EP0497642A1 - Method for manufacturing massive steel workpieces and workpieces thereby obtained - Google Patents

Abstract

A quenching treatment for attaining the desired characteristics in at least a part of the massive block or piece is defined. The rates of cooling VRC and VRP of the block at the core and at the skin during the quenching are determined. The block is made of a steel whose chemical composition has been adjusted so as to obtain a martensitic structure at the core under the effect of deformations induced by the quenching, although the rate of cooling at the core VRC of the block or piece is lower than the critical rate of martensitic transformation. Appropriate tempering is performed after the quenching. The blocks or pieces exhibit high tensile mechanical characteristics and good tenacity at all points and are weldable. The process makes it possible to define a class of steels satisfying the conditions imposed by the quenching and exhibiting a minimum equivalent carbon. <IMAGE>

Description

The present invention relates to a method for manufacturing blocks or solid pieces of steel having at all points high mechanical tensile characteristics, good toughness and good weldability.

The invention also relates to the definition of a family of steels allowing the implementation of the process and the blocks or parts obtained by the process.

Those skilled in the art know that, at least for weakly or moderately alloyed hypoeutectoid steels, the best mechanical characteristics are obtained by a quenching heat treatment followed by tempering which gives the block or the part a martensitic structure. This is how all blocks or parts with high mechanical characteristics and good toughness are obtained.

However, this method has limitations. Indeed, for a given steel, the person skilled in the art considers that the cooling rate at any point of the block or of the part during the quenching must be greater than a speed V₁ called the martensitic critical speed. However, the more massive a block or a part, the lower the cooling rate at the core, and this simply because of the laws of heat transfer.

When it is necessary to produce very massive blocks or parts, the skilled person is thus led to increase the hardenability of the steel, that is to say to decrease the critical martensitic speed and for this to increase significantly carbon and alloying content. But in doing so, it deteriorates the weldability all the more as it increases the contents of additives.

Those skilled in the art can certainly improve the weldability by keeping good tensile characteristics by acting on the conditions of realization of the income processing. But then it is unable to obtain good toughness properties in particular at low temperatures.

All these contradictions induce well-known and numerous limitations. For example, one of the best known steels, intended to make thick parts and having an elastic limit R _e0,2 greater than or equal to 900 MPa has guaranteed properties in elastic limit only for thicknesses less than or equal to 4 "or 101.6 mm, its resilience is only guaranteed for temperatures greater than or equal to - 18 ° C and it is difficult to weld.

Another example is that of cryogenic steels, for example 9% nickel steel. Such a steel allows good resilience to be obtained up to -196 ° C., but its yield strength is limited to around 600 MPa and on thicknesses less than or equal to 50 mm; on the other hand it is very weldable.

The object of the present invention is therefore to propose a method for manufacturing solid steel parts and for determining a family of steels which, going beyond the limits of the current technique, makes it possible to produce blocks or parts having a compromise elastic limit, toughness at low temperature, weldability significantly improved compared to what we know how to do today and this for very massive blocks or parts.

The object of the present invention is a method of manufacturing a block or a solid piece of steel having at all points high tensile mechanical characteristics and good toughness and which is weldable.

• on définit un traitement de trempe pour atteindre les caractéristiques mécaniques voulues dans une partie au moins du bloc ou de la pièce massive,
• on détermine les vitesses de refroidissement V_RP et V_RC du bloc ou de la pièce massive dans sa partie externe ou peau et dans sa partie centrale ou coeur, pendant la trempe, en fonction de la géométrie du bloc ou de la pièce.
• on réalise le bloc ou la pièce massive en un acier dont la composition chimique pondérale a été ajustée de manière que :
  • . l'écart entre le point B_S et le point M_S soit inférieur à 100°C, B_S étant la température de début de transformation bainitique de l'acier et M_S, la température de début de transformation martensitique,
  • . la vitesse critique V₃ de début de transformation en ferrite et perlite de l'acier soit inférieure à la vitesse de refroidissement à coeur V_RC du bloc ou de la pièce massive,
  • . la vitesse critique V₁ de transformation martensitique soit inférieure à la vitesse de refroidissement en peau V_RP du bloc ou de la pièce massive,
  • . le carbone équivalent de l'acier soit le plus faible possible,
• on effectue la trempe prédéterminée sur le bloc ou la pièce massive, de manière à obtenir une structure martensitique à coeur, sous l'effet de déformations induites dans la pièce, par la trempe,
• et on complète cette trempe par un revenu approprié.

According to the invention:

• a quenching treatment is defined to achieve the desired mechanical characteristics in at least part of the block or of the solid part,
• the cooling rates V _RP and V _RC of the block or of the solid part are determined in its external part or skin and in its central part or core, during quenching, as a function of the geometry of the block or of the part.
• the block or solid part is made of steel whose chemical composition by weight has been adjusted so that:
  • . the difference between point B _S and point M _S is less than 100 ° C., B _S being the temperature at the start of bainitic transformation of the steel and M _S , the temperature at the start of martensitic transformation,
  • . the critical speed V₃ of the start of transformation into ferrite and perlite of the steel is less than the cooling rate at the core V _RC of the block or of the solid part,
  • . the critical speed V₁ of martensitic transformation is lower than the cooling speed in skin V _RP of the block or of the solid part,
  • . the equivalent carbon in steel is as low as possible,
• the predetermined quenching is carried out on the block or the solid part, so as to obtain a martensitic structure at the core, under the effect of deformations induced in the part, by the quenching,
• and this quenching is completed by an appropriate income.

The invention also relates to a solid piece or block of steel having a fully martensitic structure after quenching and tempering, although the quenching is carried out under conditions of implementation such that the cooling rate at the core of the piece is lower than the critical rate of martensitic transformation of steel.

tout en satisfaisant au moins aux trois contraintes suivantes : $${\text{B}}_{\text{S}}{\text{- M}}_{\text{S}}\text{= 245 + 204 C + 155 Si - 57 MN - 20 Ni - 53 Cr - 62 Mo ≦ Δϑ}$$

avec Δϑ ≦ 100°C $${\text{log V₁ = 9,81 - 4,62 C}}_{\text{eq}}{\text{- 0,265 Mn - 0,52 Ni + 0,425 Cr + 0,265 Mo + 0,925 V ≦ log V}}_{\text{RP}}\text{(m)}$$

$${\text{log V₃ = 6,36 - 0,43 C}}_{\text{eq}}\text{- 0,42 Mn - 0,78 Ni - 0,19 Cr - 0,3 Mo - 2}\sqrt{\text{Mo}}{\text{< log V}}_{\text{RC}}\text{(m).}$$

m représentant la massivité du bloc ou de la pièce.Preferably, the quenching is a quenching with water. The steel used to implement the invention has a determined chemical composition by solving the mathematical programming problem in which it is sought to minimize the equivalent carbon: $${\text{VS}}_{\text{eq}}\text{= C +}\frac{\text{Mn}}{\text{6}}\text{+}\frac{\text{Cr + Mo + V}}{\text{5}}\text{+}\frac{\text{Cu + Ni}}{\text{15}}$$

while satisfying at least the following three constraints: $${\text{B}}_{\text{S}}{\text{- M}}_{\text{S}}\text{= 245 + 204 C + 155 Si - 57 MN - 20 Ni - 53 Cr - 62 Mo ≦ Δϑ}$$

with Δϑ ≦ 100 ° C $${\text{log V₁ = 9.81 - 4.62 C}}_{\text{eq}}{\text{- 0.265 Mn - 0.52 Ni + 0.425 Cr + 0.265 MB + 0.925 V ≦ log V}}_{\text{RP}}\text{(m)}$$

$${\text{log V₃ = 6.36 - 0.43 C}}_{\text{eq}}\text{- 0.42 Mn - 0.78 Ni - 0.19 Cr - 0.3 MB - 2}\sqrt{\text{Mo}}{\text{<log V}}_{\text{RC}}\text{(m).}$$

m representing the massiveness of the block or the part.

• l'une relative à la dureté : $${\text{H}}_{\text{vm}}{\text{= 232 + 293 C + 11 Si - 9,5 Mn - 4,8 Ni + 3,8 Cr + 80 Mo + 532 V ≧ H}}_{\text{Vo}}$$
  
• l'autre relative à la température de transition ductile fragile :
  

To these constraints, two additional constraints can be added either separately or simultaneously:

• one relating to hardness: $${\text{H}}_{\text{vm}}{\text{= 232 + 293 C + 11 Si - 9.5 Mn - 4.8 Ni + 3.8 Cr + 80 Mo + 532 V ≧ H}}_{\text{Vo}}$$
  
• the other relating to the brittle ductile transition temperature:
  

H _Vo and T _o are values that the skilled person chooses to define the mechanical characteristics he is looking for on the part.

This problem of mathematical programming is supplemented by constraints defining the domain of validity of the formulas, namely:

These limitations of the content of alloying elements of the steel constitute preferential conditions but it is possible to envisage the use of intervals of wider compositions.

Such a steel used by the process which is the subject of the invention makes it possible to obtain a martensitic structure at heart on blocks or parts whose massivity m is such that the cooling speed at heart V _RC (m) during quenching is less than the critical martensitic speed V₁.

To improve the toughness of such a steel, it is preferably possible to add at least 0.3% of Mo, at least 0.07% of V, at least 0.015% of Al, and limit the nitrogen content to at most 0.015%.

Preferably, this steel contains less than 0.01% of P and less than 0.01% of S.

The invention, finally, relates to any block or any part manufactured by applying the method according to the invention, using a steel defined in the context of the implementation of this method.

Figure 1 is a diagram showing the distribution of cooling rates within two plates cooled by quenching.

Figure 2 is an example of a TRC (Continuous Cooling) diagram of a steel.

FIG. 3 represents the variation in hardness of a steel as a function of the cooling rate during a quenching.

FIG. 4 represents the variation of hardness inside of hardened plates.

FIG. 5 is an explanatory diagram of the principle of the invention.

The description which follows, without being limiting, will make the invention better understood.

First of all, it is necessary to define what is meant by massiveness and the consequences of this massiveness on the behavior of a metal block during a quenching treatment.

The massiveness of a block is a dimension which characterizes the heat transfers inside this block. For a plate for example, this dimension is the thickness; for a cylinder, it is the diameter; for a block of any shape, it is a dimension that a person skilled in the art knows how to determine from the drawing of the block.

When a block previously heated to high temperature is quenched, for example with water, on the one hand the average cooling rate of the block is lower the more the block is massive, and on the other hand the difference the greater the massivity, the greater the cooling speed between the core and the surface. This is shown in FIG. 1. Consequently, for determined quenching conditions, the more massive the block, the lower the cooling rate at the core.

To study the effect of quenching on a steel block of determined mass, the skilled person uses a Transformation in Continuous Cooling diagram or TRC diagram as shown in Figure 2.

A point of the steel block a₁ whose cooling rate V _R is greater than V₁ (domain 1) will have a martensitic structure at the end of quenching; if this speed is between V₁ and V₃, the structure will be mainly bainitic; if the speed is lower than V₃ the structure will be ferrite-perlite.

To this TRC diagram, it is possible to match a diagram represented in FIG. 3 and representing a mechanical characteristic such as hardness as a function of the structure: it is for martensite that this characteristic is the highest, it decreases when the one enters the bainitic domain and arrives at a lower plateau for the ferrite-pearlite domain.

In FIGS. 2 and 3, the diagrams corresponding to two steels are shown: a₁ having a critical quenching speed V₁ and a₂, more quenching than a₁, having a critical quenching speed V₁ * <V₁.

By examining Figures 1, 2 and 3 simultaneously, we can see that with steel a₁, the thickness plate 2e₁ will be entirely martensitic, while the thickness plate 2e₂ will be, on each face, only on a depth p₂ <e₂. With a₂ steel on the other hand, the two plates will be entirely martensitic.

This makes it possible to draw the distribution curves of mechanical characteristics inside the two plates represented in FIG. 4.

Thus, when looking for high mechanical characteristics on massive parts, the skilled person is led to use steels that are all the more soaking as the part is massive. But in doing so, it comes up against the difficulties described in the introduction.

C

= 0,095 %

Si

= 0,2 %

Mn

= 0,25 %

Ni

= 8 %

Cr

= 0,2 %

Mo

= 0,3 %

permet d'obtenir une structure entièrement martensitique même à coeur sur des blocs ou des plaques de 400 mm d'épaisseur trempés à l'eau alors que sa vitesse critique de trempe V₁ est d'environ 32000°C/h, ce qui correspond à une profondeur de trempe (ou massivité) de 60 mm. Selon les principes connus de l'homme de métier pour obtenir une structure martensitique à coeur d'un bloc de 400 mm d'épaisseur trempé à l'eau, il aurait fallu que la vitesse critique de trempe ait été inférieure à 700°C/h.However, the applicant has found, surprisingly, that a steel whose chemical composition is as follows:

VS

= 0.095%

Yes

= 0.2%

Mn

= 0.25%

Or

= 8%

Cr

= 0.2%

Mo

= 0.3%

makes it possible to obtain a fully martensitic structure even at its core on 400 mm thick blocks or plates soaked in water while its critical quenching speed V₁ is around 32000 ° C / h, which corresponds to a quenching depth (or massiveness) of 60 mm. According to the principles known to those skilled in the art in order to obtain a martensitic structure at the heart of a 400 mm thick block quenched with water, the critical quenching speed should have been less than 700 ° C./ h.

The document STAHL UND EISEN, vol 96 n ° 23 of November 18, 1976, pages 1168 to 1176, indicates that significant hydrostatic constraints modify the TRC diagram by delaying the non-martensitic transformations, but the effect which is described is not sufficient to explain the phenomenon observed.

Indeed, the critical quenching speed offset caused by hydrostatic constraints is not sufficient. In addition, in a very thick plate, the stress system generated by the cooling and the martensitic transformation is not hydrostatic but has a strong shear component, particularly in the zone being transformed.

We know that shear stresses favor the martensitic transformation and raise the point M _S , temperature at the start of martensitic transformation. This rise can reach 80 ° C or 100 ° C. It is also known that the onset of martensitic transformation inhibits bainitic transformation.

The TRC diagram of the steel taken as an example has a particularity: the point M _S is approximately 380 ° and the bainitic domain of the TRC diagram comprises a plateau for the point B _S of the start of bainitic transformation located at approximately 450 ° C. . There is only 70 ° C between point M _S and point B _S.

The applicant has hypothesized that the shear stresses generated in the plate by the cooling and by the martensitic transformation raised the point M _S from 80 ° to 100 ° C so that all along the bainitic plateau the martensitic transformation begins before bainitic transformation and inhibits this transformation. Thus it is possible to obtain a martensitic structure even for cooling rates much lower than the critical quenching speed V₁ and practically up to cooling speeds close to the critical speed V₃ of transformation into ferrite-perlite.

This phenomenon has the advantage of making it possible to obtain high mechanical characteristics on solid blocks with steels having very good weldability.

To assess the weldability of a steel, the skilled person generally uses the equivalent carbon criterion: $${\text{VS}}_{\text{eq}}\text{= C +}\frac{\text{Mn}}{\text{6}}\text{+}\frac{\text{Cr + Mo + V}}{\text{5}}\text{+}\frac{\text{Cu + Ni}}{\text{15}}$$

The lower the equivalent carbon, the better the weldability, but the lower the equivalent carbon the lower the quenchability, that is to say the greater the critical quenching speed V₁.

The steel taken as an example has an equivalent carbon C _eq = 0.77 and a critical quenching speed V₁ of 32000 ° C / h. It made it possible to obtain on a 400 mm thick plate an entirely martensitic structure to the core having, after an income achieved between 450 ° C. and A _c1 , an elastic limit of around 900 MPa and a resilience at 60 ° C of 45 Joules.

If the steel had been used according to the principles known to those skilled in the art, it would have had to have a critical quenching speed of less than 700 ° C./h, or an equivalent carbon greater than or equal to 0.94, c 'is to say 22% higher than that of the steel actually used and therefore corresponding to a considerably deteriorated weldability compared to that actually observed.

The invention consists in using the phenomenon which has just been described. It will be noted that, in all that follows, the TRC diagrams, the critical quenching rates, the temperatures at the start of transformation are the diagrams and the quantities measured under the usual conditions, that is to say in the absence of constraints.

• à tremper, par exemple à l'eau, un bloc ou une pièce massifs constitués d'un acier dont :
  • . l'écart entre le point B_S et le point M_S est inférieur à une certaine valeur, inférieure à 100°C,
  • . la vitesse critique martensitique V₁ est inférieure à une vitesse de refroidissement critique du bloc ou de la pièce, par exemple la vitesse de refroidissement en peau V_RP imposée par la trempe.
  • . la vitesse critique de début de transformation ferrite-perlite V₃ est inférieure à la vitesse de refroidissement à coeur V_RC du bloc ou de la pièce.
• la composition chimique étant ajustée pour minimiser le carbone équivalent compte tenu des contraintes précédentes,
• à effectuer, après trempe, un traitement thermique de revenu, à une température comprise entre 450°C et A_C1 de façon à ajuster les caractéristiques mécaniques de traction et de ténacité.

