EP1426460A1 - Machine construction steel, process for hot forming a piece of this steel and piece obtained by this process - Google Patents

Abstract

The invention relates to a steel for mechanical construction, characterized in that its composition is, in weight percentages: 0.35% <= C <= 1.2%; 0.10% <= Mn <= 2.0%; 0.10% <= If <= 3.0%; traces <= Cr <= 4.5%; traces <= Mo <= 2.0%; traces <= Ni <= 4.5%; traces <= V <= 0.5%; traces <= Cu <= 3.5% with Cu <= Ni% + 0.6 Si% if Cu> = 0.5%; traces <= P <= 0.200, traces <= Bi <= 0.200%, traces <= Sn <= 0.150%, traces <= As <= 0.100%, traces <= Sb <= 0.150%, with 0.050% <= P % + Bi% + Sn% + As% + Sb% <= 0.200%, traces <= Al <= 0.060%; traces <= Ca <= 0.050%; traces <= B <= 0.01%; traces <= S <= 0.200%; traces <= Te <= 0.020%; traces <= Se <= 0.040%; traces <= Pb <= 0.070%; traces <= Nb <= 0.050%; traces <= Ti <= 0.050%; the remainder being iron and impurities resulting from processing. The invention also relates to a process for hot forming a piece of steel, characterized in that: a piece of steel of the above composition is obtained; the piece is heated to a temperature between the solidus and the liquidus in order to obtain a liquid phase and a globular solid phase, and a thixoforging of said piece is carried out in order to obtain said part; and cooling said part. Finally, the invention relates to a thixoforged steel part, characterized in that it was manufactured by the above process. <IMAGE>

Description

The invention relates to the steel industry, and more specifically, the manufacturing steel parts which can be used, in particular, in construction mechanical and shaped by the process called "thixoforgeage".

Thixoforgging belongs to the category of setting processes forms metals in a semi-solid state.

This process consists in carrying out a significant deformation on a plot heated between solidus and liquidus.

The steels used for this process are those conventionally used for hot forging, which is done if necessary beforehand undergoing a metallurgical operation consisting in globulating the primary structure classically dendritic. Indeed, this primary dendritic structure is not not suitable for thixoforging operations. During heating up temperatures between solidus and liquidus, micro-segregation existing between the dendrites and the inter-dendritic spaces will cause the steel fusion preferably in these inter-dendritic spaces. During the operation of shaping this entanglement of liquid and solid, the liquid phase will initially be ejected at the start of the application of effort. It will therefore be necessary to deform the solid phase and a residue of liquid largely separated from the solid phase, which will cause increased efforts. For a deformation operation in these conditions the result obtained is bad: significant segregation, faults internal.

On the other hand, when thixoforging is carried out on a steel with globular structure brought to the semi-solid state by heating to a temperature between liquidus and solidus, globular particles solids are evenly distributed in the liquid phase. By optimizing the choice of the solid / liquid proportion, one can obtain a material having a high strain rate under the effect of a large stress shear. It therefore has very high deformability.

It is however possible, in certain cases, to obtain the structure globular desired during heating prior to thixoforging, without having recourse to a globulization operation of the separate primary structure. It is the case, in particular, when operating on plots from rolled bars from continuous casting blooms or ingots. The multiples reheating and the significant deformations undergone by the steel then led to a very nested and diffuse structure where a primary structure is practically impossible to reveal. It provides a structure globular phase solid during heating prior to thixoforging.

Thixoforging thus allows, compared to conventional processes hot forging, to realize in a single deformation operation parts of complex geometry which may have thin walls (1mm or less), and this with very little shaping efforts. Indeed, under the action of external efforts, steels suitable for an operation of thixoforgeage behave like viscous fluids.

For mechanical steels, whose carbon content can vary from 0.2% to 1.1%, the heating temperature required to deformation by the thixoforging process is, for example, 1430 ° C + 50 ° C = 1480 ° C for grade C38 (measured solidus temperature + 50 ° C to obtain the right liquid phase / solid phase ratio necessary for deformation) and 1315 ° C + 50 ° C = 1365 ° C for a shade 100Cr6.

