CH619270A5 - Boron steel with high hardness and toughness characteristics - Google Patents

Abstract

This steel contains: - total boron : 30 to 100 ppm - N : 40 to 220 ppm - Al : 200 to 800 ppm and the total boron (Bt), N and Al contents expressed in ppm satisfy the following relationship: 10 N - 17 Bt - Al = - 300 +/- 200 This steel, which exhibits very high mechanical characteristics, especially both a quenchability and great toughness in the quenched state, is particularly suitable for the manufacture of articles employed in the processed state, or cemented and processed, such as shafts, gears and mechanical components of all kinds.

Description

The subject of the invention is a boron steel having, in the quenched state or in the cemented and quenched state, high hardness and toughness characteristics, thanks to optimal contents of soluble boron and insoluble boron. This steel can be used for all kinds of jobs and, more particularly, in the fields concerning structural, processing and case hardening steels.

The subject of the invention is also a method for producing this steel which makes it possible to confer on this steel simultaneously improved characteristics of hardenability and toughness thanks to controlled additions of B, AL and N.

The invention also relates to the use of this steel for the manufacture of mechanical parts having a high hardness and a high toughness.

The influence of boron on the mechanical characteristics of steels has been known for a long time. Thus in 1921, U.S. Patent 1,159,388 teaches the addition of small amounts of boron to improve the hardenability of steel. Since that date, a considerable number of studies have been published which have given very often contradictory lessons on the quantities of boron which should be introduced into steels, the conditions under which this introduction must be made, and its effect. on the properties of these steels.

There is, however, a set of results showing the action of a few tens of ppm of boron on the hardenability of steels and we can cite, among the works taking stock of this question, that of R. SCHERER and K. BUNGARDT (Revue de Metallurgy L no. 2.1953, p. 73-94), relating to the hardenability of case hardening and boron treatment steels. This work shows that amounts of boron of the order of 30 to 40 ppm make it possible either to appreciably increase the quenchability of the steels in question, or to maintain their quenchability by reducing their contents of additive elements. The existence of interactions between boron and nitrogen present in steel has been reported by various authors, such as, for example, TG DIGGES and FM REINHART (Trans. ASM, 1948-40 p. 1124-1145) .

These authors show that, in steels with a high nitrogen content, the effect of boron additions is somehow neutralized by this element. They explain it by a reaction between boron and nitrogen which gives a compound insoluble in sulfuric acid 1 / I, without effect on the quenchability. To avoid this unfavorable action of nitrogen, their tests show that it can be fixed by titanium or zirconium which have, for this element, an affinity greater than that of boron. This result can be obtained by introducing boron into the steel in the form of complex alloys containing, in general, several strongly reducing elements such as Mn, Si, Al, Ti and Zr. These elements protecting the boron against oxidation and nitriding, its action on the quenchability of the steel then reaches maximum efficiency.

Dutch patent application 130,179, filed on March 20, 1963 (priority application no. ST 18,989 of March 21, 1962 in the Federal Republic of Germany) relates to steels containing both boron and nitrogen in which the formation boron nitride improves toughness. According to this publication, it is possible, in these steels by adjusting the contents of boron and of free nitrogen, to have, on the one hand, boron in the form of insoluble BN which increases the toughness and, on the other hand, excess boron called dissolved boron which increases hardenability.

It follows from the examples that the quantity of so-called free nitrogen is calculated by deducting from the total nitrogen the quantity of nitrogen which is bound to the aluminum present in the liquid metal in the form of nitride.

It is claimed in this patent, the addition of 0.0001 to 0.03% of boron in a molten steel of still steel containing 0.001 to 0.03% of free nitrogen.

Much more recently, the problem of the optimal content of soluble boron, the only active ingredient for increasing the hardenability of steels, has been reviewed by Ryuichi HABU et al. (Tetsu to Hagahe, September 1074-60, no. 10, p. 1470-1482) in the case of low-alloy steels containing aluminum. The calculations of these researchers, confirmed by their experiments, tend to show that boron reduces aluminum nitride, even in the presence of a very large excess of aluminum to form boron nitride. As long as all of the aluminum nitride has not been reduced by boron, there is, according to the authors, in the dissolved state, only a small amount of boron, it is soluble boron which is present at levels practically not exceeding 6 to 7 ppm, as long as there is nitrogen available. For these authors, there is therefore no free nitrogen and aluminum nitride acts as a buffer; it is only when this nitride has been entirely reduced by boron that the quantity of soluble boron can increase and exceed ten ppm. Still according to these authors, the optimal content of soluble boron, to promote quenching, is 3 to 5 ppm, and to reach these levels, a high Al content of the order of 0.06 to 0.08 is required. %, in the presence of N2 contents between 0.004 and 0.012%. In practice, according to the same authors, it will suffice to introduce about ten ppm of boron into such a steel to automatically obtain 3 to 5 ppm of soluble boron. Larger amounts of total boron are without disadvantage as long as there is excess aluminum nitride.