The invention is therefore a method which consists of:

• to soak, for example with water, a solid block or piece made of steel of which:
  • . the difference between point B _S and point M _S is less than a certain value, less than 100 ° C.,
  • . the martensitic critical speed V₁ is less than a critical cooling speed of the block or the part, for example the cooling speed in skin V _RP imposed by the quenching.
  • . the critical ferrite-perlite transformation start speed V₃ is lower than the cooling rate at the core V _RC of the block or of the part.
• the chemical composition being adjusted to minimize the equivalent carbon taking into account the above constraints,
• to carry out, after quenching, an annealing heat treatment, at a temperature between 450 ° C and A _C1 so as to adjust the mechanical characteristics of traction and toughness.

The quenching conditions are determined according to the type of steel used, the targeted mechanical characteristics, the technical quenching possibilities and the geometry of the part.

The adjustment of the mechanical tensile and resilience characteristics is done using an income heat treatment, the conditions of which are determined on a case-by-case basis by applying the rules of the art.

If we call m the massiveness of a block or a part, V _RC the cooling rate at the core and V _RP the cooling speed in skin of this block or this piece, for given quenching conditions, the skilled in the art knows how to determine V _RC (m) and V _RP (m), and subsequently, we will consider this quantity as known.

To implement the process which is the subject of the invention, a family of steels defined in the context of the invention must be used. This family will now be described.

First, we will express the weldability by the equivalent carbon criterion: $${\text{VS}}_{\text{eq}}\text{= C +}\frac{\text{Mn}}{\text{6}}\text{+}\frac{\text{Cr + Mo + V}}{\text{5}}\text{+}\frac{\text{Cu + Ni}}{\text{15}}$$

• la vitesse critique de trempe martensitique V₁ $${\text{log V₁ = 9,81 - 4,62 C}}_{\text{eq}}\text{- 0,265 Mn - 0,23 Ni + 0,425 Cr + 0,265 Mo + 0,925 V}$$
  
• la vitesse critique de transformation en ferrite-perlite V₃ $${\text{log V₃ = 6,36 - 0,43 C}}_{\text{eq}}\text{- 0,42 Mn - 0,78 Ni - 0,19 Cr - 0,3 Mo - 2}\sqrt{\text{M}}\text{o}$$
  

Using the weight composition of steel, we can calculate:

• the critical martensitic quenching speed V₁ $${\text{log V₁ = 9.81 - 4.62 C}}_{\text{eq}}\text{- 0.265 Mn - 0.23 Ni + 0.425 Cr + 0.265 MB + 0.925 V}$$
  
• the critical speed of transformation into ferrite-perlite V₃ $${\text{log V₃ = 6.36 - 0.43 C}}_{\text{eq}}\text{- 0.42 Mn - 0.78 Ni - 0.19 Cr - 0.3 MB - 2}\sqrt{\text{M}}\text{o}$$
  

• la valeur du point M_S, début de transformation martensitique en l'absence de contraintes : $${\text{M}}_{\text{S}}\text{= 565 - 474 C - 155 Si - 33 Mn - 17 Ni - 17 Cr - 21 Mo}$$
  
• la valeur du point B_S correspondant au plateau de température de début de transformation bainitique : $${\text{B}}_{\text{S}}\text{= 810 - 270 C - 90 Mn - 37 Ni - 70 Cr - 83 Mo}$$
  

These critical speeds are expressed in ° C / hour.

• the value of the point M _S , start of martensitic transformation in the absence of constraints: $${\text{M}}_{\text{S}}\text{= 565 - 474 C - 155 Si - 33 Mn - 17 Ni - 17 Cr - 21 Mo}$$
  
• the value of point B _S corresponding to the temperature plateau at the start of bainitic transformation: $${\text{B}}_{\text{S}}\text{= 810 - 270 C - 90 Mn - 37 Ni - 70 Cr - 83 Mo}$$
  

From the two previous formulas we deduce that: $${\text{B}}_{\text{S}}{\text{- M}}_{\text{S}}\text{= 245 + 204 C + 155 Si - 57 Mn - 20 Ni - 53 Cr - 62 Mo.}$$

