MXPA98000154A - Steel and procedure for the manufacture of a piece of steel conformed by plastic deformation in f - Google Patents

Abstract

Steel for the manufacture of a piece of steel formed by plastic deformation whose chemical composition comprises, by weight: 0.03% == 2.2% and Al + Ti > = 3.5 x N. Procedure for the manufacture of a piece of steel formed by cold plastic deformation and part obtained

Description

STEEL AND PROCEDURE FOR THE MANUFACTURE OF A PIECE OF STEEL CONFORMED BY DEFORMATION PLASTIC IN FRIÓ DESCRIPTION OF THE INVENTION The invention relates to a steel and to a process for the manufacture of. a piece of steel formed by cold plastic deformation. Numerous steel parts, and in particular high-grade mechanical parts, are manufactured by forging-cold or cold hammering, and more generally by cold plastic deformation of hot rolled steel blanks. The steel used has a carbon content between 0.2% and 0.42% (by weight). It is alloyed either with chromium, sometimes with chromium-molybdenum, sometimes with nickel-chromium, sometimes with nickel-chromium-molybdenum, sometimes with manganese-chromium, so that it is sufficiently hardenable to permit, after hardening, a martensitic structure, structure necessary to obtain after tempering, the desirable mechanical characteristics that are, on the one hand, a high tensile strength, and on the other a good ductility. To be able to REF: 26607 be cold formed, the steel must be subjected, previously, to a thermal treatment of globulization or "maximum adulteration", which consists in keeping it at a temperature above 650 ° C for a long time, which can reach several tens of hours. This treatment gives the steel a globular pearlitic structure, easy to deform in cold. This technique has the disadvantage, in particular, of needing three heat treatments, which complicates manufacturing and increases costs. The object of the present invention is to remedy this drawback by proposing a means for manufacturing, by means of cold plastic deformation, a piece of steel mechanics, with high characteristics, without it being necessary to carry out a thermal treatment of globulization or maximum adulteration, nor a thermal treatment of tempering. For this purpose, the invention relates to a steel for the manufacture of a piece of steel formed by cold plastic deformation, the chemical composition of which comprises, by weight: 0.03% < C < 0, 16% • 0.5% < Mn < 2% O, 05% < Yes < 0, 5% 0% < Cr < 1, 8% 0% < Mo < 0, 25% 0, 001% < To < 0, 05% 0, 001% < You < 0, 05% 0% < V < 0, 15% 0.0005% < B < 0, 005% 0, 004% < N < 0, 012% 0.001% < S < 0, 09% - eventually, up to 0.005% of calcium, up to 0.01% of tellurium, up to 0.04% of selenium, up to 0.3% of lead, the rest being iron and impurities that result from processing and satisfying the chemical composition of the steel, in addition, the ratios: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V > 2.2% and Al + Ti > 3.5 x N Preferably, the chemical composition of the steel is such that: 0, 06% < C < 0, 12% 0, 8% < Mn < 1, 7% 0.1% < _ If < 0, 35% 0, 1% < Cr < 1, 5% 0.07% < Mo < 0.15% O, 001% < At 0, 035%, 0.001% < You < 0, 03% 0% < V < 0, 1% 0, 001% < B < 0, 004% 0, 004% < N < 0, 01% 0, 001% < S < 0, 09% eventually, up to 0.005% calcium, up to 0.01% tellurium, up to 0.04% selenium, up to 0.3% lead, the rest being iron and impurities that result from processing. It is preferable that the contents of impurities or residual elements are, simultaneously or separately, such that: Ni < 0.25% Cu < 0, 25% P < 0.02% The invention also relates to a process for the manufacture of a piece of steel formed by cold plastic deformation, which entails a quenching as the only thermal treatment. The term "temper" is used in the present specification in a broad sense, that is, it is a sufficiently rapid cooling to obtain a structure that is not practically ferritoperlítica and that is not essentially martensitic. In addition to tempering, the procedure consists in hot rolling a semi-steel product to obtain a hot rolled product, optionally in cutting a blank