METHOD AND STEEL ALLOY FOR PRODUCING HIGH-STRENGTH HOT FORGINGS

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 tem­ perature, machined and used without tempering. The forg­ ings can also be assembled by fusion welding a number of parts together. For manufacturing the forging, a steel is used with the following composition: the remainder being iron and the usual impurities. The tensile strength of the forgings when quenched and without tempering is at least 900 N/mm², 0.2-proof stress at least 700 N/mm² and the impact toughness at room temper­ ature 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: METHOD AND STEEL ALLOY FOR PRODUCING HIGH-STRENGTH HOT FORGINGS 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: carbon = 0.05 - 0.25 % manganese = 1.0 - 2.0 % 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 melt ing process of steel, and is liable to cause non metallic inclusions in the steel; 4. the use of manganese alone as an additive to in crease 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: carbon 0.04 - 0,14 % silicon 0.05 - 0.50 % manganese 0.56 - 0.99 % chromium 1.01 - 2.00 % aluminium 0 - 0.1 % titanium 0 - 0.05 % boron 0 - 0.015 % the remainder being iron and the usual impurities with hardness, when air-colled, of, at most, 225 LIB. 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 at 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 harden abil.ty, boron additives are used in the steel, so that the steel contains the following elements: aluminium 0.003 - 0.10 % titanium 0.003 - 0.05 % boron 0.0005 - 0.015 % Another feature of the method according to the invention is that, in addition to the above mentioned elements, sulphur may also be added in the following proportion to improve machinability: sulphur 0.02 - 0.15 % 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, with a composition as follows: carbon 0.04 - 0.14 %, advantageously 0.07 0.14 % silicon 0.05 - 0.50 % manganese 0.56 - 0.99 % chromium 1.01 - 2.00 % aluminium 0 - 0.1 %, advantageously 0.003 0.1 % titanium 0 - 0.05 %, advantageously 0.003 0.05 % boron 0 - 0.015 %, advantageously 0.0005 0.015 % the remainder being iron and the usual impurities, and in which without tempering the tcnsile strength is at least 900 N/mm2, 0.2-proof stress at least 700 N/mm.2 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 adavantageous alloying element for in creasing 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 manu facturing processes. The minimum amount of chromium is determined by economic factors. If greater hardenabili ty is required, it is economically advantageous to use boron souiticns 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 shear ing is involved. Below are the test results obtained with forgings pro duced by using the method according to the invention: Table 1 Analysis of Test Steel Cast No. C ¯ ¯ Al Si Mn S Cr Al Ti B 10.12. 0.12 0.87 0.017 1.32 0.03 0.02 0.003 2 0.13 0.14 0.99 0.016 1.36 0.004 0.03 0.002 3 0.07 0.15 0.98 0.015 1.41 0.011 0.03 0.002 4 0.11 0.14 0.57 0.025 1.29 0.043 0.02 0.002 5 0.13 0.13 0.98 0.015 1.39 0.003 - 6 0.04 0.30 0.85 0.038 1.38 0.038 0.03 0.003 Table 2 Mechanical Properties of Trial Forginqs Cast No. Rp0.2 R A Z KV Equivalent round bar m 5 bar thickness, N/mm2 N/mm2 % $ +OOC mm 1 858 1010 13.7 62.4 92 80 2 945 1265 13.0 61.0 51 25 3 878 1099 13.5 64.0 39 25 4 918 1192 14.5 61.0 52 25 5 986 1207 12.5 60.0 54 10 6 a) 765 976 11.0 53.0 93 25 6 b) 745 936 15.1 65.0 35 60 The test results were obtained with hot forgings quenched in water from a finishing forging temperature of 950 - 1050 C without tempering. The results show that with the method according to the invention it is possible to produce highstrength hot forgings by quenching the forgings inwiediately from forging temperature, with an impact strength equivalent to that of conventionally heat-treated steels. Air-cocled steel according to the invention is - as far as hardness is concerned - suitable for cold shearing.

Claims: CLAIMS 1. Method for producing high-strength hot forgings by direct quenching (in water, for instance) of the forged article from the forging temperature, and in which the forging is made of steel with a chemical composition as follows: carbon 0.04 - 0.14 % silicon 0.05 - 0.50 % manganese 0.56 - 0.99 % chromium 1.01 - 2.00 % aluminium 0 - 0.1 % titanium 0 - 0.05 % borcn 0 - 0.015 % the remainder being iron and the usual impurities and in which without tempering the tensile strength is at least 900 N/mn2, 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: carbon 0.07 - 0.14 % 3. Method according to claim 1 and 2 but in which the steel contains the following elements: aluminium 0.003 - 0.1 % titanium 0.003 - 0.05 % boron 0.0C05 - 0.015 % 4. Method according to claims 1, 2 and 3 but in which the steel also contains: sulphur 0.02 - 0.15 % 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 con tinuous-cast product is used in as-cast condition as forging billet. 7. Allby suitable for producing high-strength hot forgings by direct quenching with a composition as follows: carbon 0.04 - 0.14 % silicon 0.05 - 0.50 % manganese 0.56 - 0.99 8 chromium 1.01 - 2,00 % aluminium 0 - 0.1 % titanium 0 - 0.05 % boron 0 - 0.015 % the remaineder being iron and the usual impurities 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 roorn 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. 8. Alloy according to claim 7 but in which the C-content of the steel is: carbon 0.07 - 0.14 %. 9. Alloy according to claim 7 or 8 but in which the Al-, Ti- and B-contents are: aluminium 0.003 - 0.1 % titanium 0.003 - 0.05 % boron 0.0005 - 0.015 % 10. Alloy according to claims 7, 8 and 9 but in which the steel also contains: sulphur 0.02 - 0.15 %

Inventors: OLLILAINEN, VESA
Applicants: OVAKO OY AB
Application year: 1985
Main classification: C21D1
IPC codes: C22C38, C21D1