DE1958548A1 - Hot forming process for the production of a ferritic-pearlitic steel - Google Patents

Description

Tokyo, Japan November 21, I969

Hot forming process for the production of a ferritic-pearl-table steel

The invention relates to a heat treatment process for improving the toughness of ferritic pearls Steels.

There are already numerous and varied processes for improving the toughness of ferritic-pearlitic Steel has been proposed, for example by adjusting the hot working conditions or using a normalizing treatment.

The adjustment of the heating conditions for the steel during the hot working process, for example by Setting the treatment temperature or time is in in many ways objectionable. At a lower treatment temperature, the deformation resistance and the deformation hardening decrease of the steels, which leads to deteriorated world processing properties. Besides, it is very difficult to carry out the necessary treatment on an industrial scale, which is due to the limited treatment « or processing capacity of the rolling mills.

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Normalization treatment is known to be good Heat treatment, although there are also various technical and financial difficulties to be taken into account are. With normalization, a steel is cooled down to a normal temperature after processing and then up again austenitic or solution temperature. Here is Not only does the amount of heat required is very high, but also a lot of fluff for the subsequent necessary cooling of the steel required. In addition, the time lost between hot working and heat treatment contributes to one Reduction of the workability at. Finally, the long-term heating causes the heating process to take place Generates scale on the steel surface, which improves the surface quality of steel deteriorates.

The object of the invention is therefore primarily to eliminate the various disadvantages of the conventional processes. To solve this problem, the invention creates a process for improving the toughness of ferritic-pearlitic steels or steels with a ferritic, pearlitic structure, in which the steel initially cooled to 700 - 500 ⁰ C in order to convert the austenitic structure to about 50 - 90 $ in a ferritic, pearlitic structure, and then heated again via the A3 transition point for austenitization and finally cooled by air or by another cooling process,

The method according to the invention can be applied to all steels with ferritic beading Microstructure can be applied so that no special need to use a steel with special Composition area exists. However, if there are too large and too many alloy parts, the transition point becomes decreased, making this treatment difficult to carry out

Ö.Q 9 8 A 9/1 1 9 5

* ■ *? ■■

will. In practice, therefore, it is generally desirable to limit the range of ingredients, by weight, to be specified as follows:

Carbon:

The upper limit of the C content is set to about 0.25 % , since as the C content increases, the toughness is lowered and the transition point is lowered, making the treatment more difficult to perform. The lower limit is set at about 0.05% for structural steels which are required to have high toughness because of the limitations of industrial steelmaking.

Silicon:

The upper limit of the Si content is about 1.00 ^. Silicon is very effective as a deoxidizing agent in steel production and facilitates the treatment according to the invention, since it increases the starting temperature of the ferrite transformation. On the other hand, if more than 1.00 # Si, the toughness of the steel is lowered, so that the upper limit is set to about 1.00 % .

Manganese:

Manganese is an inexpensive but very effective element in improving the toughness of steels. With a higher percentage of manganese, however, the transition or transition point is lowered and the satisfactory application of the treatment according to the invention is hindered. The upper limit for Mn is set at about 1.50 % and the lower limit at about 0.40 % ; the latter value is generally considered to be a substantial percentage of the Mn content as a deoxidizer in steel making.

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BATH ORIGINAL

The percentage proportions of other elements are selected as follows:

Less than 0.50 % for Ni, Cr, Cu; less than 0.30 % for Mo, W; less than 0.15 % for V, Nb, Ti, Zr, Al. When these elements exceed the stated limits, they lower the transition point considerably. However, this is not desirable in the treatment according to the invention, since it reduces the toughness of the steel.

After the hot working, the maximum value becomes the intermediate cooling temperature limited to 700 ° C. A temperature above this maximum will slow down the conversion from the austenitic to the ferritic, pearlitic structure, which affects the processability (cf. Treatment 2 in Table 1).

The minimum cooling temperature is set at 500 ° C. Below At this temperature the loss of heat retained in the steel is greater. In addition, the following Heating is prolonged, so that the formation of scale is increased.

Within the specified temperature range, the extent of the transformation from the austenitic to the ferritic, pearlitic structure after hot rolling is approximately 50 - 90 #. If the amount is less than 50 % , the treatment is less effective because the after-effect of hot rolling is very pronounced, while if it is over 90 %, the rate of conversion is slower, so that the treatment time is prolonged and the process loses its industrial advantages.

