DE1263052B - Heat treatment process to achieve a tough steel - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN WTTWt PATENT OFFICE Int. CL:

C21d

EDITORIAL

German class: 18 c -1/00

Number: 1263 052

File number: Y 777 VI a / 18 c

Filing date: January 17, 1964

Opening day: March 14, 1968

The invention relates to a heat treatment method for obtaining a tough steel that less than 0.4 weight percent carbon, less than 1.5 weight percent manganese and any of the Contains elements niobium, tantalum and vanadium in an amount of 0.02 to 0.2 percent by weight.

A method for improving the strength of tough steel to which niobium or vanadium has been added has already become known. Tough steel produced by these known methods, however, quickly loses toughness as the thickness of the steel increases, so that it is difficult to achieve an impact value above 3.0 kgm / cm ² at 0 ° C in the Sharpy impact test which is usually required with structural steels. Therefore, it has hitherto been refrained from using such tough steels with a thickness of over 15 mm in practice as structural steels, and the production of tough steels of such thickness has been regarded as almost impossible.

As the usual tough steel by heat and rolling treatments at a temperature above 1250 ° C is processed, bainites grow in its crystal structure if the thickness of the steel is over 15 mm goes beyond, as a result of which the toughness of the steel decreases rapidly without a heat treatment of the Crystal grains enter.

To give an example: a killed steel with a thickness of 25 mm, which contains either niobium, tantalum or vanadium and has been treated at a temperature above 1250 ° C, shows an impact strength value of 1 to 1.5 kgm / cm ² .

It was found that acid-soluble niobium, which is mainly dissolved in the steel in solid form, the reason for improving the strength of the steel due to that caused by the solid solution Hardening forms and that acid-soluble niobium, which is mainly as carbide and partially is deposited as nitride, the crystal grains of the steel are tempered and thus its toughness is increased. It it was also found that both types of niobium occurring the specific equilibrium maintained between them, in particular by the heating temperature and heating time is determined. This equilibrium experiences practically no change during the rolling process and during the cooling process after rolling.

The content of acid-insoluble niobium increases with increasing temperature, reaching a maximum at a temperature of 900 ° C while it decreases immediately before HOO ⁰ C.

Heat treatment process to achieve
of tough steel

Applicant:

Yawata Iron & Steel Co. Ltd., Tokyo

Representative:

Dr. F. Zumstein,

Dipl.-Chem. Dr. rer. nat. E. Assmann

and Dipl.-Chem. Dr. R. Koenigsberger,

Patent attorneys,

8000 Munich 2, Bräuhausstr. 4th

Named as inventor:

Susuma Goda,

Hisashi Gondo, Kitakyushu City;

Isao Kimura, Yawata;

Suehiro Hiyoshi,

Akira Yonei,

Hiroki Masumoto, Kitakyushu City (Japan)

Claimed priority:

Japan January 18, 1963 (38-2228)

According to the invention, a slab of the steel mentioned at the beginning is now achieved in that a slab of this steel is ^{heated at a temperature of 1100 to 1200 °} C. for at least 15 minutes before hot rolling. It is achieved that the acid-insoluble elements contained in the steel niobium, tantalum or. Vanadium can be excreted in an amount of 30 to 80 percent by weight. It is useful to heat the steel after hot rolling at a temperature of 550 to 800 ° C for at least 15 minutes. In this case, the slab is heated in the usual way to about 1250 ° C. before hot rolling.

A tough steel with a thickness of over 15 mm can be produced by the measures according to the invention will.

The single figure shows an equilibrium curve between acid-insoluble niobium * that is deposited in the steel and the total niobium content at different heating temperatures.

The steels to be treated according to the invention contain niobium, tantalum or vanadium in quantities

809 518/473

from 0.02 to 0.2%, less than 0.4% C, silicon and manganese in amounts that are customary in steel production are. If a higher strength is expressly desired, then nitrogen can be used in amounts of less be provided as 0.02%.

Steel containing more than 0.40% C is not suitable for the present invention because it has a C content of more than 0.4% reduces the impact resistance. Are one or more of the elements niobium, tantalum or Vanadium is present in an amount of less than 0.02% in total, so the desired effect does not occur a. If it is more than 0.2%, the impact resistance decreases.