in the hot rolled product, and in shaping the blank or the rolled product by cold plastic deformation. The temper, which is intended to give the piece a bainitic structure, can be made both before and after the cold forming. When carried out before the cold forming, quenching can be performed both directly on the rolling charge, and after the austenitization by reheating above AC3. When performed after cold forming, tempering is performed after austenitization by reheating above AC3. The invention relates, finally, to a piece of steel obtained by cold forming, constituted by a steel according to the invention, such that the Z-stiffness of the steel is greater than 45%, preferably greater than 50%, and the tensile strength Rm exceeding 650 MPa, and even, for certain applications, exceeding 1,200 MPa. In general, and this would be desirable, the piece has an essentially bainitic structure, that is, constituted in more than 50% by bainite. The invention will now be described in more detail and will be illustrated by the following examples. The chemical composition of the steel, according to the invention, comprises, in% by weight: - from 0.03% to 0.16% and, preferably, from 0.06% to 0.12% carbon to obtain an important aptitude to be hardened by cold deformation when cold formed, to avoid the formation of coarse carbides unfavorable for ductility, and to allow a cold forming without the need for an annealing of globulization or maximum adulteration; - from 0.5% to 2% and, preferably, from 0.8% to 1.7% manganese, in order to guarantee a good ability to be cast, obtain sufficient aptitude to be tempered and the desired mechanical characteristics; - from 0.05% to 0.5% and, preferably, from 0.1% to 0.35% silicon, a necessary element to ensure the deoxidation of the steel, in particular when the aluminum content is low, but that, in very high quantity, it favors hardening that is detrimental to cold forming and to ductility; - from 0% to 1.8% and, preferably, from 0.1% to 1.5% of chromium to adjust the ability to be tempered and the mechanical characteristics at the desired level for the parts, without exceeding a value that would harden too much steel in the raw state of rolling or would lead to the formation of martensite detrimental to the ability to be cold formed and to ductility; - from 0% to 0.25% and, preferably, from 0.07% to 0.15% molybdenum to, in synergy with the boron, ensure an ability to be homogenously tempered in the various sections of the piece; - optionally, from 0% to 0.15% and, preferably, less than 0.1%. of vanadium to obtain high mechanical characteristics (resistance to traction) when these are desired; - from 0.0005% to 0.005% and, preferably, from 0.001% to 0.004% boron to increase the necessary aptitude to be tempered; - from 0% to 0.05% and, preferably, from 0.001% to 0.035% aluminum, and from 0% to 0.05%, and preferably from 0.001% to 0.003% titanium, the sum of which must be aluminum and titanium content greater than or equal to 3.5 times the nitrogen content, in order to obtain the fine grain necessary for a good cold forming ability and good ductility; - from 0.004% to 0.012% and, preferably, from 0.006% to 0.01% nitrogen, to control the grain size by forming aluminum, titanium or vanadium nitrides, without forming boron nitrides; more than 0.001% sulfur in order to guarantee a minimum ability to be worked, to allow final touches on the pieces, but less than 0.09% to guarantee a good capacity for cold forming; the aptitude to be worked, combined with a good aptitude for the conformation by cold plastic deformation, can be improved both by the addition of calcium up to 0.005%, and by the addition of tellurium up to 0.01%, in which case it is preferable that the Te / S ratio remains close to 0.1, or by the addition of selenium up to 0.05%, in which case it is preferable that the selenium content remain close to the sulfur content, or, finally, by the addition of lead to 0.3%, in which case, the sulfur content should be reduced; the rest is iron and impurities, which result from processing.

The impurities are, in particular: phosphorus, the content of which must remain preferably less than or equal to 0.02% in order to guarantee good ductility during and after cold forming; copper and nickel, cdered as residual, whose content, preferably, must remain below 0.25% for each one. Finally, the chemical composition of the steel must satisfy the ratio: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V > 2.2% guaranteeing that • the combination of the contents of manganese, chromium, molybdenum and vanadium allows obtaining the desired resistance characteristics and an essentially bainitic structure. This steel has the advantage of having a good aptitude for cold plastic deformation and allows obtaining, without it being necessary to carry out an annealing, a bainitic type structure having excellent ductility and high mechanical characteristics. In particular, the ductility can be measured by Z-stiffness that is greater than 45% and even higher than 50%. The tensile strength Rm is greater than 650 MPa and may exceed 1,200 MPa. These characteristics can be obtained both when tempering is carried out in the rolling load before the cold forming, and when it is carried out after austenitization by heating above AC3, before or after the cold forming. To manufacture a cold formed part, a steel semi-product according to the invention is provided and hot rolled after reheating above 940 ° C, in order to obtain a hot rolled product such as a bar , a billet or a machine thread. In a first embodiment, the hot rolling is finished at a temperature comprised between 900 ° C and 1050 ° C and the hot rolled product is directly tempered in the rolling load by cooling with blown air, in oil, with fog, with water or with water added with polymers, depending on its section. The product thus obtained is then cut into blanks, then cold formed, for example, by cold forging or cold hammering. The final mechanical characteristics, obtained directly after cold forming, result in particular from the cold deformation hardening generated by the cold forming operation. In a second embodiment, after the hot rolling, or the rolled product is tempered after austenitization and then cut into blanks which are formed by cold plastic deformation, or the ingots are cut before making tempering and then cold forming. In both cases, the austenitization csts of a heating between AC3 and 970 ° C, and the tempering is carried out by cooling with blown air, in. oil, in fog, with water or with water added with polymers, depending on the section of the product. The final mechanical characteristics, obtained directly after cold forming, result in particular from the cold deformation hardening engendered by the shaping operation. In this embodiment, the end of rolling conditiare not particularly important. In a third embodiment, the cold forming operation is carried out on a blank cut in the hot rolled product and the hardening is carried out after cold forming. As in the previous case, tempering is carried out after heating between AC3 and 970 ° C and by cooling with blown air, in oil, fog, water or water added with polymers. The rolling end conditions also have no particular importance. The invention, which is more particularly intended for the manufacture of mechanical parts, also applies to the manufacture of cold-drawn bars, drawn wires and unwound machine threads, with cold drawing, drawing and unwinding in particular ways. conformation by cold plastic deformation. The drawn bars and the drawn machine wires can be dehusked, deburred or rectified in order to present a surface condition free of defects. The expression "piece of cold-formed steel" covers all these products, and the term "blank" includes, in particular, any piece of bar or thread; in certain cases, the bars or threads are not cut into rough pieces before being cold formed. The invention can, in short, be used to manufacture pre-treated bars or pre-treated yarns, or more generally pre-treated steel products, intended to be used in the state for the manufacture of parts for cold forming without additional heat treatment. These steel products are tempered after hot rolling, either directly in the rolling load, or after austenization, in order to present an essentially bainitic structure (bainite> 50%). It can be de-crushed or deburred to present a surface condition free of defects. The invention will now be illustrated by examples.

First Example: A steel according to the invention was produced whose chemical composition comprised, by weight: C = 0.065% Mn = 1.33% Si = 0.34% S 0.003% P = 0.014% Ni = 0.24% Cr = 0.92% Mo = 0.081% Cu = 0.23% V = 0.003% Al = 0.02% Ti = 0.02% N = 0.008% B = 0.0035% who fulfilled, therefore , conditions: 'Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V = 2.27% > 2.2% and Al + Ti = 0.040% > 3.5 x N = 0.028% With this steel, packages were made that were hot rolled after reheating above 940 ° C to form rounds (or rods) of 16 mm, 25.5 mm and 2-4.8 mm diameter. 1) Round of 16 mm in diameter: The rolling of rounds of 16 mm in diameter has been finished at 990 ° C and the rounds have been tempered in the rolling load in the following three conditions (according to the invention): : cooling at the speed of 5.3 ° C / s equivalent to quenching with blown air, B: cooling at the speed of 26 ° C / s equivalent to oil quenching, C: cooling at the speed of 140 ° C / s equivalent to water tempering. The mechanical characteristics before the cold forming of the hardened rounds and their ability to be formed by cold plastic deformation have been evaluated by tensile tests and by torsion tests until cold break (the results of the torsion tests are expressed in "number of turns before the test piece breaks"). The results have been the following: The hardness and tensile strength, which vary considerably with the hardening conditions, are all the greater the higher the cooling rate. However, in all cases, the ductility and the capacity for cold deformation are excellent, since the Z stiffness is always substantially greater than 50%, and the number of turns until breaking is always clearly greater than 3. In order to determine the mechanical characteristics that it is possible to obtain on pieces manufactured by cold plastic deformation from these same rounds, cold torsion-traction tests have been carried out, the results of which are as follows: The cold torsion-traction test consists of subjecting a test piece to 3 turns of cold torsion to simulate the conformation by plastic deformation, before performing a tensile test at room temperature. The increase in strength corresponds to the hardening by cold deformation relative to the resistance between the state hardened by cold deformation (after 3 turns of torsion) and the normal state (before the 3 turns of torsion). The results obtained show that, even after an important cold deformation (three turns of twist), the stiffness remains higher than 50%, and that the tensile strength can exceed 1,200 MPa. The capacity of hardening by deformation in cold, measured by the increase of. resistance after deformation by cold tension, is high in all cases. 2) Round of 25.5 mm in diameter: Round rods of 25.5 mm in diameter were tempered before cold forming, after austenitization at 950 ° C, under the following conditions (according to the invention): D: cooling with blown air (average cooling speed of 3.3 ° C / s between 950 ° C and room temperature) E: cooling in oil (average cooling speed of 22 ° C / s between 950 ° C and the room temperature) F: cooling in water (average cooling speed of 86 ° C / s between 950 ° C and room temperature). The rounds have been subjected to cold forging tests consisting of the measurement of the Limit Crushing Percentage (P.A.L.) by crushing notched cylinders according to a generatrix. The Percentage of Crushing Limit, expressed in%, is the percentage of crushing above which appears the first fissure by cold forging in the press in the notch made according to the generatrix of the cylinder. By way of comparison, P.A.L. it has also been measured on a steel for cold forging according to the prior art whose composition was: C = 0.37% Mn = 0.75% Si = 0.25% S = 0.005% Cr = 1% Mo = 0 , 02% Al = 0.02% This steel according to the prior art had previously been subjected to a pelleting annealing of the pearlite to make it suitable for cold deformation. The results obtained have been the following: In view of the Limit Crushing Percentages, it appears that the steel according to the invention has an ability to shape by cold forging substantially more important than steel according to the prior art, despite a higher hardness, and this is whatever the level of resistance, even if it is high (treatment F). 3) Round of 24.8 mm in diameter: After rolling and before cold forming, rounds of 24.8 mm in diameter have been annealed after austenitization at 930 ° C under the following conditions according to the invention: G: quenching with blown air H: quenching in oil The rounds thus treated have been forged cold to manufacture automobile wheel loaders whose mechanical characteristics obtained were the following: Of these . results it follows that, whatever the initial treatment, the ductility obtained on cold forged piece is very high (Z> 50%), and, this, regardless of the resistance level. On the other hand, in both cases the rounds were completely suitable for cold forging, since the pieces have been shown to be free of any internal or external defect. With other rounds of 24.8 mm in diameter (identical to the previous ones), the same winches were manufactured by cold forging of the rough rolling mills, carrying out the hardening after the cold forming operation. The quenching was carried out in water after austenitization at 940 ° C. Under these conditions, the characteristics obtained on trunnions were the following: Rm 1.077 MPa Z = 73% These results show that with the steel according to the invention, making a hardening after cold forging a hot round of hot rolling, very good ductility (Z> 50%) can be obtained despite a high resistance level. On the other hand, the steel according to the invention has proved perfectly suitable for forming by cold forging in the raw state of rolling without needing previous globulization treatment as is practiced with the steels according to the prior art, the freeloaders have been revealed, in effect, exempt from any internal or external defect. By way of comparison, according to the prior art, it is used to manufacture the same frets, a steel of composition: C = 0.195% Mn = 1.25% Si = 0.25% S 0.005% Ni 0, 25% Cr 1, 15% Mo 0.02% Cu 0.2% Al 0.02% In order to obtain mechanical characteristics similar to those obtained with the invention, it is necessary to use the following manufacturing range: Globular annealing of steel for make it suitable for cold forming. • Cold forging of the freeloaders. • Temper in steel oil according to the prior art. . • Steel recovered according to the prior art.

Second example: Equally. Mechanical parts were manufactured by cold hammering using steels 1 and 2 according to the invention whose chemical compositions were in% by weight: which fulfill, in this way, the conditions: For steel 1: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V = 2.43 > 2.2% Al + Ti = 0.045% > 3.5 x N = 0.024% For steel 2: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V = 2.59 > 2.2% Al + Ti = 0.041% > 3.5 x N = 0.028% According to the invention, these steels have been hot-rolled in the form of bars of 28 mm in diameter. After rolling and before cold forming, the bars have been subjected to a tempering treatment in warm oil at 50 ° C after austenitization at 950 ° C. The bars have been cut to obtain blanks from which the pieces have been formed by cold hammering at the deformation percentage of 60%. The mechanical characteristics obtained on the rough pieces before the cold hammering and on the pieces after the cold hammering. They were the following: (*) = ability to harden by cold forming.

These results show that the ductility is high (Z> 50%) despite a very high percentage of cold deformation, this independent of the initial resistance level (before cold hammering) and final resistance (after cold hammering) ) of steel, even if the final resistance level is very high. They also show that the ability to harden by cold deformation, measured by the increased resistance to cold hammering, is important. On the other hand, the ability to form by cold hammering is excellent since, despite the high initial resistance levels and a high deformation (60%) in the cold, the cold hammered pieces have been shown to be free from defects. internal as well as external. These examples show that the steel and the processes that follow the invention allow to obtain a very good ductility (Z> 50%) for the manufacture of a piece formed by cold plastic deformation, without it being necessary to carry out an expensive treatment of globulization nor a treatment of tempering. This high ductility (Z> 50%) combined with mechanical properties on very high parts (Rm> 1,200 MPa) can be obtained especially thanks to the high capacity of hardening by cold deformation of the steel. In short, the very good aptitude even if the level of resistance (or hardness) of the steel and the percentage of cold deformation are high.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (11)

1. Steel for the manufacture of a piece of steel formed by cold plastic deformation, characterized in that its chemical composition comprises, by weight: 0, 03% < C < 0, 16% 0, 5% < _ Mn < 2% 0.05% < Yes < 0.5% 0% < Cr < 1, 8% 0% < Mo < 0, 25% 0, 001% < To < 0.05% 0.001% < You < 0, 05% 0% < V < 0, 15% 0.0005% < B < 0.005% 0.004% < N < 0.012% 0.001% < S < 0, 09% - eventually, up to 0.005% of calcium, up to 0.01% of tellurium, up to 0.04% of selenium, up to 0.3% of lead, the rest being iron and impurities that result from processing and satisfying the chemical composition of the steel, in addition, the ratios: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V > 2.2% Al + Ti > 3, 5 x N

2. Steel according to claim 1, characterized in that its chemical composition is such that: 0, 06% < C < 0, 12% 0, 8% < Mn < 1, 7% 0, 1% < Yes < 0, 35% 0, 1% < Cr < 1, 5% 0, 07% < _ Mo < 0, 15% 0, 001% < To < 0, 035% 0, 001% < You < 0.03% 0% < V < _ 0, 1% 0, 001% < B < 0, 004% 0, 004% < N < 0, 01% 0, 001% < S < 0, 09% eventually, up to 0.005% of calcium, up to 0.01% of tellurium, up to 0.04% of selenium, up to 0.3% of lead, the rest being iron and impurities that result from processing.

3. Steel according to claim 2, characterized in that its chemical composition is such that: Ni < 0, 25% Cu < 0.25%

4. Steel according to claim 2 or claim 3, characterized in that its chemical composition is such that: P < 0.02%

5. Process for the manufacture of a piece of steel formed by cold plastic deformation, characterized in that: - a steel semi-product according to any of claims 1 to 4 is provided, the semi-product is hot-rolled after having been reheated to a temperature higher than 940 ° C and the rolling is finished at a temperature between 900 ° C and 1,050 ° C in order to obtain a rolled product, the rolled product is tempered directly in the rolling load in order to confer an essentially bainitic structure, - possibly, a blank in the rolled product is knocked off, - and the blank or the rolled product is formed by cold plastic deformation to obtain the part having its final mechanical characteristics.

6. Process for the manufacture of a piece of steel formed by cold deformation, characterized in that: a steel semi-product according to any of claims 1 to 4 is provided, - the semi-product is hot-rolled in order to obtain a rolled product, - the rolled product is tempered after it has been. reheated above the point AC3, in order to confer an essentially bainitic structure, - eventually a blank is cut in the rolled product, - and is formed by cold plastic deformation. blank or the rolled product to obtain the part that has its final mechanical characteristics.

7. Process for the manufacture of a piece of steel formed by cold plastic deformation, characterized in that: a steel semi-product according to any of claims 1 to 4 is provided, - the semi-product is hot-rolled for the purpose to obtain a rolled product, - optionally, the blank is cut in the rolled product, the blank is formed by cold plastic deformation or the rolled product to obtain the part, and the piece is tempered after having reheated above point AC3, in order to confer an essentially bainitic structure and its final characteristics.

8. Cold formed steel part, characterized in that it is constituted by a steel according to any of claims 1 to 4, because the Z-stiffener of the steel is superior to 45% and because the tensile strength Rm of the steel is superior to 65Q. MPa.

9. . Piece according to claim 8, characterized in that the tensile strength of the steel is greater than 1,200 MPa.

10. Piece according to claim 8 or claim 9, characterized in that it has an essentially bainitic structure.

11. Hot-rolled steel product, characterized in that it is constituted by a steel according to any of claims 1 to 4 and because it has an essentially bainitic structure.