left(\mathrm{Mos}+\mathrm{ws}/2\right)-\left(\mathrm{Moh}+\mathrm{Wh}/2\right)\right)/\left(\mathrm{Moh}+\mathrm{Wh}/2\right)$$

The Mo + W / 2 criterion has been retained since it represents the cumulative hardening contribution of the Mo and W elements, both in segregated veins and outside of them. TABLE 1 C Ti Zr C * Yes mn Or Cr MB W V Nb Mo + W / 2 D Γ sMW a ₁ comp 0.31 0 0 0.31 0.2 0.7 0.4 3 0.75 0 0.10 0 0.75 825 133 a ₂ comp 0.31 0 0 0.31 0.2 0.7 0.4 3 0.55 0.4 0.10 0 0.75 825 137 a ₃ inv 0.41 0.40 0 0.31 0.2 0.7 0.4 3 0.55 0.4 0.10 0 0.75 825 106 b ₁ comp 0.6 0.4 0 0.5 0.5 0.5 0.3 6.5 2.2 0 0.3 0 2.2 999 128 b ₂ comp 0.75 0.8 0.4 0.5 0.5 0.5 0.3 6.5 2.2 0 0.3 0 2.2 999 131 b ₃ inv 0.75 0.8 0.4 0.5 0.5 0.5 0.3 6.5 1.5 1.4 0.3 0 2.2 999 98 c ₁ comp 0.80 0.25 0 0.74 0.9 0.45 0.25 3.9 2.1 0 0.28 0 2.1 1028 130 c ₂ inv 0.80 0.25 0 0.74 0.9 0.45 0.25 3.9 1.2 1.8 0.28 0 2.1 1028 121 c ₃ inv 0.95 0.85 0 0.74 0.9 0.45 0.25 3.9 1.2 1.8 0.28 0 2.1 1028 93 d ₁ comp 1.25 1 0 1 1 0.5 0.2 5 2.4 0 0.6 0 2.4 1117 127 d ₂ inv 1.25 1 0 1 1 0.5 0.2 5 2.0 0.8 0.6 0 2.4 1117 107 d ₃ inv 1.25 1 0 1 1 0.5 0.2 5 1.5 1.8 0.6 0 2.4 1117 91

The examples a ₁ , b ₁ , c ₁ and d ₁ correspond to reference steels, that is to say steels whose composition is chosen before implementing the method according to the invention. The other examples are deduced from these reference steels by the process according to the invention, except examples a ₂ and b ₂ for which the conditions relating to tungsten and titanium are not respected.

Examples ₁ , ₂ and ₃ have the same hardness. Example ₂ is deduced from Example ₁ by replacing 0.20% molybdenum with 0.40% tungsten, without adding titanium. It can be seen that the segregation rate is not significantly modified.

Example ₃ , according to the invention, is deduced from example a ₁ not only by the replacement of 0.20% of molybdenum with 0.40% of tungsten, but also by the addition of 0, 40% titanium and the consequent adjustment of carbon. It is noted that the segregation rate of this steel is very significantly reduced compared with that of Examples ₁ and ₂ .

In the same way, examples b ₁ , b ₂ and b ₃ show that the addition of titanium and zirconium without addition of tungsten has no effect (comparison b1, b2), whereas the desired effect appears in the presence of tungsten partially substituted for molybdenum (comparison b2, b3).

Examples c ₁ , c ₂ and c ₃ show that, with equal addition of tungsten, an increase in the addition of titanium has a favorable effect on the segregations.

Similarly, examples ₁ , ₂ , and ₃ show that an increase in the tungsten content has a favorable effect if the titanium or zirconium contents are sufficient.

To illustrate the effect of the ratio (Ti + Zr / 2) / W on the segregation of tungsten, one can also consider the examples corresponding to the steels of the castings ref, 5, 7, 1, 9, 6, 2, 18, 13 , 17 and 3 which all correspond to the invention, except casting ref. The main element contents of these flows are reported in Table 2; the rest of the composition being iron and impurities resulting from the preparation. TABLE 2 Casting number C Ti Zr Yes mn Or Cr MB W V Nb Ref 0.82 0 0 0.35 1.15 0.25 5.00 0.90 0.57 0.11 0.00 5 0.37 0.20 0.00 1.00 0.50 0.20 3.00 1.50 0.60 0.20 0.00 7 0.62 0.55 0.21 0.50 0.40 1.20 2.20 1.00 1.50 0.20 0.15 1 0.37 0.35 0.00 1.00 0.50 0.20 3.00 1.50 0.60 0.20 0.00 9 0.75 0.80 0.40 0.50 0.50 0.30 6.50 1.40 1.50 0.30 0.00 6 0.50 0.50 0.00 0.40 0.60 0.20 5.00 1.20 0.60 0.25 0.10 2 0.41 0.42 0.00 0.20 0.70 0.40 3.00 0.40 0.50 0.10 0.00 18 0.95 0.85 0.00 0.90 0.45 0.25 2.10 1.60 0.95 0.28 0.00 13 1.00 1.00 0.00 0.60 0.60 0.20 3.80 1.00 1.00 0.25 0.20 17 1.25 1.00 0.00 1.00 0.50 0.20 5.00 2.40 0.80 0.60 0.00 3 0.55 0.95 0.00 0.25 0.70 0.30 2.50 0.45 0.45 0.10 0.00

In Table 3, the sum Ti + Zr / 2, the contents in W, the ratios (Ti + Zr / 2) / W and the ratios Ws / W of the contents of tungsten in the segregated veins at the nominal contents in tungsten are reported. .