These last results are in contradiction with most of the teaching of the NL patent. 130 179 according to which only free nitrogen, that is to say that which is not bound to aluminum, is available to form boron nitride.

Other authors: H. TREPPSCHUH and coli (Stahl und Eisen, November 2, 1967, vol. 87, no. 22 p. 1355-1368) estimate that, in the liquid state, boron fixes the nitrogen contained in

S

10

15

25

10

35

40

45

50

55

60

65

3

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steel, even in the presence of excess aluminum. If, therefore, it is desired to have soluble boron, it is necessary to use additions of effective denitriding elements such as titanium and zirconium. However, according to the same authors, at a lower temperature in the solid metal, the affinity of Al for nitrogen becomes greater than that of boron and there can be reversion and reduction by aluminum of a part. boron nitride. The results presented by the authors show that, to obtain a little less than 10 ppm of boron in solid solution, in the presence of nitrogen and in the absence of denitrurants, an addition m of aluminum is necessary in the steel of 1 , 2 kg / tonne, or 0.12%, a quantity much higher than current practice.

Faced with these contradictory results, a person skilled in the art does not manage to draw a lesson enabling him to predict the effect of the boron additions and to adjust them as a function of a result to be achieved. This explains the failures encountered in the use of boron in steels, the relative mediocrity and the dispersion of the results obtained with, as a consequence, a lack of user confidence in these grades of steels. Indeed, efforts to promote steel grades containing: o fewer costly and rare addition elements such as Cr, Ni, Mo etc ... and having as good quenchability thanks to the addition of boron, were compromised by the great irregularity of the results obtained.

In the case, for example, of case-hardened steels, used in the automobile industry, this irregularity is expressed either by deformations of parts of too low hardness, or by intergranular cracks due to fragile compounds rich in boron.

The present invention makes it possible, quite unexpectedly, to obtain new boron steels which it is possible to prepare with very high safety and very high reproducibility and which have characteristics perfectly defined in advance, combining better hardenability and considerably increased toughness compared to those of previously known steels.

To this end, the steel according to the invention is characterized in that it contains 30 to 100 ppm of total boron, 40 to 220 ppm of nitrogen and in that the sum equivalent to aluminum, So, is included between 200 and 800 ppm, this sum being expressed by the m relation:

tenacity. The development of these steels also has the advantage of simple implementation which does not require costly treatments with special reinforcing agents such as titanium or zirconium.

The process for producing the steel according to the invention is characterized in that the contents of total boron Bt, nitrogen, aluminum and, where appropriate, V and / or Nb are adjusted after deoxidation. and / or Ta, so as to satisfy relations (1) and (2).

The inventors first of all considered that the contradictions observed in previous work could be explained, in part, by the use of imprecise methods of analysis, or systematically distorted by defective calibrations.

It is known, in fact, that the precise dosage of contents of the order of magnitude of the part per million presents great difficulties which the steelworks laboratories are not used to solving. Likewise, the inventors thought that the data in thermodynamics were insufficient to predict the possible reactions between interacting elements such as boron, aluminum and nitrogen.

From these working hypotheses, they had the simple but unexpected idea that it should be possible to discover a relationship between the aluminum, nitrogen and boron contents which would explain, to a large extent, the phenomena observed and , above all, which would make it possible to predict the characteristics of steels based on these three elements and, from there, to develop new steels with improved characteristics.

The tests carried out fully justified this initial idea and made it possible to establish a relationship which, when satisfied, in obtaining an optimal effect of boron with regard to both quenchability and toughness. It is a very simple linear relationship between the total boron Bt, nitrogen and aluminum contents expressed in ppm, which are found in steel.

This relationship is as follows:

s ° -ai + r9 + s "4 + 6-7

(1)

10 N- 17 Bt-Al = —300 ± 200 ppm We must also have:

200 <Al <800 ppm 30 <Bt <100 ppm 40 <N <220 ppm

(3)

(4)

(5)

(6)

in which Al, V Nb and Ta are the ppm contents of these elements, and in that the relation:

10 N -17 Bt —So = -300 ± 200 ppm (2)

is satisfied, Bt and N being respectively the total boron and nitrogen contents expressed in ppm and So being calculated by the relation indicated above.

In this steel, the presence of V and / or NB and / or Ta is optional. This steel is characterized not only by its total boron content, but also by the simultaneous presence, at optimal levels, of boron effective for hardenability and boron effective for tenacity.

These are steels of all types, such as structural steels, case hardening steels, processing steels in which improved characteristics are obtained in a controlled and reproducible manner of hardenability and toughness.

These new steels can be produced industrially more economically than steels with higher contents of expensive additives which they can replace by virtue of at least equal performances from the point of view of hardness and elastic limit, and, in general, superior from the point of view

As the examples will show, these relationships are only valid if certain experimental conditions are met. First of all, N, Bt and Al are respectively the contents expressed in parts per million of total nitrogen, total boron and total aluminum. Total aluminum is understood to mean that which is present in steel in the dissolved or combined state. A fraction is in the form of inclusions of alumina which have not had time to decant. The aluminum thus fixed no longer acts on the nitrogen boron balance. Experience shows that this is a cause of error which can be neglected in the exploitation of the above relationship, provided that the samples for analysis are carried out, either on the metal solidified after molding. , or on the liquid metal at the end of the settling period just before molding. In practice, the amount of aluminum thus fixed in the form of alumina is of the order of 10 to 30 ppm.

In addition, tests have shown that aluminum can be replaced, at least partially, by vanadium, niobium or tantalum which play a similar role with respect to nitrogen and boron. This replacement must be carried out in the report of the atomic weights of these various elements. It follows that, if we express the contents of aluminum, vanadium,

55

60

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4

niobium and tantalum in ppm, we can define a sum equivalent to aluminum, So, expressed in ppm, such that:

So = Al +

+ Nb

+

Your

1.9 3.4 6.7

(1)

This So value can therefore be substituted for the Aluminum content expressed in ppm in the relationship which has been defined above which is then written:

10 N - 17 Bt - So = - 300 ± 200

(2)

We notice that if V = 0, Nb = 0 and Ta = 0, simultaneously, we then have So = Al and we are brought back to the initial relation. This relation (2) is therefore the generalized form of relation (3).

As before, the three ranges of contents must be achieved:

200 <So <800 ppm 30 <Bt <100 ppm 40 <N <220 ppm

(7)

(5)

(6) "

Although the maximum value of Al is 800 ppm, in most cases it suffices to introduce Al contents not exceeding 400 ppm into the steels according to the invention; it is the same ,. for the value of So which most often does not exceed

400 ppm.

For nitrogen, the quantities which could possibly be fixed by very powerful denitrurants such as titanium or zirconium are not taken into account in the relation. In the case where it would be necessary to use such additions, it would therefore be necessary, for the application of the above relationship, to deduce the nitrogen which would be fixed by these elements in the form of stable nitrides.

Often, it is not necessary, for the implementation of the method according to the invention, to modify the nitrogen content present in the steel bath which is naturally in a range of the order of 40 to 100 ppm.

However, in the case of elaborations in a basic arc furnace, it sometimes happens that the nitrogen content is greater than the value desired for the implementation of the process according to the invention. This content can then be lowered by vacuum degassing, for example, or, which is not without drawback, by carefully adjusted additions of denitriding elements, such as titanium or zirconium, generally in the form of ferro - 4 alloys. An increase in nitrogen content can be obtained, on the contrary, by various methods: bubbling of N2, introduction of a nitrided ferro-alloy, calcium cyanamide, etc.

Although the nitrogen content of the steels according to the invention can reach up to 220 ppm, this content is generally limited to values below 150 ppm.

By total boron Bt is meant the boron which is actually present in the liquid metal. The method of analysis which is described below makes it possible to determine it. It should however be known that in this total boron thus dosed, there may exist small quantities in the form of boron oxide if this element has been introduced without precautions and if the metal bath has not been correctly deoxidized beforehand. This oxide has no effect either on the hardenability or on the toughness of the steel. If we have taken the precaution to introduce aluminum under the conditions (, o tions which have been specified, before introducing boron, experience shows that the quantities of boron liable to be oxidized in the metal bath are negligible and do not lead to uncertainties in the exploitation of the relationship between boron, nitrogen and aluminum. Boron can be introduced into the bath (, 5 of liquid steel in the form of ferrobore, in the oven or in the ladle, or possibly during degassing or, finally, in the mold.

Except in special cases, it is necessary to avoid introducing boron by means of so-called intensifier mixtures which have the purpose of deoxidizing and denitriding the bath. These mixtures contain deoxidizers and denitrurants which will, in general, have the effect of eliminating nitrogen and, if they contain aluminum, to further increase the content of this element, which may prevent satisfying the relationship given above, unless new nitrogen additions are made. It should be noted that, in most cases, the boron content of the steels according to the invention is substantially less than 100 ppm and hardly exceeds 60 ppm.

Although the theoretical bases of the invention have not yet been fully elucidated, the very detailed analytical studies which have been carried out have shown that, by producing steels in accordance with the invention, that is to say by adjusting to the end of steel production, under the conditions specified above, the boron, nitrogen and aluminum contents, so as to satisfy relation (3) or generalized relation (2) and respecting the content ranges given above, a soluble boron content of 15 to 50 ppm and also 15 to 50 ppm of insoluble boron is obtained in this steel. Soluble boron, as defined in this process, is that which is determined by the following method: etching in a quartz beaker of 100 ml of 100 mg of steel for B contents greater than 50 ppm and of 250 mg for the lower contents, by a mixture of 20 ml of S04H27N + 10 ml of P04H39N in a boiling water bath. When the attack seems to stop, it is extended by 15 min. it is cooled, 5 ml of H 2 O 2 at 55 volumes are added, the excess is removed by maintaining for 10 min. in a boiling water bath, leave to cool and adjust to 50 ml in a graduated flask. The boron present in the solution is measured, according to a method known to those skilled in the art, such as the light absorption at 6500 Å of the triple boron methylene fluoride complex extracted with dichloroethane.

The total boron content Bt is determined after the sample has been completely dissolved.

The dosing method consists of a sulfuric attack with white smoke. The vapors from the attack are collected after condensation and combined with the initial attack solution. The boron is then dosed in the solution by a method comparable to that described above. By definition, the insoluble boron content is the difference between the total boron and soluble boron contents determined in the manner just described.

The examples below describe a certain number of cases of implementation of the method according to the invention. This process actually applies to steels produced by any method, since the process according to the invention consists in adjusting a certain number of parameters of the composition of the steel, which must be carried out at the end of production. in the oven itself, or possibly in the ladle.

Example 1

Four flows were designed, marked 1A, 1B, IC and 1D. The three castings 1A, IC and 1D were in accordance with standard AFNOR 19 NCDB 2 which relates to a nickel chromium molybdenum steel comprising an addition of boron from 8 to 50 ppm approximately. Casting 1B was in accordance with standard AFNOR 20 NCD 2 which relates to a boron-free steel whose composition ranges for the other addition elements are identical to those of steel 19 NCDB 2. The operating conditions were as follows:

Casting 1A

Type 19 NCDB 2 steel was produced in a basic arc furnace in a known manner. Then, the metal was stirred in the ladle with a slag containing approximately 50% CaO and 50% A1203. AI was added as FeAl

619,270

VS

0.24

Mo

0.20

Yes

0.31

S

0.030

Mn

0.84

P

0.020

Or

0.70

Al

0.030

Cr

0.67

o2

0.0025

in the tilting jet and the B was added as a ferrobore. The metal was then degassed under vacuum. After casting in molds, rolling in blooms and forging in small profiles, samples were taken for chemical analysis and measurement of mechanical characteristics.

The chemical composition by weight (in%) is as follows:

N2 0.0080

B total 0.0050

B soluble 0.0025

B insoluble 0.0025

This steel is in accordance with the invention: the contents of N2, total B and Al check the relationship (3) and the ranges of composition (4), (5) and (6). The contents of soluble B and insoluble B found correspond to the desired result.

IB casting

Steel type 20 NCD 2 was produced in a basic arc furnace in a known manner. Then, the metal was stirred with a slag containing 50% CaO and 50% A1203. The AI was added as FeAl in the tipping jet. There was no addition of B. The metal was then degassed under vacuum. After casting in molds, rolling in blooms and forging in small profiles, samples were taken for chemical analysis and measurement of mechanical characteristics.

The chemical composition by weight (in%) is as follows:

The chemical composition by weight (in%) is as follows:

VS

0.21

Mo

0.20

n2

0.0135

Yes

0.26

S

0.010

B total

0.0055

Mn

0.85

P

0.010

B soluble

0.0012

Or

0.58

Al

0.027

B insoluble

0.0043

Cr

0.57

o2

0.0035

VS

0.22

Cr

0.55

Al

0.034

Yes

0.31

Mo

0.25

o2

0.0025

Mn

0.77

S

0.030

N2

0.0090

Or

0.63

P

0.011

B

0

This steel is therefore not in accordance with the invention since it does not contain B.

Casting 1C

Type 19 NCDB 2 steel was produced in a basic arc furnace in a known manner. The metal has not been brewed. AI was added 4ii as FeAl in the tipping jet. The metal was then degassed under vacuum. B was added in molds in the form of a grain-type alloy containing Ti, Zr, Al.

After casting in molds, rolling in blooms and forging in small profiles, samples were taken for chemical analyzes and measurements of mechanical characteristics.

The chemical composition by weight (in%) is as follows:

This steel is not in accordance with the invention because it does not satisfy 1 with relation (3).

The hardenability of the steels corresponding to these four flows was evaluated in a conventional manner by drawing the JOMINY curves. The curves are shown in solid lines in the single figure with, on the abscissa, the distances in mm and, on the ordinate, the hardnesses expressed in ROCKWELL C units.

In the same figure, the quenching curve of a conventional boron-free steel with nickel content approximately 6 times higher is plotted in dashed lines, the steel in accordance with standard AFNOR 18 NC 13, for which the nickel content was 1.35%.

Curve 1 corresponds to the steel coming from casting 1 A, the only one according to the invention, which has the highest quenchability, even higher than that of type 18 NC 13 steel, the high nickel content of which increases by significantly the cost price. The most mediocre results are those obtained with steels from castings IB (curve 2) and 1D (curve 4). This is logical because the IB flow is free of boron and the 1D flow, although it has a boron content of the same order of magnitude as that of flows 1A and 1C, has a too low content of soluble boron alone effective for the hardenability. Cast 1C (curve 3), which has a high content of soluble boron, however, has a slightly lower quenchability than that of cast 1 A. This is explained because it seems that the effect of soluble boron passes through an optimum towards 20 at 30 ppm.

The toughness of these same steels was evaluated by measuring the cracking force of case-hardened and hardened specimens. The type of test carried out is that described in the article by H. BRUGGER and G. KRAUS: Arch. Eisenhiittenw., 32, no. 8 (1961), p. 529.

The table below gives, on the one hand, the values of the cracking forces in KN thus determined and, on the other hand, the grain dimensions according to the ASTM scale measured on samples of the same flows after a Mac Quaid test. which includes a 10 hour carburizing treatment at 925 ° C.

VS

0.20

Mo

0.19

n2

0.0125

Yes

0.42

S

0.039

Ti

0.065

Mn

0.71

p

0.012

B total

0.0065

Or

0.54

Al

0.040

B soluble

0.0060

Cr

0.58

o2

0.0025

B insoluble

0.0005

Table l No of casting

Type of steel (AFNOR)

CRACKING EFFORT in KN

GRAIN SIZE after test Me QUAID-ASTM scale

This steel is therefore not in accordance with the invention because the titanium <present fixes nitrogen in the form of TIN. As there is an excess of titanium compared to the stoichiometry, there is no more nitrogen available to interact with boron and aluminum.

1A

19 NCDB 2

61

6-8

IB

20 NCD 2

47

7-8

1 C

19 NCDB 2

56

4-6

ID

19 NCDB 2

52

6-8

18 NC 13

60

7-8

ID casting

Type 19 NCDB 2 steel was produced in a basic arc furnace in a known manner. Then, the metal was stirred with a slag containing 50% CaO and 50% A1203. AI was added as FeAl in the tipping jet and B was added as ferrobore. The metal was not degassed. After casting in molds, rolling in blooms and forging in small, profiles, samples were taken for chemical analysis and measurement of mechanical characteristics.

It can be seen that the steel according to the invention corresponding to casting 1A has the maximum toughness practically equal to that of highly alloyed steel 18 NC 13 which it can therefore replace in this type of application, which results in very low savings. appreciable.

The three other castings, not in accordance with the invention, have lower toughness either due to lack of hardenability due to the absence of boron, this is the case for IB boron-free and ID borings with too weak soluble boron, or, despite sufficient hardenability of the

(i5

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too low an insoluble boron content, this is the case with casting 1C. In the latter, we also observe the existence of a coarse grain after the Me Quaid test. This poor grain control, despite a normal aluminum content, is a consequence of the nitrogen capture by the titanium.

A fifth casting was carried out. This steel of the same type as that of casting 1 A, IC and ID was produced in the same way as casting 1 A.

The chemical composition by weight (in%) is as follows:

VS

0.20

Mo

0.21

B total

0.0028

Yes

0.23

S

0.036

B soluble

0.0011

Mn

0.91

P

0.009

B insoluble

0.0017

Or

0.61

Al

0.035

Cr

0.55

n2

0.0100

This steel is not in accordance with the invention because its contents of Al, total B and N2 do not verify the relation (3).

Tests have shown that the hardenability was analogous to that of the same grade casting without B (example no. IB). In addition, its tenacity is insufficient.

Example 2

A type 18 MCDB 6 steel (AFNOR designation) was produced in the same way as the casting of Example 1 A.

Its chemical composition by weight (in%) is as follows:

VS

0.19

Mo

0.20

N2

0.0115

Yes

0.36

S

0.007

B total

0.0070

Mn

1.39

P

0.013

B soluble

0.0025

Or

0.28

Al

0.030

B insoluble

0.0045

Cr

1.03

o2

0.0030

This steel is therefore in accordance with the invention: the contents of N2, total boron and Al verify well the relation (3) and the ranges of composition (4), (5) and (6).

This steel was compared to a steel of the same grade without boron produced under similar conditions. The Jominy hardenability measurements and the toughness measurements were carried out under the same conditions as in Example 1, for the grade comprising boron.

The boron grade had a J 40 hardness (measured in a Jominy test tube 40 mm from the hardened end) 15 RC units higher than the boron-free grade. Likewise, the toughness of the boron grade was 7 KN higher than that of the boron-free grade.

Example 3

Two flows, marked 3A and 3B, of steel type 16 MCB 5 were produced in the basic arc furnace in a known manner:

Casting 3A

The metal was stirred with slag at 50% CaO and 50% A1203. The AI was added in the form of FeAl in the tilting jet and the B was added in the form of ferrobore in ingot molds. The metal was not degassed. After casting in molds, rolling in blooms and forging in small profiles, samples were taken for chemical analysis and measurement of mechanical characteristics.

The chemical composition by weight (in%) is as follows:

VS

0.17

Mo

0.03

N2

0.0130

Yes

0.24

S

0.0035

B total

0.0090

Mn

1.25

P

0.016

B soluble

0.0025

Or

0.29

Al

0.020

B insoluble

0.0065

Cr

0.93

o2

0.0040

This steel is in accordance with the invention: the contents of total N2, B and AI verify the relationship (1) and the content ranges (4), (5) and (6). The contents of soluble B and insoluble B are within the expected limits. However, it can be seen that the Al content corresponds to the minimum acceptable.

This steel was compared to a steel of the same grade without boron produced under similar conditions. The boron grade had a J 20 hardness (measured on a Jominy test tube 20 mm from the hardened end) 12 RC units higher than the io boron-free grade. The toughness of the boron grade was 5 KN higher than that of the boron-free grade.

Casting 3B

The metal was treated in the same way as for casting, 3 A, except that the Al was not added in the form of FeAl, but pure Al.

The chemical composition by weight (in%) is as follows:

0.16

Cr

1.06

Al

0.105

0.29

Mo

0.02

N2

0.0155

1.30

S

0.029

B total

0.050

0.12

P

0.023

The Al, B total and N2 contents verify the relation (3) well but the Al content is higher than the content range (4). Although the hardenability and toughness are satisfactory, there is a poorly controlled grain: 2 to 6 ASTM after Mac QUAID test.

Example 4

A 20 MB 5 type steel (AFNOR standard) was produced in a basic arc furnace in a known manner. Then, the metal was stirred with a slag containing 50% CaO and 50% A1203. The metal was then degassed under vacuum. AI and B were added in vacuo as pure Al and FeB. After casting in molds, rolling in blooms and forging in small profiles, samples were taken for chemical analysis and measurement of mechanical characteristics.

The chemical composition by weight (in%) is as follows:

0.19

Cr

0.15

Al

0.027

0.18

Mo

0.03

n2

0.0075

1.19

S

0.009

B total

0.0040

0.15

P

0.008

45 This steel is in accordance with the invention: the contents of N2, total B and Al verify well the relation (3) and the ranges (4), (5) and (6). The hardenability of this steel complies with that prescribed by standard AFNOR no. NF A 35—551. The toughness (measured on a hardened test piece) is excellent (KCU = 150 J / cm2 at n / a room temperature) above the minimum prescribed by the standard (90 J / cm2).

Example 5

A steel of type 21B 3 (AFNOR standard) was produced in the same way as the steel of example no. 4.

The chemical composition by weight (in%) is as follows:

VS

0.22

Cr

0.09

Al

0.033

Yes

0.22

Mo

0.02

n2

0.0070

Mn

0.82

S

0.0011

Or

0.19

P

0.008

B total

0.0035

This steel is in accordance with the invention: the contents of N2, total B and Al verify well the relation (1) and the ranges. The hardenability of this steel complies with that prescribed by standard AFNOR no. NF A 35—551. The toughness is good (KCU = 140 J / cm2, at ambient temperature), greater than the minimum prescribed by the standard (100 J / cm2).

VS

,Yes

Mn Ni

.m

3 5

Yes

Mn

Or

7

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Example 6

Three types of steel casting type 16 NCDB 2 (AFNOR standard) were produced in a high frequency oven in a known manner. The metal has not been brewed. It was degassed under vacuum. The AI and the B were added under vacuum, in pure Al and FeB form. After casting in molds, rolling in blooms and forging in small profiles, samples were taken for chemical analysis and measurement of mechanical characteristics. These flows are marked 6A, 6B and 6C.

Casting 6 A

The chemical composition by weight (in%) is as follows:

VS

0.16

Mo

0.22

n2

0.0130

Yes

0.30

S

0.013

B total

0.0045

Mn

0.42

P

0.008

B soluble

0.0025

Or

0.60

Al

0.075

B insoluble

0.0020

Cr

0.77

o2

0.0007

This steel is in accordance with the invention: the contents of N2, total B and Al verify well the relation (3) and the ranges of contents (4), (5) and (6). The contents of soluble B and insoluble B also conform to the description of the invention. This steel was compared to a steel of the same grade without boron produced under similar conditions. The boron grade has a J 20 hardness 10 RC units higher than the boron-free grade. The toughness is better than that of the same grade without B: the transition temperature in the KCV test is 70 ° C lower. The grain is well controlled: 6-8 ASTM after Mac QUAID test.

Casting 6B

The chemical composition by weight (in%) is as follows:

vs

0.12

Mo

0.22

. n2

0.0065

Yes

0.30

S

0.015

b total

0.0045

Mn

0.55

P

0.011 -

b soluble

0.0025

Or

0.61

Al

0.010

b insoluble

0.0020

Cr

0.75

o2

0.0020

This steel is not in accordance with the invention because if the contents of Al, total B and N2 verify relationship (3) well, the content is too low and is outside the content range (4).

The hardenability and toughness are similar to that of the steel of casting 6A but the Al content is too low. This results in a significant enlargement of the grain after Mac QUAID test: 3-8 ASTM.

Casting 6C.

The chemical composition by weight (in%) is as follows:

VS

0.13

Mo

0.22

n2

0.0060

Yes

0.35

S

0.013

B total

0.0065

Mn

0.51

P

0.019

B soluble

0.0050

Or

0.59

Al

0.043

B insoluble

0.0010

Cr

0.60

o2

0.0007

This steel is not in accordance with the invention because the contents of Al, total B and N2 do not verify the relation (3). The hardenability is correct, but the toughness is insufficient: in fact, the transition temperature KCV is 20 ° C., that is to say of the same order as the corresponding grade without boron. This can be explained by a too low content of insoluble boron.

Example 7

A type 16 MCB 5 steel was produced in a high-frequency furnace in a known manner. The metal has not been brewed. It was degassed under vacuum. Not only FeAl was introduced in vacuo in relatively small quantities, but FeV. Then FeB was added in the same way.

The chemical composition by weight (in%) is as follows:

VS

0.17

Cr

1.05

Al

0.10

S If

0.30

Mo

0.014

V

0.040

Mn

1.29

S

0.021

n2

0.0110

Or

0.09

P

0.003

B total

0.0070

, (l This steel is in accordance with the invention because its contents in total B, N2, Al and V verify the relation (2) and are inside the ranges (5), (6) and (7). Hardenability and toughness are similar to that of steel of the same grade in Example 3 A. In addition, the grain is well controlled (ASTM> 5) as long as the austenization temperature does not exceed 875 ° C Beyond that, the grain grows, which is well known for V steels.

Example 8

A steel of type 16 MCB 5 was produced in the same way: n as in Example 7, except that the addition of FeV was replaced by an addition of FeNb.

The chemical composition by weight (in%) is as follows:

VS

0.18

Cr

1.03

Al

0.011

Yes

0.31

Mo

0.014

Nb

0.101

Mn

1.30

S

0.022

n2

0.0090

Or

0.10

P

0.004

B

0.0040

This steel is in accordance with the invention because its contents in total B, N2 and Nb verify the relationship (2) and are inside the ranges (5), (6) and (7). Its hardenability and toughness are similar to that of steel of the same grade in Example no. 3 A. In addition, the grain is well controlled (ASTM> 5) up to 950 ° C.

35

Example 9

A 38 MB 5 type steel (AFNOR standard) was produced in a basic arc furnace in a known manner. Then, the metal was stirred with a slag containing 50% CaO and 50% A1203. Metal has

4o was then degassed under vacuum. AI and B were added in vacuo as pure Al and FeB. After casting in molds, rolling in blooms and forging in small profiles, samples were taken for chemical analysis and measurement of mechanical characteristics.

45 The chemical composition by weight (in%) is as follows:

VS

0.39

Cr

0.20

Al

0.020

Yes

0.18

Mo

0.02

n2

0.0050

Mn

1.21

S

0.012

B total

0.0040

Or

0.25

P

0.009

This steel is in accordance with the invention: the contents N2, total B and Al verify well the relation (1) and the ranges (4), (5) and (6). The hardenability of this steel complies with that prescribed by standard AFNOR no. NF A 35-551. The toughness, measured on a soaked test piece and returned to 550 ° C, 1 h., Is excellent (KCU = 105 J / cm2 at room temperature), greater than the minimum prescribed by the standard which is 50 J / cm2.

(, 0 Example 10

A type 38 B3 steel (Norma AFNOR) was produced in the same way as the steel of example no. 9.

The chemical composition by weight (in%) is as follows:

VS

0.39

Cr

0.20

Al

0.026

Yes

0.29

Mo

0.03

n2

0.0065

Mn

0.82

S

0.023

B total

0.0035

Or

0.16

P

0.030

619,270

This steel is in accordance with the invention: the contents of N2, total B and Al check the relationship (3) and the ranges (4), (5) and (6). The hardenability of this steel complies with that prescribed by standard AFNOR no. NF A 35-551. Tenacity, measured

8

on a specimen soaked and returned to 550 ° C, 1 h. is very good: KCU = 85 J / mm2 at room temperature, higher than the minimum prescribed by the standard which is 60 J / cm2.

VS

1 sheet of drawings

Claims (6)

619,270 1.9 3.4 6.7 (1) in which Al, V, Nb and Ta are the ppm contents of these elements, and in that the relation: 10 N -17 Bt- So = -300 ± 200 ppm (2) is satisfied, Bt and N being respectively the total boron and nitrogen contents expressed in ppm and So being calculated by the relation indicated above. 1. Boron steel characterized in that it contains 30 to 100 ppm of total boron, 40 to 220 ppm of nitrogen and in that the sum equivalent to aluminum, So, is between 200 and 800 ppm, this sum being expressed by the relation: So = Al + - ¥ • + - + 2. Steel according to claim 1, characterized in that it contains 30 to 60 ppm of total boron, 40 to 150 ppm of nitrogen, and in that the value of So is between 200 and 400 ppm. 2 3. Steel according to claim 1 or 2, characterized in that it is free of very powerful denitriding agents such as Ti or Zr. 4. Method for producing steel according to claim 1, characterized in that the contents of total boron Bt, of nitrogen, of aluminum and of V and / or Nb and / or Ta are adjusted after deoxidation , so as to satisfy relations (1) and (2). 5. Method according to claim 4, characterized in that the use of very powerful denitrurants such as Ti or Zr is prohibited. 6. Use of the steel according to claim 1 for the manufacture of mechanical parts in the quenched state, or case-hardened and quenched.