EP0538158B1 - Low alloy, resulfurized free-cutting steel - Google Patents

Abstract

Steels of particular compositions which, because of their sulphur contents and of the composition of the oxidised inclusions, are particularly suitable for screw-cutting and machining at high speed using carbide or ceramic tools. The steels contain, in weight %: C - 0.05 to 0.45 % Mn - 0.80 to 1.65 % Si - 0.10 to 0.35 % P - max. 0.030 % S - 0.050 to 0.12 % and the compositions of their oxidised inclusions are, in weight %: Al2O3 - 15 to 50 % SiO2 - 20 to 60 % CaO + MnO - 15 to 50 %. Despite the presence of sulphur, these steels retain good mechanical characteristics.

Description

The present invention relates to steels weakly alloyed with carbon-manganese and relates to steels of particular compositions which, in particular because of their sulfur content and of the composition of their oxidized inclusions, are particularly suitable for high speed machining in particularly by carbide or ceramic tools and by cutting at high cutting speed, in particular from tubes produced in said steels.

Steels are currently known whose machinability characteristics are improved by the presence of sulfur in a content range from 0.020 to 0.040% by weight. However, these steels have limited machining performance which does not allow high cutting speeds to be achieved and / or the use of carbide or ceramic cutting tools under good conditions. On the other hand, these steels with improved machinability generally keep mechanical characteristics, in particular in traction, close to those corresponding to their basic composition.

There are also known steels for bar turning semi-calmed by silicon additions containing about 0.3% of sulfur by weight. These steels can be machined at high cutting speed with carbide or ceramic cutting tools.

In some cases, even, the search for the highest performance requires the combined addition of sulfur, tellurium (or selenium) and / or lead or bismuth.

These steels generally have a large amount of inclusions of oxides and sulfides and a high oxygen content greater than 0.0100% by weight.

These steels, the essential, if not exclusive, characteristic of which is that they have properties as high as possible in bar turning have a very poor resilience and it is impossible to assemble them by arc welding. We can also highlight the toxicity problems associated with handling, particularly at the the elaboration of the liquid metal of the Tellurium, Selenium, Lead, Bismuth elements as well as during the recycling of scrap and shavings.

In addition, steels with 0.30% by weight of sulfur have low mechanical tensile properties, close to that of A37 steel according to AFNOR standard. Nuances for heat treatment have been standardized, but their bar turning performance is therefore lower.

Patent application DE 2323623 describes sulfur steels containing in% by weight: C 0.10 to 0.60; If 0.05 to 0.40; S 0.04 to 0.15; Ca 0.0005 to 0.0015 and including oxide inclusions: Ca 0 - Al ₂ O _{3 -} Si O ₂ whose composition is in the mullite domain of the corresponding ternary diagram. The contents of the components of the oxide mixture are not specified; only an O ₂ content of the liquid steel is indicated, before Ca / Si deoxidation, of between 0.01 and 0.04% by weight.

The machining results of the steels thus produced show a significant improvement in the ability to cut at high speed mainly using carbide-based tools. No precise indication is given on the mechanical properties of these steels.

US patent 4,115,111 describes free cutting steels containing in% by weight: C 0.05 to 0.65; If 0.10 to 0.40; Mn 0.25 to 1.70; Ni 0 to 4.5; Cr 0 to 3.5; Mo 0 to 1 and Ca 0.0002 to 0.0010 as well as at least 1 element of the group comprising Pb at a rate of 0.03 to 0.30 and S at a rate of 0.04 to 0.10 this steel containing plus 30 to 100 g / tonne of oxide inclusions containing in% by weight: Si O ₂ 40 to 60; Ca 0 5 to 30 and Al ₂ O ₃ 25 to 40; these inclusions having a melting point of between 1200 and 1600 ° C.

This document does not specify the exact domain of the Ca 0 - Al ₂ O ₃ - Si O ₂ ternary diagram in which the composition of the inclusions should preferably be situated. However, the melting temperature of the oxides below 1600 ° C. and the minimum Ca 0 content of 5% show that most of the domain of mullite and that adjacent to corundum are not acceptable.

Furthermore, while the maximum content of oxide inclusions is defined as being 100 g / tonne, the minimum oxygen content of 0.01% by weight specified in several places in the application DE 2323623 already corresponds to 100 g / tonne of oxygen, that is to say approximately twice the weight of oxides.