• la dureté de la martensite à l'état trempé revenu (pour un revenu standard à 600 ° C pendant 1 h) : $${\text{H}}_{\text{Vm}}\text{= 232 + 293 C + 11 Si - 9,5 Mn - 4,8 Ni + 3,8 Cr + 80 Mo + 532 V}$$
  
• la température de transition ductile/fragile lors d'un essai de résilience (après un revenu identique à celui défini ci-dessus) : $${\text{T}}_{\text{c}}\text{= 162 + 530 C² - 540 Si - 161 Mn - 48 Ni - - 138}\sqrt{\text{M}}\text{o +}\frac{\text{10 <figure-callout id="3" label="x H" filenames="imgf0001.png" state="{{state}}">x H</figure-callout>}}{\text{3}}{\text{}}_{\text{vm}}\text{(0,141 Mo + 0,053 Ni + 0,243 Mn + 0,710 Si - 0,408 C - 0,278)}$$
  

Steel characteristics can also be assessed using formulas to calculate:

• the hardness of martensite in the quenched quenched state (for a standard tempering at 600 ° C. for 1 h): $${\text{H}}_{\text{Vm}}\text{= 232 + 293 C + 11 Si - 9.5 Mn - 4.8 Ni + 3.8 Cr + 80 Mo + 532 V}$$
  
• the ductile / brittle transition temperature during a resilience test (after an income identical to that defined above): $${\text{T}}_{\text{vs}}\text{= 162 + 530 C² - 540 Si - 161 Mn - 48 Ni - - 138}\sqrt{\text{M}}\text{o +}\frac{\text{10 <figure-callout id="3" label="x H" filenames="imgf0001.png" state="{{state}}">x H</figure-callout>}}{\text{3}}{}_{\text{vm}}\text{(0.141 MB + 0.053 Ni + 0.243 Mn + 0.710 Si - 0.408 C - 0.278)}$$
  

These formulas are all valid in the following field of chemical composition:

$${\text{log V₁ < log V}}_{\text{RP}}\text{(m)}$$

$${\text{log V₃ < log V}}_{\text{RC}}\text{(m)}$$

et C_eq minimal.The steel belonging to the family concerned by the invention must therefore satisfy, for a block or a piece of massiveness m, the following system of equation: $${\text{B}}_{\text{S}}{\text{- M}}_{\text{S}}\text{≦ Δϑ}$$

$${\text{log V₁ <log V}}_{\text{RP}}\text{(m)}$$

$${\text{log V₃ <log V}}_{\text{RC}}\text{(m)}$$

and C _eq minimum.

This is a classic problem of mathematical programming.

Preferably we will take Δϑ = 100 ° C.

• soit de la dureté visée H_V et on ajoutera une contrainte supplémentaire : $${\text{H}}_{\text{Vm}}{\text{≧ H}}_{\text{V}}\text{visée.}$$
  
• soit de la température de transition souhaitée pour la résilience T_o et on ajoutera une contrainte : $${\text{T}}_{\text{c}}{\text{≦ T}}_{\text{o}}\text{.}$$
  
• soit des deux contraintes précédentes.

This equation system can be completed to take into account:

• either of the target hardness H _V and an additional constraint will be added: $${\text{H}}_{\text{Vm}}{\text{≧ H}}_{\text{V}}\text{aimed.}$$
  
• either of the desired transition temperature for the resilience T _o and a constraint will be added: $${\text{T}}_{\text{vs}}{\text{≦ T}}_{\text{o}}\text{.}$$
  
• either of the two previous constraints.

$${\text{log V₁ < log V}}_{\text{RP}}\text{(m)}$$

$${\text{log V₃ < log V}}_{\text{RC}}\text{(m)}$$

$${\text{H}}_{\text{Vm}}{\text{≧ H}}_{\text{V}}\text{visée}$$

$${\text{T}}_{\text{c}}{\text{≦ T}}_{\text{o}}$$

et l'on minimisera le carbone équivalent, C_eq minimal.Preferably, the mathematical programming problem that we solve will include five constraints: $${\text{B}}_{\text{S}}{\text{- M}}_{\text{S}}\text{≦ Δϑ}$$

$${\text{log V₁ <log V}}_{\text{RP}}\text{(m)}$$

$${\text{log V₃ <log V}}_{\text{RC}}\text{(m)}$$

$${\text{H}}_{\text{Vm}}{\text{≧ H}}_{\text{V}}\text{aim}$$

$${\text{T}}_{\text{vs}}{\text{≦ T}}_{\text{o}}$$

and we minimize the equivalent carbon, C _eq minimum.