The heating temperature and the amount of liquid phase formed are important parameters of the thixoforging process. Ease to get the "right" temperature and dispersion interval possible around this temperature to limit variations in the amount of liquid phase depend on the solidification interval. More this interval is large the easier it is to adjust the heating parameters.

For example, this solidification interval is 110 ° C for a grade C38, and 172 ° C for grade 100Cr6. So it's much more easy to work with this last shade which has a temperature of solidus low: 1315 ° C, and a large solidification range: 172 ° C.

Very high shaping temperatures, significant deformations which are used in the thixoforging process, result in thermally stressing the deformation tools in frequently extreme conditions. This leads to the use for these tools of alloys with very high mechanical properties when hot, or ceramic materials. Difficulties in making certain geometries or of tools (inserts) of large volumes and the costs of their realization can slow down the development of the thixoforging process.

The aim of the invention is to propose new steel grades better suited to thixoforging than those conventionally used, in that they would reduce the stresses on the deformation tools. These new shades should not, moreover, degrade the properties mechanical parts obtained.

• 0,35% ≤ C ≤ 1,2%
• 0,10% ≤ Mn ≤ 2,0%
• 0,10% ≤ Si ≤ 3,0%
• traces ≤ Cr ≤ 4,5%
• traces ≤ Mo ≤ 2,0%
• traces ≤ Ni ≤ 4,5%
• traces ≤ V ≤ 0,5%
• traces ≤ Cu ≤ 3,5% avec Cu ≤ Ni% + 0,6 Si% si Cu ≥ 0,5%
• traces ≤ P ≤ 0,200, traces ≤ Bi ≤ 0,200%, traces ≤ Sn ≤ 0,150%, traces ≤ As ≤ 0,100%, traces ≤ Sb ≤ 0,150%, avec 0,050% ≤ P% + Bi % + Sn% + As% + Sb% ≤ 0,200%,
• traces ≤ Al ≤ 0,060%
• traces ≤ Ca ≤ 0,050%
• traces ≤ B ≤ 0,01 %
• traces ≤ S ≤ 0,0200%
• traces ≤ Te ≤ 0,020%
• traces ≤ Se ≤ 0,040%
• traces ≤ Pb ≤ 0,070%
• traces ≤ Nb ≤ 0,050%
• traces ≤ Ti ≤ 0,050%

   le reste étant du fer et des impuretés résultant de l'élaboration.To this end, the subject of the invention is a steel for mechanical construction, characterized in that its composition is, in weight percentages:

• 0.35% ≤ C ≤ 1.2%
• 0.10% ≤ Mn ≤ 2.0%
• 0.10% ≤ If ≤ 3.0%
• traces ≤ Cr ≤ 4.5%
• traces ≤ Mo ≤ 2.0%
• traces ≤ Ni ≤ 4.5%
• traces ≤ V ≤ 0.5%
• traces ≤ Cu ≤ 3.5% with Cu ≤ Ni% + 0.6 Si% if Cu ≥ 0.5%
• traces ≤ P ≤ 0.200, traces ≤ Bi ≤ 0.200%, traces ≤ Sn ≤ 0.150%, traces ≤ As ≤ 0.100%, traces ≤ Sb ≤ 0.150%, with 0.050% ≤ P% + Bi% + Sn% + As% + Sb% ≤ 0.200%,
• traces ≤ Al ≤ 0.060%
• traces ≤ Ca ≤ 0.050%
• traces ≤ B ≤ 0.01%
• traces ≤ S ≤ 0.0200%
• traces ≤ Te ≤ 0.020%
• traces ≤ Se ≤ 0.040%
• traces ≤ Pb ≤ 0.070%
• traces ≤ Nb ≤ 0.050%
• traces ≤ Ti ≤ 0.050%

the remainder being iron and impurities resulting from processing.

According to a variant of the invention, its Si content is between 0.10% and 1.0%.

The Mn% / Si% ratio is preferably greater than or equal to 0.4.

• on se procure un lopin d'acier de la composition précédente ;
• on lui applique éventuellement un traitement thermique lui procurant une structure primaire globulaire ;
• on le réchauffe à une température intermédiaire entre sa température de solidus et sa température de liquidus, dans des conditions telles que la fraction solide présente une structure globulaire ;
• on réalise un thixoforgeage dudit lopin pour obtenir ladite pièce ;
• et on effectue un refroidissement de ladite pièce.

