EP0191873B1

EP0191873B1 - Method and steel alloy for producing high-strength hot forgings

Info

Publication number: EP0191873B1
Application number: EP85101721A
Authority: EP
Inventors: Vesa Ollilainen
Original assignee: Ovako Oy AB; Ovako Steel Oy
Current assignee: Ovako Bar Oy
Priority date: 1985-02-16
Filing date: 1985-02-16
Publication date: 1989-06-28
Also published as: EP0191873A1; ATE44290T1; DE3571254D1

Abstract

This invention concerns a method and a steel alloy for producing high-strength hot forgings, the forging billet being cold-sheared from cast or hot-rolled steel and the finished forging being immediately quenched from the forging temperature, machined and used without tempering. The forgings can also be assembled by fusion welding a number of parts together. For manufacturing the forging, a steel is used with the following composition: <IMAGE> the remainder being iron and the usual impurities. The tensile strength of the forgings when quenched and without tempering is at least 900 N/mm<2>, 0.2-proof stress at least 700 N/mm<2> and the impact toughness at room temperature is at least 25 J measured on a V-notched impact test-piece. The hardness of steel, when air cooled, is at most 225 HB.

Description

This invention concerns a method and a steel alloy for the production of high-strength hot forgings, the billets being cold sheared from cast or hot-rolled steel and the finished forging being immediately quenched from the forging temperature, machined and used without tempering. The forgings can also be assembled by fusion welding a number of parts together.
Existing methods of producing hot forgings involve immediate heat treatment of the forging (quenching from forging temperature in water, for instance). Such a method is presented, for example, in European Patent Application EP 52308. Existing methods involve using, for the forgings, a steel of the following composition:
With existing methods, too, the forgings can be immediately heat treated by quenching in water, machined and used without tempering.
The weaknesses of existing methods relate to the compositions of the steel used, the high upper limit of the carbon content and the high manganese content. With existing methods it is necessary, in the case of forgings with a large cross-section, such as front axle beams of lorries, to use steel with a carbon content above 0.15% and with a manganese content of about 1.5% in order to obtain the necessary strength. Any lack of hardenability in the steel used has to be compensated by increasing the carbon content for thick forgings. This gives rise to various problems, the more serious being that:

1. a high carbon and manganese content increases the hardness of bar billets so that they are too hard for cold shearing;
2. the high manganese content results-on account of the tendency of segregation of manganese-in a banded structure which reduces fatigue strength and machinability;
3. the high manganese content also results in more pronounced consumption of refractories in the melting process of steel, and is liable to cause non metallic inclusions in the steel;
4. the use of manganese alone as an additive to increase hardness causes not only martensite but also an upper bainitic microstructure to form in large-cross-section forgings. The softness and brittleness of low-carbon upper bainite reduce the fatigue strength and increase the risk of brittle fracture;
5. if the carbon content is above 0.15%, there is a danger that the forging will tear during fusion welding.