This resolution is done in the field of validity of the formulas as defined above.

In addition, toughness can be affected by co-aggregation at the joints of the phosphorus grains accompanied by manganese, chromium, nickel. But phosphorus can be trapped inside the grains by molybdenum to as long as it does not react with carbon, these reactions taking place during tempering.

• au moins 0,3 % de Mo pour piéger le phosphore,
• au moins 0,070 % de V pour piéger le carbone par formation de précipités de carbures de vanadium.
• au moins 0,015 % d'Al
• au plus 0,015 % d'azote,

de façon, d'une part à former des nitrures d'aluminium pour contrôler le grain au cours de l'austénitisation, d'autre part à piéger suffisamment d'azote à l'aide de l'aluminium pour favoriser la réaction du vanadium avec le carbone de préférence à une réaction avec l'azote et de ce fait laisser du molybdène libre pour piéger le phosphore.In order to improve the toughness of the steel, we add simultaneously:

• at least 0.3% of Mo to trap phosphorus,
• at least 0.070% of V to trap the carbon by formation of precipitates of vanadium carbides.
• at least 0.015% Al
• at most 0.015% nitrogen,

so, on the one hand to form aluminum nitrides to control the grain during the austenitization, on the other hand to trap enough nitrogen using aluminum to promote the reaction of vanadium with carbon preferably in reaction with nitrogen and therefore leave free molybdenum to trap phosphorus.

It is also desirable to limit the contents of P and S.

$$\text{S < 0,01 \%}$$

afin d'améliorer la résilience.We will preferably have: $$\text{P <0.01\%}$$

$$\text{S <0.01\%}$$

in order to improve resilience.

C

= 0,101 %

Si

= 0,198 %

Mn

= 0,273 %

Ni

= 5,943 %

Cr

= 0,205 %

Mo

= 0,471 %

V

= 0,083 %

Cu

= 0,107 %

P

< 0,010 %

S

< 0,010 %.

One thus obtains, for example, a steel whose weight composition is:

VS

= 0.101%

Yes

= 0.198%

Mn

= 0.273%

Or

= 5.943%

Cr

= 0.205%

Mo

= 0.471%

V

= 0.083%

Cu

= 0.107%

P

<0.010%

S

<0.010%.

Such a steel has a critical martensitic speed V₁ of: 71 ° C / s which corresponds to the cooling rate at the heart of a 25 mm thick plate soaked in water.

However, this steel makes it possible to obtain a martensitic structure at the heart of plates with a thickness of at least 40 mm, which, for those skilled in the art, would have required a critical quenching speed V₁ of less than 28 ° C / s.

The example steel has a carbon equivalent of C _eq = 0.702%.

If the critical speed V₁ had been less than 28 ° C / s, the equivalent carbon would have been:
C _eq = 0.788 or 12% more and the weldability would have deteriorated by the same amount.

After quenching and tempering at 620 ° C of the example steel, the following mechanical characteristics are obtained:
R _e0,2 2 = 995 MP _a , R _m = 1048 MP _a ,
K _Cv = 81 Joules at - 85 ° C.

Which is an excellent result.

• limiter les teneurs en As, Sb, Sn pour améliorer la ténacité,
• ajouter du Ti, Zr, Nb, B, Co, W pour améliorer les caractéristiques de traction ou augmenter la trempabilité sans modifier le carbone équivalent (c'est notamment le cas pour le B).

In addition, to improve the performance of these steels we can:

• limit the contents of As, Sb, Sn to improve the toughness,
• add Ti, Zr, Nb, B, Co, W to improve the traction characteristics or increase the hardenability without modifying the equivalent carbon (this is particularly the case for B).

The invention also relates to all the blocks or parts produced using the process which is the subject of the invention using a steel according to the invention.

These blocks or parts have, after quenching and tempering, a fully martensitic structure, although the cooling rate at the core V _RC of the part during quenching is less than the critical rate of martensitic transformation.

