GB2195658A - Production of steel - Google Patents

Abstract

A steel composition consisting by weight essentially of: 0.01 to 0.20% carbon; up to 1.0% silicon; 0.50 to 2.25% manganese; up to 1.5% chromium; up to 0.05% titanium; up to 0.10% niobium; 0.005 to 0.015% nitrogen; up to 0.06% aluminium; balance iron apart from incidental impurities. Interrupted quenching may be carried out to achieve the desired microstructure, ie the quench is terminated before completion of transformation to a martensitic structure. <IMAGE>

Description

SPECIFICATION Production of steel This invention relates to the production of steel.

High strength steel forgings with weights between approximately 2 and 100kg are used widely as safety-critical components in the automotive and general engineering industries. They are usually cooled from forging, then reheated for hardening and finally tempered. The resulting product has a suitable combination of strength and toughness which results from heat treatment.

Steel forgings suffer competition from alternative materials, such as improved cast irons, and developments have been made to include a reduction or elimination of heat treatment costs by using steels which will air-harden from the forging temperature. As an alternative, steels may be hardened by quenching immediately after forging and subsequently tempered conventionally, this process being known as 'direct quenching'.

The present invention sets out to define a steel composition and a cooling route used in combination with this composition which enables the tempering operation associated with direct quenching to be eliminated, fully or in part. This latter objective is achieved while preserving the metallurgical advantage of strength and toughness of a quenched and tempered product over airhardening steels and cast products. Low carbon steels in accordance with the invention can be welded readily and are, therefore, suitable for use in components involving joining forgings to fabrications or pressings.

According to the present invention in one aspect, there is provided a steel of composition consisting by weight essentially of: 0.01 to 0.20% carbon; up to 1.0% silicon; 0.50 to 2.25% manganese; up to 1.5% chromium; up to 0.05% titanium; up to 0.10% niobium; 0.005 to 0.015 h nitrogen; and up to 0.06% aluminium.

The phosphorous level should be kept low, below 0.015% and, preferably, below 0.010%.

A hardenability-effective addition of boron up to a maximum of 0.005% by weight may be included.

A preferred steel composition for the achievement of 900 to 1150 N/mm2 tensile strength at a section size up to 50mm consists by weight essentially of: 0.05 to 0.08% carbon; 0.10 to 0.5% silicon; 0.5 to 1.6% manganese; 0.5 to 1.5% chromium; up to 0.05% titanium; up to 0.10% niobium; 0.005 to 0.012% nitrogen; up to 0.06% aluminium; 0.002 to 0.005% boron balance iron apart from incidental impurities, the ratio of titanium to nitrogen being not less than 4:1 by weight %.

According to the present invention in another aspect, there is provided a method of treating a steel component of a composition as sepcified in any one of the preceding four paragraphs in which the component is subjected to quenching for a period of time which is controlled by sensing the temperature of the component and terminating the quench before completion of transformation to a martensitic structure.

According to the present invention in a further aspect, there is provided a steel component which has been subjected to a controlled quenching as indicated above.

In another aspect, the invention provides a steel containing between 0.03 and 0.10% by weight carbon and an addition of manganese and/or chromium sufficient to achieve for the critical section size of the product, tensile properties of between 700-1100 N/mm2 on quenching together with satisfactory impact toughness without further temperating or other heat treatment.

The impact toughness, and particularly the ratio of proof to tensile strengths of the steel, may be improved by quenching components produced from the steel in such a way that the duration of the quench is controlled in dependence upon the measured temperature of the component.

Preferably, the quench is terminated just before the start of transformation of the steel to martensite.

A preferred steel includes by weight 0.05% C, 1.83% Mn, 0.10% Cr, 0.0017% B, 0.049% Ti and is capable of giving a tensile strength of approximately 1010 N/mm2 at 19mm bar size on water quenching without further heat treatment.

A further preferred steel includes by weight 0.08% C, 2.01% Mn, 0.11% Cr, 0.0026% B, 0.044% Ti and is capable of giving a tensile strength of 1105 N/mm2 at 19mm bar size on water quenching without further heat treatment. Such a steel is also capable of giving a tensile strength of 995 N/mm2 at 50mm bar size.

When removed from the quenching medium prior to completion of transformation the steel referred to in the preceding paragraph is capable of giving a 0.2% proof strength of 885 N/mm2 at a tensile strength level of 1140 N/mm2 in 19mm diameter bar.

A further preferred steel contains by weight 0.05% C, 1.84% Mn, 0.11% Cr; when tempered at 300"C after quenching, such a steel is capable of giving a 0.2% proof strength of 740 N/mm2 at a tensile strength level of 900 N/mm2 in 19mm diameter bar.

A further preferred steel contains by weight 0.05% C, 1.03% Mn, 1.0% Cr, 0.0018% B and 0.058% Ti. When processed under standard production conditions this composition can produce after quenching a tensile strength of 830 N/mm2, 0.2% PS of 685 n/mm2 and Charpy Impact Toughness of 45 J at room temperature for samples taken from a region of the component with ruling section of 20 mm.

The steel referred to in the preceding paragraph when given a low temperature temper at between 200 and 300"C after quenching gives a 0.2% proof strength of 735 Nmm2 with a tensile strength of 830 N/mm2 and room temperature Charpy Energy of 43J.

A further preferred composition contains by weight 0.05% C, 1.04% Mn, 0.49% Cr, 0.0016% B and 0.051% T. When processed under standard production conditions and air cooled from forging this composition gives a tensile strength of 460 N/mm2, 0.2% proof strength of 320 Nmm2 and a room temperature Charpy Impact value of 230 J, for samples taken from a region of the forging with a ruling section of about 14mm.

However, when the steel forging referred to in the preceding paragraph was quenched in water for only 5 seconds it gave a tensile strength of 805 N/mm2, 0.2% proof strength of 695 N/mm2 and a room temperature Charpy Impact value of 70 J.

A further preferred composition contains by weight 0.08% C, 1.9% Mn and 0.11% Cr. When water quenched from rolling at a section size of 15 mm, this steel gives a 0.2% proof strength of 855 N/mm2 with a tensile strength of 1150 N.mm2 and with a room temperature Charpy Impact energy of 95 J.

The section sizes at which these high strength levels can be obtained in this steel are limited.

However, they can be extended by using a hardenability effective addition of boron.

A further preferred composition contains by weight 0.08% C, 2.0% Mn, 0.0026% B and 0.044% T. When water quenched from rolling at a section size of 20mm, a tensile strength of 1150 N/mm2 with a room temperature Charpy Impact Engergy of 28 J was produced.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates dilatometric curves which define transformation as a function of cooling rate and show the hardenability effect of boron in steels containing 0.08% C and 2% manganese; Figure 2 diagrammatically illustrates the relationship which exists between steel hardness levels and carbon content for steels with fully martensitic structures; and Figure 3 illustrates diagrammatically the relationship which exists between the manganese and chromium contents of steel in accordance with the invention and the maximum bar diameters at which tensile strengths of 900 or 950 N/mm2 can be obtained.

In a typical forging cycle, bar or billet stock is reheated to temperatures between 1200 and 1275 C prior to forging to shape and trimming in either a drop hammer or press. After trimming, conventional forgings are allowed to cool in an uncontrolled fashion, for example, within skips or during passage down a conveyor.

In the process according to the present invention, forged components are transferred to a quenching bath and are immersed in an agitated quenching medium which may be water, water containing suitable additives, or oil.

The duration of the quenching cycle may be controlled, by pre-setting the immersion time on the basis of initial trials. A preferred method is to monitor the temperature of individual components. This ensures that the actual quench duration for each component is optimised, irrespective of any minor variations in temperature arising in the forging or hot shaping operation.

One embodiment of this method is to pick up each component from the end of the working operation in a robotic handling device fitted with a clamp containing a temperature sensor which contacts the workpiece. The control signal from this sensor is used to determine the start of quenching and the duration of immersion in the quenching bath. This transfer system may be used to divert components for remedial treatment, in the event that their temperatures on receipt from the working process are too low for an effective quench, or in the case of a failure in operation of the quenching bath. (For example, failure of the quenchant circulation pumps).

A difficulty which must be overcome in the use of controlled, interrupted quenching treatments is the variability in cooling rate arising from the changes in section found in many forged components. To some extent this can be overcome by selecting the control temperature sensor location, to ensure that optimum transformation conditions are achieved at the location in the component which is critical for the service application.

However, a feature of the steel compositions proposed herein is that they give useful mechanical properties within the normal spread obtained by conventional heat treatment over a range of cooling rates. This is shown by the OCT characteristics given in Fig. 1, when there is a slow -transition in the type of microstructure obtained with decreasing cooling rate.

Experience has shown that the carbon content, cooling rate and quench duration can be used to control the strength achieved.

Reference will now be made to Tables 1 to 7 below.

For the steels whose compositions are shown in Table 1, Table 2 shows that by reducing the carbon content from 0.08 to 0.05%, the tensile strength was lowered from 1010 to 895 N/mm2 when fully martensitic structures were obtained on quenching. The influence of carbon content up to 0.2% C on strength in the as-quenched condition is illustrated in Fig. 2.

However, even where the microstructures are not fully hardened attractive combinations of strength and toughness can still be obtained; Table 3 refers. These samples were quenched immediately after hot working at section sizes varying from 19 to 50mm, giving strength levels varying from 1105-995 N/mm2. By slowing down the cooling rate still further, by interrupting the quench prior to completion of transformation then air cooling to ambient temperature, strength levels as low as 700 N/mm2 can be obtained readily, with good levels of impact toughness.

Interrupted quenching can also be used to influence the properties of the steels even when the cooling rates are such that fully martinsitic microstructures are formed. The principal difference between the properties of these steels and those of conventional medium carbon quenched and tempered steels is that, for a given level of tensile strength, the low carbon, as quenched steels, give lower 0.2% proof stress values, with PS/TS ratios of typically 0.75 compared with 0.8-0.9 for the medium carbon steels. Interrupting the quench, and also cooling slowly through the martensite transformation temperature range allows some relief of transformation stresses and auto-tempering to occur.

Providing that the cooling rate is sufficient to suppress the formation of non-martensitic transformation products, then reducing the quenching time so that the material is removed from the quenching bath at higher temperatures leads to an improvement in 0.2% PS with little effect on tensile strength. This effect is demonstrated by the results given in Table 4. A further improvement in the proof to tensile strength ratio may be obtained by retarding the cooling rate further after removal from the quenching bath. In experimental trials this has been achieved by transferring the forgings to a bath of insulating material such as vermiculite.

In some cases forgings may be subject to a low temperature heat treatment after forging (e.g.

for stress relief or hydrogen removal after plating). At temperatures up to 300"C this will also produce in these steels an improvement in 0.2% PS with no significant effect on tensile strength as shown by the results given in Table 5.

The steel hardenability can be controlled by adjusting the total Mn and Cr content as illustrated in Fig. 3. The carbon content may be selected from Fig. 2 to give an upper limit of strength set by the achievement of a fully martensitic microstructure. The alloying elements, principally Mn and Cr may then be set at levels to give the hardenability appropriate to the section size of the component. The relationship between the Mn and Cr level required and bar section size for water quenching 0.05% C-Boron steels is shown in Fig. 3.

A further method of controlling hardenability is to use a boron treatment, whereby the maximum ruling section for hardening is increased. It has been observed that addition of boron is accompanied by a drop in toughness, although the levels obtained remain acceptable; see Table 6.

The range of strengths available using steels of this type in forgings produced under normal production conditions are illustrated in Table 7. For steel 6 in the water-quenched condition a tensile strength of 830 N/mm2 with a 0.2% proof strength of 685 N/mm2 and room temperature Charpy Energy of 45 J was obtained at a ruling section of 20 mm. A subsequent low temperature temper increased the 0.2% proof strength to 735 N/mm2 with no change in tensile strength or toughness.

Using a lower hardenability material (steel 7) a tensile strength of 460 N/mm2 and a room temperature impact energy of 230 J was produced at 15 mm ruling section, for material air cooled from forging.

With a 5 second water quench after forging the tensile strength level increased to 805 N/mm2 with a Charpy Impact energy at room temperature of 70 J.

Niobium or titanium in boron-free steels may be used in certain hot working operations to aid control of the austenite grain size prior to quenching.

Steels in accordance with this invention may be used with enhanced sulphur levels, and/or the presence of additives to modify the sulphide inclusion particle shapes, for the improvement of machinability.

TABLE 1

Steel C Si Mn P S Cr 1 0.08 0.27 2.01 0.015 0.016 0.11 2 0.05 0.17 1.83 0.010 0.013 0.10 3 0.05 0.l25 1.84 0.010 0.015 0.11 0.08 0.22 1.90 0.015 0.017 0.11 5 0.08 0.27 2.01 0.015 0.016 0.11 6 0.05 0.39 1.03 0.012 0.015 1.02 .05 0.37 1.04 0.014 0.015 0.49 Table 1 continued: :

Steel Mo Ni Al N B Ti 1 0.05 0.09 0.036 0.008 0.0026 0.44 2 0.06 0.15 0.030 0.010 0.0017 0.049 3 0.06 0.15 0.022 0.010 - 0.017 4 0.06 0.09 0.032 0.006 - 5 0.05 0.09 0.036 0.008 0.0026 0.044 6 0.04 0.12 0.035 - 0.0018 0.058 7 0.04 0.16 0.033 - 0.0016 0.051 TABLE 2 Influence of Carbon Content on Mechanical Properties 0.2% PS TS E RA% RT Charpy PS/TS Steel N/mm2 N/mm2 2mm 'V' NOTCH ENERGY, J 1 845 1105 14 4 64 35 0.76 2 770 1010 15 67 74 0.76 TABLE 3 Influence of Cooling Rate on Properties of Steel 1 0.2 PS TS E RA% RT Charpy PS/TS Section N/mm2 N/mm2 2mm 'V' jize mm NOTCH ENERGY, J 19 845 1105 14 64 35 0.76 30 805 1055 15 66 47 0.76 50 790 995 13 67 57 0.79 TABLE 4 Influence of Quenching Time in Water of Steel lin Fully Hardened Microstructure

Quench 0.2% PS TS E RA RT Charpy PS/TS ing N/mm2 N/mm2 2mm 'V' Time NOTCH s ENERGY, J 30 s + AC 835 1140 16 68 28 0.73 10 s + AC 850 1135 16 68 32 0.75 7 s + AC 860 1135 14 68 37 0.76 7 s + VC 885 1140 15 68 34 0.78 AC = Air Cool VC = Vermiculite Cool TABLE 5 Influence of Low Temperature Heat Treatments on the Properties of Steel

ondition 0.2% PS TS E% RA% RT Charpy PS/TS N/mm N/mm 2mm 'V' NOTCH ENERGY, J As Quenched 685 930 16 68 144 0.74 VQ + 1h 1000C 675 925 17 63 162 0.73 VQ + 1h 150 C 710 925 16 62 128 0.76 VQ + 1h 200 C 725 920 16 63 134 0.79 VQ + lh 2500C 740 925 14 60 122 0.80 VQ + 1h 3000C 740 900 14 62 128 0.82 TABLE 6 Influence of Boron Treatment on Properties and Hardenability teel Section 0.2%PS TS E% RA% RT PS/TS Size N/mm N/mm Charpy mm Energy J 4 15 855 1145 14 68 95 0.75 5 20 845 1150 15 65 28 0.73 TABLE 7 Properties Obtained on Forgings under Normal Production Conditions Steel Section Cooling 0.2%PS TS E% RAS RT Size N/mm2 N/mm2 Charp mm Energy J 6 20 WQ 685 830 12 65 45 6 20 EQ + 735 830 11 65 43 1Hr.

2000C 7 14 AC 320 460 38 80 230 7 14 WQ 695 805 - 75 70 5 sec.

Claims (12)

1. A steel of composition consisting by weight essentially of: 0.01 to 0.20% carbon; up to 1.0% silicon; 0.50 to 2.25% manganese; up to 1.5% chromium; up to 0.05% titanium; up to 0.10% niobium; 0.005 to 0.015% nitrogen; up to 0.06% aluminium; balance iron apart from incidental impurities.

2. A steel as claimed in Claim 1 including up to 0.015% by weight per cent.

3. A steel as claimed in Claim 1 or Claim 2 further including boron up to a maximum of 0.005% by weight.

4. A steel of 900 to 1150 n/mm2 tensile strength at a section size up to 50mm and consisting by weight essentially of: 0.05 to 0.08% carbon; 0.10 to 0.5% silicon;

1.3 to 1.6% manganese; 0.8 to 1.2% chromium; up to 0.05% titanium; up to 0.10% niobium; 0.005 to 0.012% nitrogen; up to 0.06% aluminium; 0.002 to 0.005% boron balance iron apart from incidental impurities, the ratio of titanium to nitrogen being not less than 4:1 by weight per cent.

5. A method of treating a steel component of a composition as claimed in any one of Claims 1 to 4 in which the component is subjected to quenching for a period of time which is controlled by sensing the temperature of the component and terminating the quench before completion of transformation to a martensitic structure.

6. A steel component produced by the method as claimed in Claim 5.

7. A steel containing between 0.05 and 0.10% by weight carbon and an addition of managanese and/or chromium sufficient to achieve for the critical section size of the product, tensile properties of between 700-1100 N/mm2 on quenching together with satisfactory impact toughness without further tempering or other heat treatment.

8. A steel as claimed in Claim 7 wherein the ratio of proof to tensile strengths of the steel is improved by controlling the duration of the quench in dependence upon the measured temperature of the component.

9. A steel as claimed in Claim 8 wherein the quench is terminated just before the start of transformation of the steel to martensite.

10. A steel including by weight 0.05% C, 1.83% Mn, 0.10% Cr, 0.0017% B, 0.049% Ti which is capable of giving a tensile strength of approximately 1010 N/mm2 at 19mm bar size on water quenching without further heat treatment.

11. A steel including by weight 0.08% C, 2.01% Mn, 0.11% Cr, 0.0026% B, 0.044% Ti which is capable of giving a tensile strength of 1105 n/mm2 at 19mm bar size on water quenching without further heat treatment.

12. A steel and a method of producing the same substantially as herein described with reference to the accompanying diagrammatic drawings.