The object of the method according to this invention is to present a method of producing hot forgings which does not exhibit the deficiencies inherent in existing methods.
A feature of the method covered by this invention is that, for manufacturing the forging, a steel is used of the following composition:
with hardness, when air-cooled, of, at most, 225 HB. The shearing of the billets from the cast bar or from the hot-rolled semi-products can be performed cold.
The method according to the invention is also characterized by the fact that the tensile strength of forgings obtained by this method is a least 900 N/mm², when quenched from forging temperature and without tempering, 0.2-proof stress at least 700 N/mm² and the impact toughness at room temperature at least 25 J measured on a V-notched specimen.
Another feature of the method covered by this invention is that, on large diameter forgings, such as the front axle beams of lorries, in order to increase the hardenability, boron additives are used in the steel, so that the steel contains the following elements:
Another feature of the method according to the invention is that, sulphur may also be added in the following proportion to improve machinability:
A further feature of the method according to the invention is that, because of the favourable composition of the steel, the forgings can be assembled by fusion welding from a number of parts.
The as-cast billets can be used directly as forging blanks. The invention also concerns a steel alloy suitable for producing high-strength hot forgings by direct quenching. A feature of this alloy is that the composition of the alloy is as follows:
and in which without tempering the tensile strength is at least 900 N/mm², 0.2-proof stress at least 700 N/mm² and the impact toughness at room temperature at least 25 J measured on a V-notched impact bar, and hardness, when air cooled, does not exceed 225 HB, when the forging produced from this alloy is intended to be immediately quenched (in water for instance) from the forging temperature.
As mentioned above the alloy according to the invention may also contain 0.02-0.15% sulphur to improve machinability.
With the method and alloy according to the invention it has been possible to avoid the weak points inherent in existing techniques by using steels with a favourable combination of alloying elements.
The minimum carbon content is determined by the minimum tensile strength required, while the maximum is determined by the maximum hardness of the air-cooled steel and the need for weldability and machinability in the quenched condition.
Silicon is an element which greatly strengthens iron and its content should be kept as low as possible for segregation.
During quenching, manganese also promotes the formation of brittle upper bainite in the microstructure together with martensite. After cooling in air, on the other hand, manganese greatly increases the amount of pearlite, thus strengthening the steel and making cold shearing difficult. For these reasons and also having regard to steel production techniques, the manganese content should not exceed about 1%.
Chromium is an advantageous alloying element for increasing hardenability. Chromium does not strengthen ferrite and its effect on strength is slight in the case of slow cooling.
Chromium promotes formation not only of martensite but also of a tough and ductile fine lamellar pearlite. Little chromium segregation occurs during solidification and chromium gives rise to no problems even in the manufacturing processes. The minimum amount of chromium is determined by economic factors. If greater hardenability is required, it is economically advantageous to use boron additions which implies successful aluminium and/or titanium alloying. Alloying with boron does not in any way increase the strength of air-cooled steel and is therefore advantageous particularly when cold shearing is involved.
Below are the test results obtained with forgings produced by using the method according to the invention:
The test results were obtained with hot forgings quenched in water from a finishing forging temperature of 950-1050⁰C without tempering. The results show that with the method according to the invention it is possible to produce high-strength hot forgings by quenching the forgings immediately from forging temperature, with an impact strength equivalent to that of conventionally heat-treated steels. Air-cooled steel according to the invention is-as far as hardness is concerned-suitable for cold shearing.

Claims

1. Method for producing high-strength hot-forgings, which consists of the following process steps: (1) heating a billet up to the forging temperature of 800 to 1300°C, (2) forging into a component and (3) direct quenching the component from the forging heat (into water, for instance), and in which the forging is made of a steel with a chemical composition comprising:

and in which without tempering the tensile strength of the forged component is a least 900 N/mm², 0.2-proof stress at least 700 N/mm² and the impact toughness at room temperature at least 25 J measured on a V-notched impact bar, and hardness, when air cooled, does not exceed 225 HB.

2. Method according to claim 1 but in which the steel contains:

3. Method according to claim 1 and 2 but in which the steel contains the following elements:

4. Method according to claims 1, 2 and 3 but in which the steel contains:

5. Method according to claims 1 to 4 but in which the forgings are assembled by fusion welding.

6. Method according to claims 1 to 4 but in which a continuous-cast product is used in as-cast condition as forging billet.

7. Alloy suitable for producing high-strength hot forgings by direct quenching comprising:

and in which without tempering the tensile strength of the forged component is at least 900 N/mm², 0.2-proof stress at least 700 N/mm² and the impact toughness at room temperature at least 25 J measured on a V-notched impact bar, and hardness, when air cooled, does not exceed 225 HB.

8. Alloy according to claim 7 but in which the C-content of the steel is:

9. Alloy according to claim 7 or 8 but in which the Al-, Ti- and B-contents are:

10. Alloy according to claims 7, 8 and 9 but in which the steel contains: