WO1985004906A1

WO1985004906A1 - Method for producing steel bars or rod wire and corresponding products

Info

Publication number: WO1985004906A1
Application number: PCT/FR1985/000094
Authority: WO
Inventors: Bernard Heritier; Philippe Maitrepierre; Jaime Rofes-Vernis
Original assignee: Ugine Aciers
Priority date: 1984-04-24
Filing date: 1985-04-24
Publication date: 1985-11-07
Also published as: FR2563236A1; JPS60238419A; EP0179832A1; FR2563236B1

Abstract

Method for producing bars or rod wire having in the raw hot transformation state specially high mechanical characteristics. The method for producing steel bars or rod wire having after hot bonding and cooling a bainitic structure and characteristics: R >= 1000 Mpa, E >= 600 Mpa, and resilience KCU at 20oC >= 60 J/cm2, utilizes a steel containing substantially (% by mass): C 0.07-0.13; Mn 3.0-4.5; Si 0.1-1.0; Nb 0.03-0.12; B and/or Ti verifying either of the two following conditions: a) B >= 0.0010% and B + Ti/10 = 0.0020 to 0.0040%; b) Ti <= 0.010% and B = 0.0020 to 0.0080% and comprises a hot bonding terminated under 950oC. The products obtained by the method of the invention are used for example in the raw state of hot bonding as bars for oil drilling devices, and in a drawing temper state for machining, welding or cold or lukewarm conformations.

Description

METHOD FOR MANUFACTURING STEEL BARS OR MACHINE WIRE AND RELATED PRODUCTS

The method of the present invention relates to a method of manufacturing bars or wire rod of steel having in the raw state of hot transformation mechanical properties that are especially high, and further having after tempering, a set of mechanical characteristics and properties. more interesting to use than that traditionally obtained with heat treated steel bars.

PRESENTATION OF THE PROBLEM There is a great need for bars with high mechanical characteristics, for example "petroleum bars", used in particular in petroleum drilling devices, having the following mechanical characteristics:

- breaking load greater than or equal to 965 MPa (i.e. 140,000 psi or 98.4 kg / mm ² );

- resilience KCU long direction at 20 ° C greater than 60 J / cm ² (i.e. 3430 in. lb / in ² ).

There is also a significant need for pretreated bars, with a breaking load greater than 900 MPa and of good resilience, which can be implemented in various ways among which mention may be made of machining, welding and cold or lukewarm conformations. , such as mastering.

In both cases, the bars often have a diameter greater than 80 mm, and for all requirements the diameters or thicknesses can range from 4 mm to 210 mm, whether they are erect or slightly straight bars. or crown wires.

In the first case, CrMo steel bars are commonly used (typical composition: C 0.42%, Cr 1%, Mo 0.25% - designation "42CD4" according to standard AFNOR NF A 35-557 of June 1975 ), close to grade AISI 4145H, in the quenched-tempered state.

In the second case, that of the pretreated bars, we also use Cr-Mo steels such as "35CD4" (C 0.35%, Cr 1%, 0.25% Mo) or "42CD4" already mentioned, in the quenched-tempered state with the most tempering temperature often between 550 and 650 ° C. These hardening steels are difficult to weld and do not lend themselves well to cold conformation. Processing operations other than machining require a final heat treatment.

In the two cases examined, relatively expensive steels are thus used because of their Cr and Mo contents and because of the necessary heat treatments. In addition, these steels are poorly suited to processing operations other than machining.

The Applicant has sought to develop an economical manufacturing process which makes it possible to satisfy these two types of need (bars with high mechanical characteristics such as "petroleum bars" on the one hand, and pretreated bars on the other hand), products obtained satisfying the various requirements of users: mechanical resistance, resilience, machinability, ability to be cold or lukewarm, weldability. -

KNOWN PRIOR ART

In US Pat. No. 3,518,080, steels having very good mechanical characteristics in the raw state of hot rolling are described, having the following composition: 0.01 to 0.04% C; 2.2 to 6% Mn; If 1% maximum; Al soluble 0.2% maximum; 0.01 to 0.2% of at least one element from the group consisting of (Nb, V, Zr); impurities and Fe = the balance. The mechanical characteristics measured on laminated bars of cross section 12 x 12 mm, are at the level of 4 to 5% Mn: elastic limit at 0.2% = 76 to 84 kg / mm ² and breaking load = 88 at 104 kg / mm ² . It is indicated that the mechanical resilience of the steels is "relatively independent of the end of rolling temperature", and examples of the end of rolling temperature between 860 and 770 ° C. are given. However, these are straight section bars or thin thickness vis-à-vis most of the bars studied in the present application.

In METALLURGICAL TRANSACTIONS - Vol.2, Dec. 1971, "The Effect of the Transformation Substructure on the Mechanical Properties of a Low-Carbon Manganese Martensitic Steel "p.3490, it is stated about the same steels at 3.5 and 4.5% Mn + 0.1% Nb, that" the resilience properties deteriorate considerably when the carbon content exceeds about 0.04% ". Although the steels below meet the technical requirements set out at the beginning of this description, their very low C content entails the obligation of an expensive special preparation.

STATEMENT OF THE INVENTION The Applicant has discovered during its studies and its tests a surprising fact of great importance: a weak addition of B to a steel at 4% Mn, 0.1% Nb and 0.1% C provides good resilience properties. It is also possible to use an addition of Ti, but the quantity of Ti seems critical and its adjustment is therefore difficult in practice. Therefore, as an alternative to adding B alone, an addition of B and Ti is recommended. The effect of B and / or Ti on the resilience is explained by a probable action on the nitrogen contained, such as the fixation of free nitrogen. Furthermore, the Applicant has clarified that, in the case of the steels of the invention, the forging or rolling carried out below 950 ° C. at the end of the hot working has a great influence on the resilience.

Finally, the applicant has shown that using an income treatment it is possible to adapt the mechanical characteristics to the specifications of the client, possibly increasing the elastic limit, and improving the machinability and the suitability for conformation. .

An essential advantage of the steel of the invention is that it can be produced in an electric steelworks, and obtained directly by hot working in a wide range of dimensions, from 4 mm to 210 mm in diameter or thickness.

The invention thus relates to a method of manufacturing bars or steel wire rod having, after hot working and cooling, an essentially bainitic structure and high minimum characteristics: - breaking load: R ≥ 1000 MPa

- elastic limit at 0.2%: E ≥ 600 Mpa

- resilience at 20 ° C: KCU ≥ 60 J / cm this steel, preferably produced in an electric steelworks, containing in% by mass:

- C 0.07 to 0.13%; Mn 3.0 to 4.5%; If 0.1 to 1.0%; Nb 0.03 to 0.12%; B and / or Ti satisfying one or the other of the two following conditions:

b) Ti≤ 0.010% and B = 0.0020 to 0.0080%

N ≤ 0.015%; Ni + Cr + Mo + Cu ≤ 1.0% (with preferably Ni≤ 0.3%;

Cr≤ 0.5% and Mo ≤ 0.1%); Al≤ 0.08%; S ≤ 0.090%; Te≤ 0.015%;

P ≤ 0.020%; other elements and Fe: the balance; and the end of the hot working is preferably carried out below 950 ° C. The "other elements" are in the usual contents for an elaboration in electric steelworks from scrap, their total is usually less than 0.5%. At the end of the hot working, the magnification of the grain of the austenite is then avoided and a bainitic structure resulting from the transformation of the hardened or fine-grained austenite into bainite is obtained after cooling, with a greatly improved resilience.

Preferential composition intervals have been determined for the following elements, taken separately or in any combination:

- C: preferably 0.08 to 0.11%

- Mn: preferably 3.1 to 3.6% when one is satisfied with the following minimum mechanical characteristics in the raw state of hot rolling or forging:. R ≥ 1000 MPa; E ≥ 600 MPa; resilience at 20 ° C. KCU ≥ 60 J / cm ² and preferably 3.6 to 4.3% with S + Te ≤ 0.015% to obtain higher mechanical characteristics in the same state:. R ≥ 1100 MPa; E ≥ 750 MPa; resilience at 20 ° C. KCU ≥ 70 J / cm ²

- If: preferably 0.2 to 0.9%; there does not seem to be a marked influence of Si on the mechanical characteristics in this range of contents and we can therefore maintain it; - Nb: preferably 0.05 to 0.10%. The role of Nb in manganese steels is similar to that observed in microalloyed steels: according to the publications made, Nb delays the recrystallization of austenite and allows hardening by precipitation. It is also known that the addition of Mn produces an increase in the solubility of Nb in austenite (publication by MGABKEN, J.WEISS and JJJONAS in Acta Metallurgica, 29, 1981, p.121).

- The addition of B and / or Ti which surprisingly improves resilience has no perceptible effect on the microstructure in optical micrography. A likely mechanism is an action on nitrogen, which for its part should preferably be kept at a fairly low level because it has an unfavorable effect on resilience. The action of B and / or Ti is not well understood, tests show that the addition of B is preferable to the addition of Ti for industrial production. The additions of B and of Ti can however be combined while keeping Ti preferably between 0.010 and 0.030%, within the framework of the first condition (a):

The preferred range B = 0.0030 to 0.0070% corresponds to the main tests carried out.

- N: preferably between 0.003 and 0.010%, which is easily obtained in an electrical steelworks, equipped with a degasser.

- The additions of S, Te, can be adjusted as follows to obtain improved machinability in the raw state of hot working or in the annealed state, possibly with straightening, stretching, or cold or temperature conformation below 300 ° C after hot working or after tempering: S 0.015 to 0.090% and Te ≤ 0.015% with S + Te = 0.020 to 0.090%

- To improve machinability, it is also possible to use an addition of Ca in a content preferably comprised between 0.001 and 0.004%. Other secondary additions known for their favorable effect on machinability can be used without departing from the scope of the present invention.

- Preferential content intervals are also indicated for other impurities, they correspond to a normal tightening of these contents in electric steelworks, with a view to better quality reproducibility:. Ni + Cr + Mo + Cu ≤ 0.8%. Al 0.02 to 0.06%. P ≤ 0.015%

As the test results show below, it is particularly important for the resilience of the steel products of the invention that hot working, usually rolling or forging, ends below 950 ° C, or better below 900 ° C. Taking into account the practical possibilities of deformation, the end of the hot working is thus carried out below 950 ° C or 900 ° C and usually above 700 ° C, or possibly at a lower temperature while remaining above above 600 ° C for products with a smaller cross section. This important feature of the process of the invention is characterized by a condition relating to the reduction ratio of section "S" being the straight section before

the beginning of the working below 950 ° C and "s" being the cross section obtained at the end of working. In the case of a significant change in shape of the cross section without significant variation in the surface of this cross section, for example in the case of a flattening or an ovalization, it will undoubtedly be more correct to apply the conditions below to the thickness ratio.

In the case of standard transformations by forging or by hot rolling, in which section reductions play a key role, the following practical conditions should preferably be observed:

- large bars of diameter or thickness at the end of hot working from 80 to 210 mm approximately: section reduction ratio by working between 950 and 700 ° C greater than 1.2 and typically

nt between 1.2 and 2.5;

- bars of diameter or thickness at the end of hot working from 20 to 80 mm approximately: section reduction ratio by

wrought between 950 and 700 ° C greater than 1.5 and typically between 1.5 and 3.5; - rods or wire rods of diameter or thickness at the end of hot working from 4 to 50 mm approximately: section reduction ratio "" by working between 950 ° C and 600 ° C greater than 2 and typically

only between 2 and 5. Typical intervals shown below to match

to wrinkles below 950 ° C practicable with known means, these wrinkles allow the optimal results to be obtained.

Thus, a resilience at room temperature typically between 70 and 120 J / cm ² is obtained with a C content of the order of 0.1%, at the same time as high mechanical R and E characteristics, depending on the setting of the hot rolling or forging. A tempering treatment of 2 to 8 h at temperature between 400 and 650 ° C is used to modify the properties. Such treatment:

- to lower,

- increases E when its temperature does not exceed a limit temperature between 450 and 550 ° C, temperature depending on the composition and the working conditions,

- increases the elongation A% and therefore improves the ductility,

- lowers the resilience of the bars to 4% Mn according to the invention from an tempering temperature of about 450 to 500 ° C., but increases it for higher tempering temperatures, - improves machinability.

In practice, a time of 3h to 5h at temperature is preferred for this tempering treatment.

Tempering can thus be used for various uses, notably involving machining, welding and / or cold or lukewarm conformations, at a temperature usually below 300 ° C. A particular case is the manufacture of bolts, screws or nuts by cold or warm striking or by stamping, from bars or wire rod with a diameter between 4 and 50 mm, or on these same drawn bars or wire rod or drawn, with a diameter of between 3.3 and 50 mm.

The relative insensitivity of the characteristics (R, E and resilience) to the cooling conditions, favorable to the weldability, as well as the other particularities of the process and of the product of the invention will be illustrated and commented on by means of the description of the main test results. PRESENTATION OF THE MAIN TEST RESULTS

FIG. 1 represents the variations in the breaking load "R" and the elastic limit at 0.2% "E" as a function of the final forging temperature, for steels according to the invention and for reference steels without B.

FIG. 2 represents the variations of the resilience KCU at 20 ° C. as a function of the temperature of final forging, for the same steels.