The aim of the present invention is to propose a family of carbon-manganese steels which can be used in particular for mechanical construction having an excellent turning ability and giving excellent performance at high speed machining by carbide or ceramic tools. , performance superior to that obtained with steels with improved machinability mentioned above while retaining good mechanical characteristics and not having the drawbacks of free-cutting steels at 0.30% by weight of sulfur mentioned above.

The object of the invention is in particular to create, by the use of a specific process, a family of clean free cutting steels, not requiring the use of toxic elements, covering a wide range of mechanical characteristics and having resilience characteristics much better than those of free-cutting steel at 0.30% by weight of sulfur.

The process according to the invention is a process for the preparation of a steel suitable for high-speed turning in which a carbon-manganese steel is produced containing in% by weight C 0.05 to 0.45; Mn 0.80 to 1.65; S 0.050 to 0.12; P ≤ 0.030 and in which, after the initial phase of preparation in an arc furnace, then the transfer of the liquid steel into a ladle, this steel is treated to deoxidize it by introduction into a ladle filled with silico- calcium whose composition and quantity are determined so as to obtain in steel, a content in% by weight of: If 0.10 to 0.35 and Ca 0.001 to 0.010, the inclusions of oxides distributed in the blank containing in% by weight: Al ₂ O ₃ 15 to 50; If O ₂ 20 to 60 and Ca O + Mn 0 15 to 50, the composition of these inclusions being located inside the domain comprising the anorthite and the gehlenite of the ternary diagram Al ₂ O ₃ -Si O ₂ - Ca O , the Mn O content being added to Ca O and limited to 10% of all the oxides, the aluminum being entirely in the form oxidized and the content in% by weight of the total oxygen being between 0.0015 and 0.0080 the remainder being constituted by the usual impurities and iron.

The process claimed according to the invention makes it possible to prepare carbon-manganese steels with a limited sulfur content, the pocket deoxidation conditions of which are filled with Ca / Si cored wire, are precisely controlled by limiting the total oxygen content between 0.0015 and 0.0080% by weight and by virtue of the addition of an excess of Ca such that the steel obtained contains 0.0010 to 0.0100% by weight of Ca in solution.

Carbone -

0,05 à 0,45 %

Manganèse -

0,80 à 1,65 %

Silicium -

0,10 à 0,35 %

Phosphore -

inférieur à 0,030 %

ainsi que les impuretés habituelles, le solde étant du fer.The carbon-manganese steels suitable for free cutting of the family prepared by the invention have the following composition expressed in weight percentages:

Carbon -

0.05 to 0.45%

Manganese -

0.80 to 1.65%

Silicon -

0.10 to 0.35%

Phosphorus -

less than 0.030%

as well as the usual impurities, the balance being iron.

Soufre -

0;050 à 0,12 %

Calcium -

0,001 à 0,010 %

l'aluminium étant entièrement sous forme oxydée, et les inclusions non métalliques d'oxydes comprennent en pourcentage pondéraux :

Al ₂ O₃

15 à 50 %

Si O₂

20 à 60 %

CaO + MnO

15 à 50 %

la teneur totale en oxygène étant comprise entre 0,0015 et 0,0080 %.They also contain:

Sulfur -

0; 050 to 0.12%

Calcium -

0.001 to 0.010%

aluminum being entirely in oxidized form, and the non-metallic inclusions of oxides comprise in percentage by weight:

Al ₂ O ₃

15 to 50%

If O ₂

20 to 60%

CaO + MnO

15 to 50%

the total oxygen content being between 0.0015 and 0.0080%.

The non-metallic inclusions are located on the representation diagram of the Al ₂ O ₃ - Si O ₂ - MnO / CaO system in the Anorthite or Gehlenite domain. Such a representation diagram of the Al ₂ O ₃ - Si O ₂ - MnO / CaO system can be found for example in "Phase diagrams for ceramists" Edited and Published by the American Ceramic Society 1964 figure 630 page 219. MnO and CaO are oxides whose role is considered similar. Insofar as the percentage of MnO is low (less than 10%), it is therefore possible to cumulate CaO + MnO to refer to the diagram of the system Al ₂ O ₃ -Si O ₂ - CaO where only CaO is represented and take the value of CaO + MnO for a value of CaO.

≤ 0,48 %.Advantageously for the implementation of steel by welding by an electric arc welding process with filler metal, the carbon content is limited to 0.22% by weight and the sum carbon content plus content in manganese / 6 at 0.48% by weight, ie C% + $\frac{\text{Mn\%}}{\text{6}}$

≤ 0.48%.

Advantageously also the sulfur content is between 0.080% and 0.12% by weight.

The composition described above makes it possible, while obtaining good mechanical properties under tension, to obtain a type of favorable inclusion of oxides and sulphides, distributed homogeneously in the metal and in a limited and controlled quantity. This set of characteristics makes these steels suitable for bar turning in excellent conditions, this despite the relatively low sulfur content, in particular for bar turning using carbide or ceramic tools.

It is particularly advantageous for manufacturing steel to use the rotary continuous casting process, as is known.

This process is for example described in the article of the CIT metallurgical review, February 1981 pages 119-135.

Such a process makes it possible to ensure the homogeneity of the metal, both in terms of the chemical composition and the distribution of the inclusions, over the entire length of casting.

The production of steel which is then continuously cast, which is advantageously made in a ladle after fusion in an electric arc furnace, will be carried out with care so as to always keep the metal homogeneous and protected from oxidation, the metal being, in its final phase treated with Calcium, by the introduction into the pocket of liquid metal of a cored wire as described in European patent n ° 87 420052.0 based on silico-calcium.

It is particularly advantageous to use the steel according to the invention in the form of a tube, for example of drawn tubes, this reducing the turning times by eliminating the drilling operation of the internal hole.

By using a tube, it is also certain to avoid the central segregations which could be found at the heart of the bars, segregations which, as already explained above, are very reduced if the rotary continuous casting process is used.

In this way the use of tubes instead of bars is an additional guarantee for the product intended for bar turning.

By way of non-limiting example, the results of a steel according to the invention have been compared with pre-existing standardized steels.

• Acier A selon :
  TU52b (norme AFNOR NFA 49310)

  C :

  0,178 %

  Si :

  0,265 %

  Mn :

  1,270 %

  S :

  0,006 %

  P :

  0,014 %

  Al :

  0,024 %

  Ca :

  0,0015 %

  O total :

  0,0022 %

The compositions are as follows in percentages by weight:

• Steel A according to:
  TU52b (AFNOR standard NFA 49310)

  VS :

  0.178%

  Yes :

  0.265%

  Mn:

  1.270%

  S:

  0.006%

  P:

  0.014%

  Al:

  0.024%

  Ca:

  0.0015%

  O total:

  0.0022%

• Acier B (version acier à usinabilité améliorée de l'acier A).
  selon
     TU 52 b q2 (norme AFNOR NFA 49310)

  C :

  0,180 %

  Si :

  0,183 %

  Mn :

  1,340 %

  S :

  0,028 %

  P :

  0,016 %

  Al :

  0,022 %

  Ca

  0,0016 %

  O total :

  0,0022 %

The oxide inclusions consist of calcium aluminates Al ₂ O ₃ - CaO or spinels Al ₂ O ₃ - Mgo associated with mixed sulfides (Ca, Mn) S. There are also isolated inclusions of manganese sulfides MnS in small quantities.

• Steel B (steel version with improved machinability of steel A).
  according to
  TU 52 b q2 (AFNOR standard NFA 49310)

  VS :

  0.180%

  Yes :

  0.183%

  Mn:

  1.340%

  S:

  0.028%

  P:

  0.016%

  Al:

  0.022%

  It

  0.0016%

  O total:

  0.0022%

Here the oxide inclusions consist of calcium aluminates Al ₂ O ₃ - CaO or spinels Al ₂ O ₃ - MgO associated with mixed sulfides (Ca, Mn) S. Besides these inclusions, there are numerous inclusions of manganese sulfide.

C :

0,172 %

Si :

0,191 %

Mn :

1,430 %

P :

0,018 %

S :

0,090 %

Al :

≤ à 0,005 %

Ca :

0,0014 %

O total :

0,0026 %

ayant des inclusions d'oxydes dont la composition moyenne est la suivante en pourcentages pondéraux :

Al ₂ O₃ :

34 %

Si O₂ :

30 %

CaO :

36 %

Steel C according to the invention.

VS :

0.172%

Yes :

0.191%

Mn:

1.430%

P:

0.018%

S:

0.090%

Al:

≤ 0.005%

Ca:

0.0014%

O total:

0.0026%

having oxide inclusions the average composition of which is as follows in percentages by weight:

Al ₂ O ₃ :

34%

If O ₂ :

30 %

CaO:

36%

This composition is located in the Gehlenite domain of the previously mentioned Al ₂ O ₃ - Si O ₂ - CaO ternary diagram.

In this steel C the sulfur is essentially combined in the form of MnS sulphides.

These three steels were subjected to a rolling test on a drawn-out tube with carbide tool, referenced SAFETY TPMR 16 n ° 030476S4 from the company SAFETY, uncoated and without lubricant.

• la durée pour créer une usure de 0,4 mm de l'arête de l'outil à une vitesse de coupe de 180 m/min (durée de vie)
  et
• la vitesse de coupe en m/min donnant une usure de 0,4 mm de l'arête de l'outil en 20 minutes (VB 20-0,4).

We compared:

• the time required to create 0.4 mm wear on the edge of the tool at a cutting speed of 180 m / min (lifetime)
  and
• the cutting speed in m / min giving a wear of 0.4 mm of the edge of the tool in 20 minutes (VB 20-0.4).

The results are as follows: lifetime VB 20-0.4 Steel A 10 minutes 160 m / min Steel B with improved machinability 25 mins 220 m / min Steel C of the invention 87 mins 300 m / min

It can be seen that the steel according to the invention (steel C) presents results far superior to those obtained with existing improved machinability steel (steel B) and without common measure with steel not intended for machining (steel AT).

We also compared by a productivity test with carbide tools with identical criteria a free-cutting steel at 0.30% by weight of sulfur with the steel C of the invention.

In 6 hours, almost identical production was found for the two steels, this despite the differences in unfavorable mechanical characteristics for the steel C, steel of the invention:

R ρ 0,2

539 MPa

Rm

558 MPa

A % 5,65 √S

11,4 %

Steel 0.30% sulfur by weight according to NFA 35561 (grade S300).

R ρ 0.2

539 MPa

Rm

558 MPa

A% 5.65 √S

11.4%

R ρ 0,2

704 MPa

Rm

765 MPa

A % 5,65 √S

8,4 %

Steel C of the invention

R ρ 0.2

704 MPa

Rm

765 MPa

A% 5.65 √S

8.4%

This clearly shows that the steel according to the invention is particularly suitable for high-speed turning with carbide or ceramic tools.

The steels according to the invention therefore have the advantage of having much better machinability properties than those of steels with improved machinability and of the same order of magnitude as those of free-cutting steels with 0.30% by weight of sulfur without having , especially in terms of mechanical characteristics and implementation of the disadvantages of these steels at 0.30% by weight of sulfur.

Claims (5)

1. A process for preparing a steel suitable for high-speed free-cutting which comprises producing a carbon-manganese steel containing in % by weight: C 0.05 to 0.45; Mn 0.80 to 1.65; S 0.050 to 0.12; P ≤ 0.030 and, after the initial phase of production in an arc furnace and then transfer of the liquid steel into a ladle, treating said steel to deoxidise it by the introduction into the ladle in the form of a coated wire of a silico-calcium whose composition and quantity are so determined as to produce in the steel a content in % by weight of: Si 0.10 to 0.35 and Ca 0.001 to 0.010, the inclusions of oxides which are distributed in the blank containing in % by weight: Al₂O₃ 15 to 50; SiO₂ 20 to 60 and CaO + MnO 15 to 50, the composition of said inclusions being within the area comprising anorthite and gehlenite of the ternary diagram Al₂O₃ - SiO₂ - CaO, the content of MnO being added to CaO and limited to 10% of the total of the oxides, the aluminium being entirely in oxidised form and the content in % by weight of the total oxygen being between 0.0015 and 0.0080, the balance being formed by the usual impurities and iron.
2. A process according to claim 1 characterised in that the content of carbon is less than or equal to 0.22% by weight, the sum C + $\frac{\text{Mn}}{\text{6}}$

   

   being less than or equal to 0.48% by weight.
3. A process according to claim 1 or claim 2 characterised in that the content of sulphur is not less than 0.080% by weight.
4. A process according to one of claims 1 to 3 characterised in that the steel is cast by the continuous rotary casting process.
5. A steel tube for free-cutting characterised in that the base steel was prepared using the process according to any one of claims 1 to to 4.