HIGH STRENGTH HIGH TOUGHNESS STEEL

Abstract: A relatively low cost, high strength, high toughness bar and sheet steel, which is substantially non-susceptible to the formation of delayed surface cracks in the as-rolled condition and its method of preparation, are provided, the constitution of the steel on a percentage mass to mass basis being as follows: C = 0,21 - 0,28 Mn = 0,80 - 1,80 Cr = 1,60 - 2,10 Si = 0,35 maximum Al = 0,02 - 0,05 P and S each = 0,025 maximum Fe = the balance; the steel being characterised in that its composition is such that, upon air cooling following rolling, the transformation temperature of the steel during the cooling is at a sufficiently high level to ensure that there is sufficient thermal contraction possible after the transformation has been completed to accomodate at least the thermal expansion which had taken place during the transformation.

Description: THIS invention relates to a high strength high toughness steel, and its method of manufacture. Such steel, particularly in the form of round bars, can be utilised in the manufacturing of bolts, chains, agricultural implements such as spades, etc. The steels which have thus far been manufactured for the aforesaid purpose, suffer from the disadvantages that they either include a relatively high concentration of the relatively expensive alloying elements such as molydenum, nickel and chromium and/or that they require special heat treatments in their manufacture. Apart from the fact that such a high alloy content makes the steel expensive, it has also been found that such steels are more susceptible to the development or delayed surface cracks, especially in the case of round bars. It is accordingly an aim of this invention to provide a novel steel which can be used in the aforesaid applications, and a method for its manufacture, with which the aforesaid problems may be overcome or at least minimised. The present invention provides a method of manufacturing a relatively low cost, high strength, high toughness bar and sheet steel which is substantially non-susceptible to the formation of delayed surface cracks in the as rolled condition, including the steps of providing a steel composition on a percentage mass to mass basis within the following range: C = 0,21 - 0,28 Mn = 0,80 - 1,80 Cr = 1,60 - 2,10 Si = 0,35 maximum Al = 0,013 - 0,057 P and S each = 0.025 maximum Fe = the balance; hot rolling the steel; air cooling the steel; and then subjecting the steel to a heat treatment entailing heating the steel to an austeniting temperature of around 900°C, followed by cooling the steel product by quenching with water or oil or, where the steel product is relatively thin, by air cooling the steel product. In this manner the development of residual stresses on the surface of the steel, which has been found to be the main cause of delayed surface cracking, are avoided, while the properties of hardness, toughness and tensile strength required for the aforesaid purpose, are retained. Preferably, the method further comprises the step of tempering the heat treated product at a temperature of around 225°C for one hour per 25 mm thickness. The present invention also provides a relatively low cost bar and sheet steel which has the following constituion on a percentage mass to mass basis: C = 0,21 - 0,28 Mn = 0,80 - 1,80 Cr = 1,60 - 2,10 Si = 0,35 maximum Al = 0,013 - 0,057 P and S each = 0.025 maximum Fe = the balance; and which has been hot rolled, air cooled, and subjected to a subsequent heat treatment entailing heating the steel to an austeniting temperature of around 900°C, followed by cooling the steel product by quenching with water or oil, or where the steel product is relatively thin, by air cooling the steel product, the steel having a hardness of 470-520 Vickers, a yield limit of 1250-1350 MPa; a tensile strength of 1500-1650 MPa and a charpy toughness of 30-60 joule at 20°C; the steel being furthermore substantially non-susceptible to the formation of delayed surface cracks. It is believed that the resultant residual stress on the surface of a bar made of such steel is primarily dependent on the total volume change of the core subsequent to that instant when the surface of the bar has transformed to form a solid "cylinder" of martensite or bainite. Prior to that critical instant, high surface residual stresses cannot develop because the maximum value of residual stresses that can be accommodated in the surface structure (which is still austenite prior to that instant) is equal to the yield strength of the structure and in the case of austenite, this value is rather low. However, as soon as a solid "cylinder" of martensite/bainite has formed on the surface, much higher residual stresses can develop due to the high yield strength of these structures. If the total volume change of the core subsequent to that instant is positive, the expansion of the core will result in detrimental residual tensile surface stresses. Conversely, if the total subsequent volume change of the core is negative, the contraction of the core will result in compressive surface stresses, which are beneficial. The effect of residual stresses on the surfaces of both air cooled and water quenched steel bars in relation to the development of delayed surface cracks, in indicated in figure 1 of the enclosed drawings, which reflects experimental results obtained by the Applicant. As will be noted, there is a good correlation between high residual tensile stresses and crack occurence. Applicant has found that the restriction of the chromium content of the steel to the stated range is critical in order to ensure both low residual stresses in the as rolled condition and good toughness and strength after the final heat treatment of the product. The present invention will now be described in greater detail by way of example only with reference to the accompanying drawings in which Figures 1 to 3 are graphs showing experimentally determined relationships between various parameters of steels made in accordance with the invention. The interrelationship between residual surface stresses (and hence crack development) and chromium content is shown in the enclosed figure 2 of the drawings which reflects the results obtained experimentally with three bars of different diameters made of steel according to the invention. As will be noted from figure 2, the residual stress level on such a steel increases dramatically with increased chromium content. On the other hand, as indicated in the following table, the Charpy-properties of the steel are fairly poor when the chromium content is below 2%. Properties of the experimental steels D2 and D5 (32mm rounds, water quenched and tempered at 200°C for one hour) compared with an existing steel QT4. Steel Hardness (HV30 kgf) Charpy properties at Tensile properties Rp (0,2%) MPa Rm (MPa) area % Rad of % el - 10°C 20°C D2 473 26 35 - 48 1218 1501 30,5 11,6 D5 502 42 47 1356 1643 55,4 12,7 QT4 508 35 41 1279 1578 12,6 Chemical compositions of existing and experimental steel types. Steel C % Mn % P % S % Si % Cu % Cr % Al % QT4 0,24 1,55 0,014 0,001 0,18 - 3,58 0,013 D2 0,25 1,26 0,009 0,007 0,31 0,03 0,95 0,012 D5 0,27 1,13 0,010 0,005 0,28 0,05 1,93 0,057 Experimental steel D2 is not in accordance with the present invention since it has Cr and Al amounts which are lower than required by the steel composition of the invention. It has accordingly been found that at higher chromium levels than that of the stated range, delayed surface cracking occurred in the as rolled condition, while at lower chromium levels than that or the stated range, adequate tensile and impact strength levels for the stated purpose could not be realised after heat treatment of the final product. It will be appreciated that the chromium level of a steel according to the invention is much lower than that of existing steels utilised for the same purpose. Applicant has however found that the achievement of the required properties can be enhanced through an appropriate selection of the concentration of the other elements, particularly the manganese, within the aforesaid range. Furthermore, apart from a cost advantage, another advantage of such low chromium content is that the steel of the invention need not be heated to the same relatively high temperatures usually required for similar steels during their heat treatment. The effect of changes in the carbon content of the steel on impact energy levels is shown in the enclosed figure 3, which reflects results obtained experimentally. From this it will be noted that an increase of carbon content of a 20mm bar from 0,24 to 0,31%, gives a decrease in Charpy values at 20°C from 60 to 20 Joule. Applicant has found that the best Charpy properties were obtained with water quenched and tempered (250°C, one hour) 20mm bars, in which case a 20°C Charpy value of 49 - 64 Joule was obtained. Even at fairly low Charpy test temperatures, very good Joule values (25 - 50J at -10°C) were still obtained. Applicant has found that the Charpy properties of the oil quenched samples were poor, which could possibly be attributed to bainite formation during the typical slow cooling in the M s -temperature region. In one method for the preparation of a steel according to the invention, which will now be described by way of example, a steel melt of a constitution chosen within the aforesaid range was prepared and allowed to solidify. It was then reheated to approximately 1250°C, rolled into the required shape, and allowed to cool. The solidified steel product was reheated to ± 900°C for one hour per 25mm thickness, whereafter it was quenched with water or oil, but preferably water, or, where the material was very thin, merely by air cooling. For optimum toughness the steel was then tempered at a temperature in the order of +/- 250°C for one hour per 25 mm thickness in order to obtain a product with the optimum properties within the aforesaid stated range. This is, however, an optional step and applicant has found that without it an acceptable product was still possible although its toughness value was slightly lower than that given above. In a further experiment involving full production melt, round bars of 9, 16, 20 and 32 mm diameter were rolled from steel according to the invention. Some of the properties of this steel are reflected in the following table: C Z % Mn Z % P Z % S Z % Si Z % Cr Z % Al Z % H % Specification 0,21/ 0,26 0.90/ 1,25 0,025 max 0,025 max 0,10/ 0,35 1,60/ 2,0 0.02/ 0,05 Pit analysis 0,24 1,18 0,013 0,010 0,16 1,87 0,018 1,5 ppm Leco product analysis 0,24/ 0,31 1,05/ 1,20 0,013/ 0,015 0,007 0,010 0,16/ 0,17 1,76/ 1,80 0,013 0,014 As is well known to the man skilled in the art, the term 'pit analysis' refers to an analysis of chemical composition performed on the molten metal prior to forming a product and the term 'Leco product analysis' refers to an analysis of chemical composition performed on a finished metal product. The principal residual surface stresses of these bars in various heat treatment conditions were determined, and are compared in the following table to that of production bars of conventional ones having a higher Cr analysis of 4%. The low residual stresses of the steel according to the invention bars in the air-cooled condition resulted in the bars not developing cracks in either the as-rolled, oil quenched or water quenched condition. Extensive optical, dye penetrant, magnetic fluorescent particle and metallographical examinations were done on a number of such bars and, except for cracks associated with rolling defects in the front ends of the bars, the bars were free of defects. Some in-line quenched 20mm bars, however, developed cracks. Tensile properties in various heat treatment conditions were determined according to ASTM and are given in the following table. The good combinations of strength and ductility in the samples tempered at 200-250°C should be noted. Tensile properties in various heat treatment conditions Section size and heat treatment Yield stress Rp 0,21 (MPa) Ultimate tensile stress (MPa) % Elongation % Reduction in area 20 mm water quenched (WQ), tempered (T) at 200°C 1257 1583 14,3 56 20 mm oil quenched (OQ), T 200°C 1253 1633 13,3 53 20 mm WQ T250 1194 1470 12,1 66 32 mm WQ T200 1356 1701 12,1 56 32 mm OQ T200 1180 1502 14,4 56 20 mm WQ T675 727 823 19,7 72 32 mm WQ T675 747 851 19,8 72 Other properties which were determined are given in the following table . Heat treatment condition and section size Charpy properties Vickers hardness (30 kgf) Test temperature (°C) Joule value 20 mm OQ T250 -10 20,30 480 20 30,30 32 mm OQ T250 -20 16,20 490 20 29,35 20 mm OQ T400 20 21,21 32 mm WQ T250 501 32 mm WQ T200 546 32 mm OQ T200 475 20 mm OQ T350 20 9 It will be appreciated that the invention provides a steel and a method for its preparation, of relatively low cost, but with a sufficiently high strength and toughness to make it suited for the aforesaid stated purpose and with which the problems stated in the preamble of this specification encountered with existing steels intended for the same purpose are overcome or at least minimised.

Claims: A method of manufacturing a relatively low cost, high strength, high toughness bar and sheet steel which is substantially non-susceptible to the formation of delayed surface cracks in the as rolled condition, including the steps of providing a steel composition on a percentage mass to mass basis within the following range: C = 0,21 - 0,28 Mn = 0,80 - 1,80 Cr = 1,60 - 2,10 Si = 0,35 maximum Al = 0,013 - 0,057 P and S each = 0.025 maximum Fe = the balance; hot rolling the steel; air cooling the steel; and then subjecting the steel to a heat treatment entailing heating the steel to an austeniting temperature of around 900°C, followed by cooling the steel product by quenching with water or oil or, where the steel product is relatively thin, by air cooling the steel product. The method of claim 1 including the step of tempering the heat treated product at a temperature of around 225°C for one hour per 25 mm thickness. A relatively low cost bar and sheet steel which has the following constitution on a percentage mass to mass basis: C = 0,21 - 0,28 Mn = 0,80 - 1,80 Cr = 1,60 - 2,10 Si = 0,35 maximum Al = 0,013 - 0,057 P and S each = 0.025 maximum Fe = the balance; and which has been hot rolled, air cooled, and subjected to a subsequent heat treatment entailing heating the steel to an austeniting temperature of around 900°C, followed by cooling the steel product by quenching with water or oil, or where the steel product is relatively thin, by air cooling the steel product, the steel having a hardness of 470-520 Vickers, a yield limit of 1250-1350 MPa; a tensile strength of 1500-1650 MPa and a charpy toughness of 30-60 joule at 20°C; the steel being furthermore substantially non-susceptible to the formation of delayed surface cracks.

Inventors: BADENHORST, RUDOLF PHILIPPUS, MOSTERT, ROELOF JOHANNES
Applicants: ISCOR LIMITED
Application year: 1988
Main classification: C22C38
IPC codes: C22C38, C21D9, C21D6