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If structures other than ferritic-pearlitic structure increase in the finished steel product, the toughness of the steels is reduced. For this reason, the maximum value is set at around 90 % »

In addition, after the A3 transition point, the Steels can be cooled at an accelerated rate depending on their composition range and the steel thickness.

The advantages of the method according to the invention emerge even more clearly from the following example,

example

Table 1 shows a comparison between the properties obtained by various treatments of a 20 mm thick steel plate with 0.14 % C, 0.37 % Si, 1.26 % Mn, 0.012 % P, 0.010 # S and 0.05 % Al be achieved.

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Tabel

Types of treatment

1. Air cooling after hot forming (from final deformation temperature)

Tensile strength test

(JIS No. 4 steel) • Stretch- tensile strength- elongation limit % kgjmm kg / mm

32.9 51.1

38.8 mm V-notch impact austenite conversion

exam - share

Tr 15 at the intermediate

Trs

m ⁰ C

-10

cool

-73

thickness

of the tinder

0.10

° 2. Cooled to 700 C after

Thermoforming, heated to 900 ° C for 15 min and then 35, air-cooled

51.2

38.1 9.4 -10

-74

0.17

■ fin

· 3 to 650 ^w C cooled after thermoforming, heated for 15 min 33.4 to 900 ⁰ C and then air cooled

50.8

40.5 -56

-78

65

0.18

4. Cooled to 550 ° C

after hot forming, 38.2 51.0 heated to 900 ° C. for 15 min and air-cooled

5. At normal temperature

cooled after hot 36.3 51.9

forming, heated to 900 ° C for 15 min and air-cooled ^ normalize)

40.5 18.4 -61

28.8 20.0 -63

-99

-100

90

100

0.19

CD Cn OO

0.25

In Table 1, Treatment 1 shows the condition of hot rolled Stahls, while treatment 5 corresponds to a conventional normalization treatment. Treatments 3 and 4 contain the process according to the invention and clearly demonstrate the great improvement in toughness compared to others Treatments.

More precisely, in treatment 3, the steel is cooled to 650 ° C. after the hot working. The austenite structure fraction achieved is 65 % and represents a significant improvement over treatment 1. It gives almost the same value for energy absorption Eo and a brittle fracture percentage Trs of 50 % as are produced by conventional normalization treatment 5. Furthermore, in treatment k, in which a steel is cooled to 55O ° C, the 15 foot-pound transition temperature Tr 15 (15 ft - Ib transition temperature TR 15) is practically the same as in the conventional treatment 5 «

As can be seen from the foregoing description, the invention thus provides a process for the production of steels which have various advantages over conventional steels. Here, the amount of heat applied can be reduced, because the heat stored by the steel after hot deformation can be used effectively for reheating. The operational The efficiency of a heat treatment furnace can be improved by drastically reducing the heat treatment time, while its efficiency by reducing the size of the overpass the required space of the steels and a reduction in the time lost between deformation and heat treatment can be. Another advantage shown in Table 1 is that the heating time is shortened the formation of scale on the steel surface is reduced.

The invention offers numerous technical and economic aspects Benefits in improving the toughness of steels.

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Claims (4)

Claims with ferritic-pearlitic structure, characterized in that firstly a steel of the deformed steel to a temperature of about 700 is hot deformed at austenitic structure state, - 500 ⁰ C is cooled to 50 - to convert 90% of the austenitic structure, then the steel to a Temperature is heated above the Aj5 transition point for renewed conversion into the austenitic structure and 'the steel is finally cooled, the finished steel acquiring a practically 90 # ferritic-pearlitic structure. 2. The method according to claim 1, characterized in that the steel contains 0.05-0.25 % C, less than 1.00 % Si and 0.40 1.50 % Mn. 2. The method according to claim 1 or 2, characterized in that the steel contains one or more of the following alloy components: less than 0.50 % Ni, Cr and / or Cu; less than 0.30 % Mo and / or W and less than 0.15 % Nb, V, Ti, Zr and / or Al. 4. The method according to any one of the preceding claims, characterized characterized in that the steel is cooled with air GG9849 / 1195