The invention also relates to a process for hot forming a piece of steel, characterized in that:

• a piece of steel of the above composition is obtained;
• it is optionally applied a heat treatment giving it a globular primary structure;
• it is reheated to a temperature intermediate between its solidus temperature and its liquidus temperature, under conditions such that the solid fraction has a globular structure;
• thixoforging said piece is produced to obtain said part;
• and cooling said part.

Said thixoforgging preferably takes place in an area of temperatures where the fraction of liquid matter present in the plot is between 10 and 40%.

Said cooling is preferably carried out in calm air.

Said cooling can be carried out at a speed lower than that which a natural air cooling would provide.

The invention also relates to a thixoforged steel part, characterized in that it was produced by the above process.

As will be understood, the invention essentially consists in add to a steel for mechanical construction of usual composition a or elements chosen from phosphorus, bismuth, tin, arsenic and antimony, or even silicon, in defined proportions. These analytical modifications make steel particularly well suited for in the form of the part which it constitutes by the thixoforgging process.

The invention will be better understood on reading the description which follows, given with reference to attached FIG. 1 which shows the proportion of phase liquid in steel as a function of temperature for a reference steel and for a steel according to the invention and in FIG. 2 which shows the same quantity for another reference steel / steel pair according to the invention.

To reduce the stress on the tools during thixoforging and make it easier, the skilled person has a first solution which consists, as we said, in lowering the working temperatures thanks to a addition of carbon. This solution lowers the liquidus temperatures and solidus. However, it has the disadvantage of having a significant influence on the mechanical properties of steel.

The inventors imagined that a beneficial effect on the stresses could be obtained by adding elements with a strong tendency to segregation at grain boundaries. This strong segregation is not usually not wanted. Indeed, the fusion of such segregated zones at a temperature lower than the solidus, usually called the burn temperature, is harmful to conventional hot forming operations: rolling and forging.

For a given forging or rolling temperature, lower at the solidus temperature for the matrix of the metal to be deformed, the presence of liquid areas due to segregating elements with low melting points, even with very small volumes (a few%), at the joints of solid grains, go lead to the disintegration of the shaped material: this is the solid part which controls the deformation mechanisms for these shaping processes, and the efforts required for shaping lead to breaks in material (total or partial) harmful to the production of the product and its properties. In the case where the liquid phase is greater than 10%, which is the case in thixoforgeage, the material is two-phase, which leads to a very different behavior during deformation: solid particles are included in liquid and if there are contacts (called bridges) between the solid particles, the very low forces required to break them are only not causes of ruin of the material.

In the case of thixoforging where the burn temperature is largely exceeded, the merger of the segregated zones creates liquid pockets which promote and accelerate the formation of liquid phase within the steel. We have therefore interest in promoting it.

We can thus obtain the quantity of liquid phase necessary for the correct unfolding of thixoforgging at a temperature below that usually necessary when not adding at least one of the phosphorus, bismuth, tin, arsenic or antimony elements when the sum content of these elements is at least 0.050%.

The sum of the elements phosphorus, bismuth, tin, arsenic and antimony should not exceed 0.200% to avoid the aforementioned problems hot rolling or forging to obtain the piece intended for undergo thixoforgging.

Of course, if arsenic is added during the preparation of the liquid metal, all necessary precautions must be taken so that the toxic vapors released are captured so as not to poison the steelworks staff. In fact, the presence of arsenic results most often the addition of copper or tin, which arsenic accompanies generally as an impurity. As arsenic is an element very strongly segregating, it is necessary to take it into account to ensure that conjugation with the other segregating elements, it does not lead to the effects harmful to hot processing which have been cited.

The carbon content of the steels according to the invention can vary between 0.35% and 1.2%. On this condition, one can obtain metallurgical structures, mechanical properties and desirable properties of use for thixoforged steel parts usable in mechanical engineering. Content carbon must be chosen according to the intended use.

The silicon content of the steels of the invention can typically vary between 0.10 and 1.0%, but can go up to 3.0% if we are looking for an effect particularly accentuated by the addition of segregating elements and if the cost of the massive addition of silicon does not seem prohibitive to the manufacturer. As the carbon, silicon lowers the solidus and liquidus and widen the solidification interval. It also has a synergistic effect on the segregation of other elements. It also improves the metal flow.