The analyzes of the test flows are presented in 3 tables:

- Table I: flows produced in a laboratory HF oven at (Mn + Nb) and (Mn + Nb + B)

- Table II: cast in a laboratory HF oven at (Mn + Nb + B) and (Mn + Nb + Ti), with a control at Cr-Mo

- Table III: industrial casting with analytical variants concerning S and Te.

Bars or wire rod with a diameter between 4 and 50 mm can be straightened or drawn (elongation between 3 and 20%) or drawn (total elongation up to 200%), cold or warm, preferably after tempering.

1st series of tests These tests concern the influence of B and the influence of the temperature at the end of hot working.

The ingots from the castings of Table I were in all cases (for this series of tests and for the two following series) roughed into 40 x 40 mm squares. Then the ingots were brought to 1150 ° C for 30 min and cooled naturally to the forging temperature (T _f ), and then underwent rapid forging to the square of 25 x 25 mm and were cooled either under vermiculite, either with water. The law (time, cooling temperature under vermiculite) of a 25 mm steel square corresponds substantially to that of a 150 mm diameter bar in air.

The first series of tests uses the ingots corresponding to the

6 flows from the top of Table I.

The mechanical characteristics measured in the raw state of forging are presented in Table IV below.

The results (R, E) are shown in Figure 1 and the average resiliency observed in each case are shown in Figure 2. With regard to these mechanical characteristics, these 6 flows at 0.1% C - 4% Mn and 0.1% Nb approximately are roughly comparable by their analysis, with the exception of the only variants concerning the presence or absence of B and the adjustment of the Si content to the level 0.9 to 1% for the two flows "19" and "24". The inherent influence of the B content does not appear to be appreciable when the temperature of the final working, here a forging, is above 950 to 1000 ° C, whether it is E and R or the KCU resilience. When the temperature of the final working drops to 950 to 880 ° C (flows "19", "89" and "24") with in all cases cooling under vermiculite which corresponds substantially to air cooling a large bar (0 150 mm), we see that, for shades with B as for shades without B, the characteristics E and R gradually increase from 100 to 200 MPa.

As for the KCU resilience at 20 ° C, it also improves in both cases, but much more significantly for the B grades (castings "89" and "24"). For these flows, we thus go from the level "50 J / cm ² " for 1000 ° C, instead of "22-24 J / cm ² " for the casting "88" without B, to a resilience level from 100 to 125 J / cm ² for final working temperatures between 920 and 880 ° C. Water cooling (forging at 890 ° C) significantly improves E and a little R, without reducing the resilience, this for the two flows "19" without B and "24" with B. The possible influence of "strong If ", with perhaps an increase in E and R and a limited decrease in resilience (comparison of castings" 89 "and" 24 "for forgings at 900 ° C on the one hand and at 920 and 880 ° C on the other hand), does not seem annoying and we withdraw from these tests the teaching that the Si content of the steel of the invention can be limited to 1.0% and preferably to 0.9%.

2nd series of tests This series of tests concerns the influence of analytical variations on Mn, C, Si, V, the latter being tested on the nuance of comparison without B (casting "18").

The 5 flows at the bottom of Table I were used as well as the 3 flows "19""89" and "24" already used in the 1st series of tests. The hot transformation method is the same as in the 1st series of tests, the rapid forging or final hot working temperature is 880 to 900 ° C, the cooling is done under vermiculite. These tests, the results of which appear in Table V, are in part a confirmation of the 1st series of tests.

The variation in the Si content from 0.24% (casting "26") to 0.87% (casting "24") does not seem to have a significant effect on the mechanical properties. The low C content (0.062% casting "25") gives under these hot working conditions characteristics "E" and "R" lower, but still of a good level, and the resilience is improved. Finally, the lowering of the Mn content to 3.5% and to 3% leads to a gradual lowering of the E, R and KCU characteristics, especially sensitive for E and the KCU resilience: this for 3% Mn is comparable to that obtained with 4% Mn steel without B. (casting "19").

The addition of V, which has no interference with the addition of B and is tested in casting "18" with 4% Mn without B, has no appreciable effect in this raw state of wrought hot.

3rd series of tests

This third series of tests relates to the influence of an annealing heat treatment on the mechanical characteristics of the raw products of hot working.

The 25 x 25 mm straight section bars cooled under vermiculite after the final hot forging, bars obtained in the two preceding series of tests, are subjected to incomes therefrom lasting 4 hours. The tempering temperatures and the mechanical characteristics obtained are given in Table VI. The influence of the income is determined by comparison between the characteristics after income (Table VI) and those before income (Table IV or Table V), for each casting and for the same final forging temperature (T.). The tempering temperature (T) varies from 500 to 650 ° C for 2 batches forged at 1000 ° C (castings "88" and "89"), and is 450 ° C and 500 ° C for the other batches which are forged at 880 ° C or 900 ° C. The study of these results shows that, for flows at 4% Mn without B and for casting with B:

- The elastic limit E is improved, especially for the lowest tempering temperature tested, that is to say 450 ° C; the improvement is particularly significant in the case of the addition of 0.1% V (casting "18");

- the breaking load R is lowered in all cases, it decreases when the tempering temperature increases;

- the elongation increases from 10-11% to 15-16%, except for the lowest Mn contents: castings "36" and "37" to which correspond a better elongation in the raw forging state, this elongation is then slightly improved by tempering at 450 or 500 ° C;

- the resilience is often little modified by tempering at 450 ° C and lowered by tempering at 500 ° C: this is the case with 4% Mn "19", "18", "26" and "25" flows . The flow "24" (4% Mn + B strong Si) gives a resilience lowered from the tempering to 450 ° C. The bars of the "37" casting at 3% Mn + B have their resilience greatly improved by tempering at 450-500 ° C: they pass from an average resilience of the order of 70 J / cm ² to the state crude forging (table V) at a resilience of the order of 110-120 J / cm ² after tempering at 450 or 500 ° C (table VI). The "36" casting at 3.5% Mn + B has an intermediate behavior: the average resilience of the bars obtained goes from 80-

85 J / cm ² in the raw forging state at around 90 J / cm ² after tempering at 450 ° C or 500 ° C. Finally, the bars of the "88" and "89" castings forged at 1000 ° C and having a low resilience in the raw forging state have their improved resilience from the tempering temperature of 500 ° C, especially for the bars of the casting "88" without B which have a particularly low resilience in the raw forging state. However, the resilience of these bars "88" and "89" is only greatly improved after tempering at 650 ° C., and we then have mechanical characteristics (E, R) which are too weak vis-à-vis the need studied. (case of pretreated bars).

According to all these results, only the bars having the highest mechanical characteristics in the b'rut state of hot working, bars at 4% Mn having undergone final forging clearly below 950 ° C (at 880 ° C ) - have their resilience lowered by tempering to 500 ° C, the compromise (E, R, KCU) obtained then for bars such as "24" and "26" can be interesting for certain applications: E particularly high, resilience greater than 70 J / cm ² .

These tempering tests confirm the importance of carrying out the hot working, with a final working sufficient below 950 ° C. and better still below 900 ° C., to obtain high mechanical characteristics, both at state returned to the rough state of hot working.

The results in Table VI suggest that tempering between about 450 and 650 ° C may be advantageous prior to cold or warm drawing or cold or warm conformation (for example between 100 and 300 ° C). These cold or lukewarm treatments then make it possible to enhance E and R.

The set of results also shows that a temperature lower than 450 ° C, at least 400 ° C, may in some cases be preferable for tempering. Typically, for the castings according to the invention laminated or final forged below 950 ° C. and having an Mn content greater than 3.6%, content preferably between 3.6 and 4.3%, the tempering temperature range 400-475 ° C is especially useful for improving the yield strength and elongation without greatly reducing the breaking load and resilience, while the range 475-600 ° C is particularly useful when after the tempering, a cold deformation or conformation is made.

4th series of tests

These tests relate to the possible replacement of boron by titanium. As the surprising effect of B on the resilience of the steel studied is probably due to its action on nitrogen, it was interesting to study whether titanium, a denitriding agent, also had an effect.

The laboratory flows carried out (table II) are of 3 kinds: flows at 4% Mn + B, flows at 4% Mn with staggered Ti additions, and a comparison flow of the "35CD4" type (AFNOR NF A35 standard -557).

The ingots of the 4% Mn castings were first transformed into square bars of cross section 50 x 50 mm by forging at 1000 ° C, then cooled in air. They then underwent a controlled forging according to the following stages: heating to 1150 ° C., beginning and end of forging at 900 ° C., the forging of each ingot for approximately 90 s and the cross section obtained being a square of 25 × 25 mm (the ratio defined previously is then 4), then cooling under

verjiiculite from around 750 ° C. The mechanical characterization (table VII) was made in the raw state of forging. The ingot of comparison n ° 18 (AFNOR "35CD4 or AISI 4137) was forged into a 25x25 mm square at 1100 ° C and air-cooled, then underwent the following conventional heat treatment: austenitization 30 min at 850 ° C. quenching oil quenched for 4 h at 575 ° C. the mechanical characteristics given in table VII correspond to the quenched-quenched state thus obtained.

Examination of Table VII and Table II leads to the following conclusions:

- the addition of Ti has an optimal effect on the resilience for 0.03% Ti, but this effect is clearly weaker at the levels of 0.05% and 0.015% Ti, and moreover the R and E characteristics obtained w are lower than in the case of the addition of B (flows "100" and "101")

- Adjustment of the Ti content to obtain the desired effect therefore seems difficult, and the addition of B is preferable to the addition of Ti for industrial application.

These results tend to confirm here the hypothesis of a denitriding action of Ti or at least of an action similar to that of B, it is advantageous in practice to associate B and Ti while avoiding Ti contents greater than 0.030 % and maintaining a minimum B content of 0.010%, with the condition already indicated above:

- the castings at "Mn + B" n ° 100 and 101 compare favorably, in their raw forging state, with the witness "35CD4" n ° 018 in the quenched-tempered state, with regard to; * characteristics mechanical at room temperature. On the other hand, their resilience at -50 ° C is a little less good. It was verified that this resilience at -50 ° C was, however, better than that of a "42CD4" steel at 0.46% C quenched-income.

5th series of tests

It relates to the conditions of hot working of large bars from an industrial casting, and on the other hand to the improvement of the machinability by adding S and / or Te.

In the same industrial casting, 3 analytical variants were made (Table III). The ingots obtained were transformed by hot rolling into bars with a diameter of 170 mm and a diameter of 105 mm. The mechanical characteristics were made in the raw rolling state and then after tempering for 4 hours at 525 ° C., and machining tests were carried out in these two states. Only the analyzes of the portions of ingots thus characterized are shown in Table III. There is also in each of the 3 variants calcium in a content of between 0.002 and 0.003%.

The mechanical characteristics and the results of machining tests are given in Tables VIII, IX and X.

In the first 3 series of tests made from the laboratory castings in Table I, we noted the significant influence of the final working carried out below 950 ° C, in this case a rapid forging performed at initial temperatures included as appropriate between 950 and 880 ° C, reduction of area of the report endessous 950 ° C being still equal to 40 ^{^2/25 2} = 2.56.

In the present tests, concerning large bars, the hot working conditions are very different:

- bars of ∅ 170 mm ("2P" and "9P"): heating to 1200 ° C and roughing in blooming, finishing on product with section 191x200 mm; then rolling in a billet cage, starting at 1020 ° C, engagement of the pre-oval 150x200 mm at 910 ° C, round engagement at 835 ° C and end of rolling at around 800 ° C; the ratio on this large section is at least equal to the ratio between

the oval section 150x200 mm and the circular section ∅ 170 mm, i.e. at least equal to approximately 1.24.

- bars of ∅ 105 mm ("1P" and "19P"): heating to 1200 ° C and roughing in blooming, finishing on 160x164 mm product, then rolling starting at 930-960 ° C, with engagement of the pre-oval 120x160 mm at 920 ° C, round engagement at around 820 ° C, and end of rolling around 800 ° C; the ratio here is at least equal to the ratio between the oval section

120 x 160 mm and the circular section 0 105 mm, and at most equal to the ratio between the section 160 x 164 mm and this same circular section 0 105 mm, this ratio is therefore between 2.2 and 3.

In both cases, the bars were air-cooled. The mechanical characteristics in the hot-rolled raw state are shown in Table VIII and the characteristics in the 4 hour tempered state at 525 ° C are shown in Table IX. It is observed that the bars ∅ 105 mm without addition of S or Te have characteristics of a very good level, with a resilience at 20 ° C of 100-110 J / cm ² . Bars ∅ 170 mm undoubtedly less corrected below 950 ° C have a lower resilience of the level of 70 J / cm ² . The hardness is homogeneous in the product section according to the determinations made on ∅ 170 mm in the tempered state (Table IX). On the other hand, the resilience is weaker at mid-radius than closer to the surface, that is to say only 12.5 mm or 25.4 mm from this surface.

The temperature of 525 ° C. is ill suited for the income of these bars ∅ 170 mm in the event that good resilience is sought at heart.

The previous tests (Table VI) show how to choose the tempering conditions to optimize the properties.

Table X groups the contents of certain elements particularly important for machinability (C, S, Te), the mechanical characteristics, averages of the results of Tables VIII and IX (R, E, A%, Z%, KCU at +20 ° C) and the result of the machining test. The Z% necking during the tensile test is indicated in these 3 tables in addition to the elongation A%, caractest as A% an interesting criterion of ductility and deformability.

Table IX shows that the necking remains high across the cross section of the bars ∅ 170 mm marked (2P) and (9P). Table X shows that in the case of bars (9P), elongation and relatively weak necking in the raw rolling state, the tempering at 525 ° C. makes it possible to increase these characteristics and to obtain a very interesting compromise in properties, taking into account the improvement in machinability.

Machining tests

The machining tests, leading to the results "V _B30 _ _0.3 " shown in table X, consist of turning tests carried out according to standard NF A 03-655 on samples of peeled bars of length approximately 600 mm, by successive passes with interruption of at least 5 minutes after each pass, under the following cutting conditions:

- feed 0.25 mm / turn - depth of cut 1.5

- SNUN carbide plate 12-04-08 according to ISO name, type P20 carbide according to ISO classification of carbides.

The machinability criterion chosen is the wear criterion of tool V _B30-0.3 according to standard NF A 03-655, that is to say the cutting speed leading to wear in clearance of 0 , 3 mm in 30 minutes.

The results given in table X show the influence of the additions "S + Te" on the machinability in the raw rolling state: influence already sensitive for 0.02% S and 0.005% Te (cutting speed increased by 17 , 5%), much greater influence for 0.067% S and 0.009% Te (185 m / min compared to 132 m / min or + 40%).

They also show the favorable influence of income on machinability, resulting in an increase in cutting speed of between +20 and + 40%.

The typical values observed with a "35CD4" steel according to AFNOR standard NF A 35-557 have been reported, with addition of sulfur (S 0.03%) to improve machinability. This steel is commonly used in the form of pre-treated bars in the quenched-tempered state.

Examination of Table X makes it possible to conclude that, even with this small addition of S, the rough rolling bars according to the invention give, with a much higher breaking load, a machinability as good as the "35CD4" processed. And for the same level of mechanical resilience and a comparable “S + Te” content, the bars according to the invention in the tempered state are machined much better: this is the case for the “9P” bars in the state returned to 525 ° C (Table X) or to a slightly lower temperature (450 to 500 ° C) so as to have a slightly higher breaking load R.

Sixth series of tests

The sixth series of tests relates to hot rolling and drawing of a 13 mm diameter machine wire according to the invention.

The wire rod crown (1 ton) laminated to the diameter 13 mm comes from the ingot portion "1P" of industrial casting, the analysis of which is given in Table III.

The hot rolling conditions are summarized as follows: - hot rolling in billets of straight section 108x108 mm and cooling - reheating to 1150-1200 ° C and hot rolling with engagement of the round of diameter 25 mm at 920 ° C and end of rolling at 800-850 ° C with a diameter of 13 mm with winding and air cooling. - the ratio characterizing the wrinkling below 950 ° C is

at least equal to

The mechanical characteristics in the raw rolling state and after tempering for 4 h at 470 ° C. are shown in Table XI, as well as the characteristics of the tempered-drawn wire after successive passes of drawing. After 4 wire drawing passes, 165% of total elongation or cumulative elongation is thus reached, and the mechanical characteristics obtained then are very advantageous since they combine an elastic limit at 0.2% "E" greater than 1000 MPa and a load of rupture greater than 1100 MPa at a still significant ductility reserve if we judge by the necking coefficient "Z%". This compromise in mechanical characteristics is obtained from 70 to 80% of total elongation, and the mechanical tests supplemented by Vickers hardness measurements at mid-radius (the hardnesses given in table XI are the means of 10 measurements) show that the quality obtained is excellent. These wire drawing results show the good ability to cold deformation of the steel of the invention after tempering, and make it possible to conclude that bars or wires of this quality can be straightened or drawn cold or "warm" unless 300 ° C, diameters commonly between 3.3 and 20 mm and up to 50 mm. This aptitude for cold deformation after tempering also allows conformations by cold striking or lukewarm, or by cold or lukewarm stamping, in particular the manufacture of bolts, screws or nuts with high mechanical characteristics.

Claims

1. Method of manufacturing bars or steel wire rod having after hot working and cooling a bainitic structure and high mechanical characteristics (R, E, KCU):

- breaking load R ≥ 1000 MPa - elastic limit at 0.2% E ≥ 600 MPa -_- KCU reflectance at 20% ≥ 60 J / cm ² characterized in that a steel is produced containing in% by mass:

b) Ti ≤0.010% and B = 0.0020 to 0.0080%;

N ≤ 0.015%; Ni + Cr + Mo + Cu ≤ 1.0%; Al≤ 0.08%; S ≤ 0.090%; Te≤ 0.015%; P ≤ 0.020%; other elements and Fe: the balance; and in that the end of the hot working is carried out below 950 ° C.

2. Method according to claim 1, characterized in that the steel produced contains:

- C 0.08 to 0.11%; Mn 3.1 to 4.3%; If 0.2 to 0.9%; Nb 0.05 to 0.10%; B 0.0030 to 0.0070%; N 0.003 to 0.010%; Ni + Cr + Mo + Cu≤ 0.8%; Al 0.02 to 0.07%; S ≤0.090%; Te≤0.015%; P≤ 0.015%; other elements and Fe: the balance.

3. Method according to claim 1 or 2, characterized in that the steel produced contains 3.1 to 3.6% Mn.

4. Method according to claim 2, in which the steel bars obtained have a breaking load R ≥ 1100 MPa, an elastic limit E ≥ 750 MPa and a resilience KCU ≥ 70J / cm ² , characterized in that the steel produced contains 3.6 to 4.3% Mn and S + Te ≤ 0.015%.

5. Method according to any one of claims 1 to 3, characterized in that the steel produced contains S = 0.015 to 0.090% and Te ≤ 0.015% with S + Te = 0.020 to 0.090%.

6. Method according to any one of claims 1 to 5, wherein the bars obtained have a diameter or a thickness between 80 and 210 mm, characterized in that the end of the hot working is carried out between 950 ° C and 700 ° C and produces a reduction in section ratio greater than 1.2, "S" being the cross section

before the start of the ^S wrought below 950 ° C and "s" being the cross section obtained at the end of the wrought.

7. Method according to any one of claims 1 to 5, in which the bars obtained have a diameter or a thickness between 20 and 80 mm, characterized in that the end of the working is carried out between 950 and 700 ° C and product a ratio section reduction greater than 1.5.

8. Method according to any one of claims 1 to 5, in which the bars or machine wires obtained have a diameter or a thickness of between 4 and 50 mm, characterized in that the end of the hot working is carried out between 950 and 600 ° C and produces a reduction in section ratio greater than 2.

9. Method according to any one of claims 1 to 8, characterized in that the bars or raw machine wires of hot working undergo a tempering between 400 ° C and 650 ° C with a holding time at temperature between 2h and 8h.

10. The method of claim 9, characterized in that the bars or machine wires obtained, of diameter or thickness between

4 and 50 mm are then stretched or straightened cold or warm at a temperature below 300 ° C.

11. Method according to claim 9, characterized in that the bars or machine wires obtained, of diameter or thickness between

4 and 50 mm are cold or warm drawn at a temperature below 300 ° C with a total elongation less than or equal to 200%.

12. Method according to any one of claims 9 to 11, characterized in that the bars or machine wires obtained by hot working followed by tempering, possibly with drawing, straightening or a wire drawing before or after tempering, of diameter or thickness then between 3.3 and 50 mm, are transformed by cold forming, or warm at temperature below 300 ° C into bolts, screws or nuts.

13. Rod, wire rod or shaped piece of bainitic structure steel containing in% by mass:

b) Ti ≤ 0.010% and B = 0.0020 to 0.0080%

N≤ 0.015%; Ni + Cr + Mo + Cu ≤ 1.0%; Al ≤ 0.08%; S ≤ 0.090%; Te ≤ 0.015%; P ≤ 0.0020%; other elements and Fe: the balance.

14. Bar, wire rod or shaped part according to claim 11, characterized in that it has as mechanical characteristics:

- breaking load R ≥ 900 MPa - elastic limit E ≥ 600 MPa

- resilience at 20 ° C KCU ≥ 60J / cm ²

15. Rod or wire rod or shaped part according to claim 13, characterized in that it contains in% by mass: C 0.08 to 0.11%; Mn 3.6-4.3%; If 0.2 to 0.9%; Nb 0.07 to 0.11%; B 0.0030 to 0.0070; N 0.003 to 0.010%; Ni + Cr + Mo + Cu ≤ 1.0%; Al 0.02 to 0.07%; S ≤ 0.090%; Te ≤ 0.015%; P ≤ 0.015%; other elements and Fe: the balance, and in that it has mechanical characteristics:

- breaking load R ≥ 1100 MPa - elastic limit E ≥ 750 MPa

- resilience at 20 ° C KCU≥ 70J / cm ² .

16. Bar, wire rod or shaped part according to any one of claims 12 to 14, characterized in that it or. it contains in% by mass:

S 0.015 to 0.090% and Te ≤ 0.015% with S + Te = 0.020 to 0.090%.

17. Bolt, screw or nut according to any one of the claims 12 to 15.

18. Bar, wire rod or shaped part obtained by the process of any one of claims 1 to 12.