The values of the ratio Ws / W have been plotted on the graph of the figure, according to the values of the ratio (Ti + Zr / 2) / W. TABLE 3 casting no Ti + Zr / 2 W (Ti + Zr / 2) / W Ws / W ref 0 0.57 0 2.7 5 0.2 0.6 0.33 1.95 7 0.66 1.5 0.44 1.55 1 0.35 0.6 0.58 1.73 9 1 1.5 0.67 1.15 6 0.5 0.6 0.83 1.48 2 0.42 0.5 0.84 1.52 18 0.85 0.95 0.89 1.12 13 1 1 1 1.09 17 1 0.8 1.25 0.94 3 0.95 0.45 2.11 0.79

On the graph we see that the ratio Ws / W becomes substantially less than 2 as soon as the ratio (Ti + Zr / 2) / W exceeds 0.2. We also see that Ws / W decreases regularly when (Ti + Zr / 2) / W increases, whereas it is 2.7 for the reference casting which contains neither titanium nor zirconium.

The invention is also illustrated by the examples corresponding to the analyzes indicated in Table 4 which also indicates the ratio Ws / W, which in all cases is less than 1.6 and can even reach 0.67. TABLE 4 Casting number C Ti Zr Yes mn Or Cr MB W V Nb Ws / W 21 0.82 0.41 tr 0.9 0.6 1.5 2.7 2.2 0.5 0.25 tr 1.51 22 0.95 0.83 0.2 0.8 0.6 0.2 4.1 2.3 0.3 0.25 tr 0.67 23 0.94 0.92 tr 0.7 1.3 0.2 3.2 2.5 0.5 0.4 tr 0.84 24 0.81 0.42 tr 0.9 0.8 0.2 4.4 1.4 0.7 0.15 0.20 1.65 25 0.72 0.4 tr 0.9 0.3 0.2 5.5 1.6 0.5 0.15 tr 1.52 26 0.79 0.71 0.4 0.9 0.6 0.2 4.4 1.5 0.5 0.15 tr 0.85 27 1.02 0.38 tr 0.9 0.9 0.3 2.1 1.5 0.5 0.15 tr 1.54 28 0.8 0.44 tr 0.2 0.6 1.4 2.7 2.1 0.5 0.25 tr 1.42 29 0.95 0.85 tr 0.3 0.6 0.2 4.0 2.3 0.4 0.20 tr 0.77 30 0.95 0.88 tr 0.2 1.4 0.2 3.1 2.6 0.5 0.4 tr 0.83 31 0.8 0.42 tr 0.3 0.9 2.1 4.7 1.5 0.7 0.15 tr 1.57 32 0.7 0.4 tr 0.3 0.3 1.2 3.5 1.4 0.5 0.15 0.25 1.47 33 0.8 0.9 tr 0.2 0.4 0.3 3.2 1.5 0.5 0.15 tr 0.82 34 1 0.44 tr 0.5 0.4 0.2 4.5 1.2 0.5 0.15 tr 1.44 35 0.71 0.41 tr 0.4 1.6 0.2 6.1 1.2 0.5 0.75 tr 1.46 36 0.91 0.92 tr 0.1 0.9 0.4 5.7 0.6 0.8 0.65 tr 1.03 37 1.25 0.95 tr 0.9 0.6 1.7 4.1 3.1 0.9 0.35 0.35 1.03

These examples also highlight the effect of the silicon content on the thermal conductivity of steel, and therefore, the interest of imposing a low silicon content when the steel is intended to produce tools for which a good thermal conductivity is desired. This effect is illustrated by the pairs of examples 21 and 28, 22 and 29, 23 and 30. In each of these pairs, the examples essentially differ only in the silicon contents. The thermal conductivities are as follows: Example 21: Si = 0.9% thermal conductivity = 20.6 W / m / K Example 28: If = 0.2% thermal conductivity = 25.1 W / m / K Example 22: If = 0.8% thermal conductivity = 21.3 W / m / K Example 29: If = 0.3% thermal conductivity = 24.4 W / m / K Example 23: If = 0.7% thermal conductivity = 20.7 W / m / K Example 30: If = 0.2% thermal conductivity = 23.6 W / m / K

It is thus seen that a low silicon makes it possible to increase the thermal conductivity significantly. In the case of the examples, this increase ranges from about 15% to about 25%.