Claims (8)

1.- Method of manufacturing a block or a solid piece of steel having at all points high mechanical traction characteristics and good toughness and which is weldable, characterized by the fact: - that a quenching treatment is defined to achieve the desired mechanical characteristics in at least part of the block or of the solid part, - that the cooling rates V _RP and V _RC of the block or of the solid part are determined in its external part or skin and in its central part or core, during quenching, as a function of the geometry of the block or of the part . - that the block or solid part is made of steel whose chemical composition by weight has been adjusted so that: . the difference between point B _S and point M _S is less than 100 ° C., B _S being the temperature at the start of bainitic transformation of the steel and M _S , the temperature at the start of martensitic transformation, . the critical speed V₃ of the start of transformation into ferrite and perlite of the steel is less than the cooling rate at the core V _RC of the block or of the solid part, . the critical speed V₁ of martensitic transformation is less than the cooling speed in the skin V _RP RP of the block or of the solid part, . the equivalent carbon in steel is as low as possible, that the predetermined quenching is carried out on the block or the solid part, so as to obtain a martensitic structure at its core, under the effect of deformations induced in the part, by the quenching, - and that this quenching is completed by an appropriate income. 2.- Method according to claim 1, characterized in that the quenching is a quenching with water. 3.- Solid piece or steel block having at all points high mechanical tensile characteristics and good toughness after quenching and tempering and which also has good weldability, characterized in that the structure of the piece is entirely martensitic since its external part or skin, up to its central part or core, although the quenching is carried out under processing conditions such that the cooling speed at core V _RC of the part is less than the critical speed of martensitic transformation , the chemical composition of the steel being chosen and adjusted so that the equivalent carbon of the composition is as low as possible and that the martensitic transformation can occur at its core under the effect of the deformation induced by the quenching. 4.- Part according to claim 3 obtained by the method according to claim 1 or claim 2, characterized in that the chemical composition by weight of the steel satisfies the following mathematical programming system: $${\text{B}}_{\text{S}}{\text{- M}}_{\text{S}}\text{= 245 + 204 C + 155 Si - 57 Mn - 20 Ni - 53 Cr - 62 Mo ≦ Δϑ with Δϑ = 100 ° C}$$

$${\text{log V₁ = 9.81 - 4.62 C}}_{\text{eq}}{\text{- 0.265 Mn - 0.52 Ni + 0.425 Cr + 0.265 MB + 0.925 V ≦ log V}}_{\text{RP}}\text{(m)}$$

$${\text{log V₃ = 6.36 - 0.43 C}}_{\text{eq}}\text{- 0.42 Mn - 0.78 Ni - 0.19 Cr - 0.3 MB - 2}\sqrt{\text{M}}{\text{o <log V}}_{\text{RC}}\text{(m).}$$

$${\text{VS}}_{\text{eq}}\text{= C +}\frac{\text{Mn}}{\text{6}}\text{+}\frac{\text{Cr + Mo + V}}{\text{5}}\text{+}\frac{\text{Cu + Ni}}{\text{15}}$$

minimal and in that:

5.- Piece according to claim 4, further characterized in that its chemical composition by weight satisfies the relationship: $${\text{H}}_{\text{Vm}}{\text{= 232 + 293 C + 11 Si - 9.5 Mn - 4.8 Ni + 3.8 Cr + 80 Mo + 532 V ≧ H}}_{\text{Vo}}\text{,}$$

H _Vm representing the hardness of the part and H _Vo a value fixed in advance as a function of the elastic limit sought for the part. 6.- Part according to any one of claims 4 and 5, characterized in that the chemical composition by weight also satisfies the relationship: $${\text{T}}_{\text{vs}}\text{= 162 + 530 C² - 540 Si - 161 Mn - 48 Ni - 138}\sqrt{\text{M}}\text{o + 10}\frac{{\text{H}}_{\text{Vm}}}{\text{3}}{\text{(0.141 MB + 0.053 Ni + 0.243 Mn + 0.710 Si - 0.408 C - 0.278) ≦ T}}_{\text{o}}\text{,}$$

T _c being the ductile, brittle transition temperature of the steel and T _o a desired value of the transition temperature. 7.- Part according to any one of claims 4 to 6, characterized in that the steel also contains:
more than 0.3% of MB
more than 0.07% of V
a proportion of Al greater than or equal to 0.015%
a proportion of nitrogen less than 0.015%. 8.- Part according to any one of claims 3 to 7, characterized in that the steel contains:
less than 0.01% P
less than 0.01% of S.

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