The manganese content can be between 0.10 and 2.0%. She must be adjusted according to the required mechanical properties, in conjunction with carbon and silicon contents. It has relatively little influence on liquidus and solidus temperatures. But, if the fluidity is high because of a high silicon content (for example 1% or more), a content of Manganese too weak gives the metal mechanical properties insufficient during cooling during continuous casting, resulting in risk of cracking. Such cracks may also appear, for the same reasons, during cooling following thixoforging, all the more so as the large variations in thickness of the part lead to significant differences in local cooling rates. We thus create stresses likely to favor the appearance of cracks if the properties steel mechanics are insufficient. For these reasons, it is preferable that the Mn% / Si% ratio is greater than or equal to 0.4.

The chromium content can be between traces and 4.5%.

The molybdenum content can be between traces and 2.0%.

The nickel content can be between traces and 4.5%.

The adjustment of the contents of chromium, molybdenum and nickel allows ensure the mechanical properties of the parts produced: resistance to rupture, yield strength and resilience.

The vanadium content is between traces and 0.5%.

For some applications where resilience is not important, this element makes it possible to obtain steels with very high mechanical characteristics can be substituted for steels rich in chromium and / or molybdenum and / or nickel, more expensive.

The copper content can be between traces and 3.5%. This element increases mechanical characteristics, improves the corrosion resistance and lower the solidus temperature. It should be noted that if copper is present in high quantities (0.5% and more), the nickel and / or silicon are present in sufficient quantities to avoid problems with hot rolling or forging. It is considered that if Cu% ≥ 0.5%, Cu% ≤ Ni% + 0.6 Si% must be present.

With regard to the segregating elements whose presence is typical of the invention, the sum of the phosphorus, bismuth, tin contents, arsenic and antimony must be at least 0.050% and must not exceed 0.200%. These elements can be present alone or in combination. If they are alone (i.e. the other elements of the list are only present trace amounts), there must therefore be at least 0.050% phosphorus, or 0.050% bismuth, or 0.050% tin, or 0.050% arsenic or 0.050% antimony.

The aluminum and calcium contents, deoxidizing elements, are between traces and respectively 0.060% for aluminum, 0.0050% for calcium.

Boron, a quenching element, has its content between traces and 0.010%.

The sulfur content is between traces and 0.200%. A high content promotes the machinability of the metal, especially if added elements such as tellurium (up to 0.020%), selenium (up to 0.040%) and lead (up to 0.070%). These machinability elements have little influence on the solidus and liquidus temperatures. When sulfur is added in notable quantities, it is good to have a Mn% / S% ratio of at least 4 so that hot rolling takes place without the formation of defects.

Niobium and titanium, when added, help control grain size. Their maximum admissible contents are 0.050%.

Examples of steel compositions according to the invention, and of reference steels which can be used with profit to manufacture thixoforged parts are given in table 1, together with the mechanical characteristics Re (elastic limit) and Rm (tensile strength ) obtained on thixoforged parts after cooling in still air. The percentages are by weight and expressed in 10 ^-3 %, Re and Rm are expressed in MPa. Compositions of samples of steels according to the invention and of reference steels (in 10 ^-3 %) and their mechanical characteristics (in MPa). No. VS mn Yes Cr MB Or V Cu S al P Re rm 1 502 1391 200 164 <5 152 <5 194 315 <0.3 15 423 773 2 493 1451 990 156 <5 152 2 201 302 1 26 510 852 3 505 1420 256 166 <5 159 <5 196 287 3 55 455 856 4 526 1478 255 156 <5 150 <5 200 315 2 97 482 866 5 508 1425 220 164 <5 155 121 203 306 7 58 583 877 6 500 1209 279 153 <5 155 7 204 83 21 99 484 871 7 508 1178 202 108 <5 158 6 204 70 25 187 528 885 8 496 1454 945 156 <5 158 <5 202 291 <0.3 55 498 877

In these examples, the steels of the invention (Nos. 3 to 8) have undergone a addition of phosphorus bringing the content of this element between 0.050 and 0.200%. Through compared to the two reference steels with low phosphorus content (0.015 and 0.026%), there is no deterioration in mechanical properties.

Table 2 shows the composition of a reference steel and a steel according to the invention which is comparable to it, except that phosphorus and a little more silicon have been introduced into it. Compositions of samples of a reference steel and of a steel according to the invention (in 10 ^-3 %) VS mn Yes Cr MB Or Cu V P S al reference 392 1383 523 193 29 87 118 88 8 56 25 invention 396 1405 620 158 21 85 151 89 96 85 2

• pour l'acier de référence, de 1437 à 1468°C ;
• pour l'acier de l'invention, de 1427 à 1463°C.

FIG. 1 represents the liquid phase / solid phase ratio in these steels as a function of the temperature. For the reference steel, the solidus temperature measured is 1415 ° C. whereas it is 1375 ° C. for the steel of the invention. The liquidus temperatures measured are 1525 and 1520 ° C respectively. The addition of phosphorus and silicon therefore played a significant role in the solidus temperature only, but this was enough to significantly increase (by 35 ° C) the solidification interval. It should also be noted that the temperature range in which the liquid fraction of the steel is between 10 and 40% and which is usually considered to be the most favorable for thixoforging, is:

• for the reference steel, from 1437 to 1468 ° C;
• for the steel of the invention, from 1427 to 1463 ° C.

We therefore observe a reduction of around 5 to 10 ° C in this interval, and a 5 ° C widening of its magnitude, all things that go in the sense of less stress on the tools during thixoforgging, and a greater ease of obtaining conditions favorable to the smooth running of the operation. This effect would be accentuated if we increased the amount of phosphorus added, or if we also added other segregating elements in the limits that have been said.

Table 3 shows the composition of a reference steel and a steel according to the invention which is comparable to it, except that phosphorus, silicon, manganese have been introduced therein (to compensate for the addition of silicon, so as to maintain a suitable Mn% / Si% ratio) and sulfur. Compositions of samples of a reference steel and of a steel according to the invention (in 10 ^-3 %) VS mn Yes Cr MB Or Cu P S al reference 0.377 0,825 0.19 0,167 0,039 0.113 0.143 0,007 0,009 0,022 invention 0.396 1,405 0.62 0.158 0,021 0,085 0,151 0,095 0,085 0,002

• pour l'acier de référence, de 1470 à 1494°C,
• pour l'acier de l'invention, de 1428 à 1464°C.

FIG. 2 represents the liquid phase / solid phase ratio in these steels as a function of the temperature. For the reference steel, the solidus temperature measured is 1430 ° C., while it is 1378 ° C. for the steel of the invention. The liquidus temperatures measured are 1528 ° C and 1521 ° C respectively. The solidification interval has therefore been widened by 45 ° C. The temperature range in which the solid fraction of steel is between 10 and 40% is:

• for the reference steel, from 1470 to 1494 ° C,
• for the steel of the invention, from 1428 to 1464 ° C.

There is therefore a drop in the order of 30 to 42 ° C of this interval and a 12 ° C increase in its magnitude.

About the determination of solidus and liquidus to be taken into account for the implementation of the invention, it is necessary note that they may not always coincide with those that we calculates from steel composition using available formulas conventionally in the literature. Indeed, these formulas are valid in the case of a transition from liquid steel to solid steel during solidification and steel cooling and for cooling rates of a few degrees per minute.

In the case of measurements made for application to thixoforgeage the measurements must be carried out starting from solid steel and going towards liquid steel, i.e. in the case of reheating and then steel melting. The tests are also carried out with conditions increase in temperature of the order of several tens of degrees per minute, corresponding to the heating conditions prior to the operation of Thixoforging.

Conventionally, carrying out the thixoforgging operation on the steels of the invention must be preceded by a heat treatment of globalization of the primary plot structure if a globular structure is not not already present and if experience shows that it cannot be obtained during the reheating of the plot for its thixoforging. Obtaining such globular structure before thixoforging for a steel of composition and history data can be checked if the piece is suddenly cooled before to have carried out its thixoforgeage. We then observe the structure as it was before cooling.

Regarding the next room cooling operation its thixoforgeage, this cooling must be carried out in calm air, and not forced manner, in the case, frequent for this kind of parts, where the part exhibits very large cross-sectional variations, for example when thin walls (1 to 2mm) are connected to thick areas (5 to 10mm or more). The use of supply air is, in this case, to be avoided because there is a risk then to introduce very significant residual constraints between thin walls and thick areas. This would result in surface defects degrading the properties of the thixoforged part.

In some cases it may be necessary to slow the cooling of the parts to favor the structural homogeneity of the different parts of the room. We can, for this purpose, pass the part in a temperature-regulated tunnel in the range 200-700 ° C for example.

But if the thixoforgée piece does not present such variations of large section, it may be tolerable to perform air cooling breath. Such cooling can be favorable for obtaining a structure homogeneous metallurgy in the part section and good mechanical characteristics.

Claims (8)

Steel for mechanical construction, characterized in that its composition is, in weight percentages: 0.35% ≤ C ≤ 1.2% 0.10% ≤ Mn ≤ 2.0% 0.10% ≤ If ≤ 3.0% traces ≤ Cr ≤ 4.5% traces ≤ Mo ≤ 2.0% traces ≤ Ni ≤ 4.5% traces ≤ V ≤ 0.5% traces ≤ Cu ≤ 3.5% with Cu ≤ Ni% + 0.6 Si% if Cu ≥ 0.5% traces ≤ P ≤ 0.200, traces ≤ Sn ≤ 0.150%, traces ≤ As ≤ 0.100%, traces ≤ Sb ≤ 0.150%, with 0.050% ≤ P% + Sn% + As% + Sb% ≤ 0.200%, traces ≤ Al ≤ 0.060% traces ≤ Ca ≤ 0.050% traces ≤ B ≤ 0.01% traces ≤ S ≤ 0.200% traces ≤ Te ≤ 0.020% traces ≤ Se ≤ 0.040% traces ≤ Pb ≤ 0.070% traces ≤ Nb ≤ 0.050% traces ≤ Ti ≤ 0.050% the remainder being iron and impurities resulting from processing. Steel according to claim 1, characterized in that its Si content is between 0.10% and 1.0%. Steel according to claim 1 or 2, characterized in that the Mn% / Si% ratio is greater than or equal to 0.4. Process for hot forming a piece of steel, characterized in that : a piece of steel of composition is obtained in percentages by weight: 0.35% ≤ C ≤ 1.2% 0.10% ≤ Mn ≤ 2.0%, preferably with Mn% / Si% ≥ 0.4 0.10% ≤ If ≤ 3.0%, preferably 0.10% ≤ If ≤ 1.0%. traces ≤ Cr ≤ 4.5% traces ≤ Mo ≤ 2.0% traces ≤ Ni ≤ 4.5% traces ≤ V ≤ 0.5% traces ≤ Cu ≤ 3.5% with Cu ≤ Ni% + 0.6 Si% if Cu ≥ 0.5% traces ≤ P ≤ 0.200, traces ≤ Sn ≤ 0.150%, traces ≤ As ≤ 0.100%, traces ≤ Sb ≤ 0.150%, with 0.050% ≤ P% + Bi% + Sn% + As% + Sb% ≤ 0.200%, traces ≤ Al ≤ 0.060% traces ≤ Ca ≤ 0.050% traces ≤ B ≤ 0.01% traces ≤ S ≤ 0.200% traces ≤ Te ≤ 0.020% traces ≤ Se ≤ 0.040% traces ≤ Pb ≤ 0.070% traces ≤ Nb ≤ 0.050% traces ≤ Ti ≤ 0.050% the rest being iron and impurities resulting from the production, it is optionally applied a heat treatment giving it a globular primary structure; it is reheated to a temperature intermediate between its solidus temperature and its liquidus temperature, under conditions such that the solid fraction has a globular structure; thixoforging said piece is produced to obtain said part; and cooling said part. A method according to claim 4, characterized in that said thixoforging takes place in a temperature zone where the fraction of liquid material present in the piece is between 10 and 40%. Method according to claim 4 or 5, characterized in that said cooling is carried out in still air. Method according to claim 6, characterized in that said cooling is carried out at a speed lower than that which would be obtained by natural air cooling. Thixoforged steel part, characterized in that it was manufactured by the method according to one of claims 4 to 7.

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