Claims (15)

1. Steel which has high mechanical strength and high wear resistance and whose chemical composition comprises in % by weight: $$0.35\%\le \mathrm{C}\le 1.47\%,$$

   

   $$0.05\%\le \mathrm{Si}\le 1.5\%,$$

   

   $$\mathrm{Mn}\le 1.95\%,$$

   

   $$\mathrm{Ni}\le 2.9\%,$$

   

   $$1.1\%\le \mathrm{Cr}\le 7.9\%,$$

   

   $$0\%\le \mathrm{Mo}\le 4.29\%,$$

   

   $$0.21\%\le \mathrm{W}\le 4.9\%$$

   

   $$0.61\%\le \mathrm{Mo}+\mathrm{W}/2\le 4.4\%$$

   

   $$0\%\le \mathrm{Ti}\le 1.49\%$$

   

   $$0\%\le \mathrm{Zr}\le 2.9\%$$

   

   $$0.21\%\le \mathrm{Ti}+\mathrm{Zr}/2\le 1.49\%$$

   

   - optionally one or more elements selected from vanadium, niobium and tantalum, at contents such that V ≤ 1.45 %, Nb ≤ 1.45 %, Ta ≤ 1.45 % and V + Nb/2 + Ta/4 ≤ 1.45 %,

   - optionally up to 0.1 % of boron,

   - less than 0.005% of sulphur,

   - optionally up to 0.01% of calcium ,

   - optionally up to 0.5 % of rare earths,

   - optionally up to 1 % of aluminium,

   - optionally up to 1 % of copper,

   the balance being iron and impurities resulting from the production operation,
   the composition complying with the following conditions: $$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)/\mathrm{W}\ge 0.20$$

   

   $$\left(\mathrm{Ti}+\mathrm{Zr}/2\right)\mathrm{x\; C}\ge 0.07$$

   

   $$0.3\%\le \mathrm{C*}\le 1.42\%$$

   

   $$800\le \mathrm{D}\le 1150$$

   

   with $$\mathrm{D}=540{\left(\mathrm{C}*\right)}^{0.25}+245{\left(\mathrm{Mo}+\mathrm{W}/2+3\mathrm{V}+1.5\mathrm{Nb}+0.75\mathrm{Ta}\right)}^{0.3}+125{\mathrm{Cr}}^{0.20}+15.8\mathrm{Mn}+7.4\mathrm{Ni}+18\mathrm{Si}$$

   

   and $$\mathrm{C}*=\mathrm{C}-\mathrm{Ti}/4-\mathrm{Zr}/8,$$

   

   and, furthermore, if C* ≥ 0.51 % and if 2.5% ≤ Cr ≤ 3.5%, then W ≤ 0.85 % if Mo < 1.21 % and W/Mo ≤ 0.7 if Mo ≥ 1.21 %.
2. Steel according to claim 1, characterized in that: $$\mathrm{C}*\le 1.1\%$$

   
3. Steel according to claim 1 or 2, characterized in that: $$\mathrm{W}\le 0.85\%$$

   
4. Steel according to any one of claims 1 to 3, characterized in that: $$\mathrm{Si}\ge 0.45\%$$

   
5. Steel according to any one of claims 1 to 3, characterized in that: $$\mathrm{Si}<0.45\%$$

   
6. Steel according to any one of claims 1 to 5, characterized in that: $$\mathrm{Mo}+\mathrm{W}/2\ge 2.2\%$$

   
7. Steel according to any one of claims 1 to 6, characterized in that: $$\mathrm{Cr}\ge 3.5\%$$

   
8. Steel according to any one of claims 1 to 7, characterized in that: $$\mathrm{C}\le 0.85\%$$

   
9. Steel according to any one of claims 1 to 7, characterized in that: $$\mathrm{C}>0.85\%$$

   
10. Steel according to any one of claims 1 to 9, characterized in that: $$\mathrm{Ti}+\mathrm{Zr}/2<0.7\%$$

    
11. Steel according to any one of claims 1 yo 9, characterized in that: $$\mathrm{Ti}+\mathrm{Zr}/2\ge 0.7\%$$

    
12. Method for producing a steel workpiece according to any one of claims 1 to 11, characterized in that:

    - a liquid steel is produced having the desired composition with the contents of titanium and/or zirconium in the bath of molten steel being adjusted, with local excess concentrations of titanium and/or zirconium in the bath of molten steel being prevented at all times;

    - the steel being cast in order to obtain a semi-finished product;

    - then, the semi-finished product is subjected to a forming processing operation by means of plastic deformation in the hot state and, optionally, a thermal processing operation in order to obtain the workpiece.
13. Method according to claim 12, characterized in that the addition of titanium and/or zirconium is carried out by progressively adding titanium and/or zirconium to a slag which covers the bath of liquid steel and by allowing the titanium and/or zirconium to diffuse slowly in the bath of liquid steel.
14. Method according to claim 12, characterized in that the addition of titanium and/or zirconium is carried out by a wire comprising titanium and/or zirconium being introduced into the bath of liquid steel, with the bath being agitated.
15. Workpiece of steel according to any one of claims 1 to 11, which workpiece is capable of being obtained by means of the method according to any one of claims 12 to 14.

Images