As can be seen in the figure, the acid-insoluble niobium gradually increases with increasing heating temperature and reaches the maximum at about 900 ⁰ C and then gradually drops again in a temperature range immediately before HOO ⁰ C from. It is therefore possible to control the amount of deposition by adjusting the heating temperature.

As already mentioned, acid-insoluble niobium is mainly in the form of carbide or partially deposited by nitride. The amount deposited is 30 to 80 percent by weight, expediently 50 percent by weight.

The curve shown in the figure has two parts, showing the deposition region of niobium is insoluble in acid in an amount of 30 to 80% in the temperature ranges from 550 to 800 and 1100 to 1220 ⁰ C.

A heating time of at least 15 minutes is necessary for the separation of acid-insoluble Ensure niobium, tantalum or vanadium in an amount of 30 to 80 percent by weight.

The invention is to be explained in more detail using an example.

Table I shows the chemical compositions of steel samples were prepared in a Siemens-Martin furnace, and Table II shows the relationship between the steel compositions and the impact value at 0 ⁰ C, at the following treatment:

The steels were heated to a temperature of 1200 ⁰ C and immediately thereafter when rolled at a finishing temperature of 900 ⁰ C.

Table I Alloy components (percent by weight)

sample carbon Si manganese niobium Tantalum Vanadium nitrogen A. 0.09 0.07 1.07 0.04 0.02 0.004 B. 0.18 0.06 0.82 0.05 - -. 0.004 C. 0.18 0.09 1.13 - - 0.06 0.005 D. 0.19 0.10 1.22 0.07 - - 0.015 E. 0.18 0.05 1.25 0.06 - - 0.003 F. 0.19 0.08 1.28 -. --- 0.07 0.016 G 0.19 0.08 1.22 0.07 - 0.05 0.016 H 0.35 0.08 0.91 0.04 - - 0.006 I. 0.42 0.08 0.97 0.04 - - 0.005 J 0.17 0.07 1.35 - -. 0.48 0.004 K 0.15 0.06 1.28 -. - 0.23 0.004 L. 0.16 0.07 1.24 - 0.07 - 0.004

Samples D, F and G have added nitrogen. No nitrogen was added to samples containing greater than 0.006% N _2.

Table II
Mechanical properties

(The steel was heated to a temperature of 1200 ⁰ C and at a final treatment temperature of

900 ⁰ C rolled, thickness 25 mm)

sample Yield point tensile strenght strain Impact resistant
speed at 0 ⁰ C (kg / mm ² ) (kg / mm ² ) (° / o) Gcgm / cm ^a ) A. 35 49 32 15th B. 39 55 28 6th C. 41 54 27 9 D. 43 59 24 5 E. 42 55 27 8th F. 48 65 21 4th G 40 59 24 4th H 44 62 20th 4th I. 49 65 17th 1 J 50 68 18th 4th K 51 70 15th 2 L. 43 56 26th 7th

As can be seen from Table II, manganese improves the impact resistance value without affecting the Strength is impaired. However, if the manganese content is above 1.5%, the steel becomes brittle, and the weldability is impaired. The content should therefore be less than 1.5 percent by weight. Nitrogen improves the strength of steel, but sets the impact resistance value as it increases Content and makes the steel brittle if the content rises above 0.02%.

Table ΠΙ shows the relationships between the heating and rolling conditions of samples E and their mechanical properties.

55

Table III

Thickness 25 mm ] 1100 Heating temperature (° Q 1200 1220 1250 Machining 50 (2) 53 (2) 54 (2) 59 (1) temperature 51 (8) 1150 54 (4) 55 (4) 59 (1) ⁶ ° when rolling 52 (15) 52 (2) 55 (8) 56 (4) 60 (1) ( ⁰ C) 52 (14) 52 (6) 55 (8) 56 (3) 61 (1) 1050 53 (15) 53 (10) 56 (10) 57 (4) 60 (1) 1000 54 (15) 53 (10) 57 (9) 56 (4) 61 (2) 65 950 54 (11) 900 55 (13) 850 800

The numbers in brackets mean the impact strength ^{value at 0} ° C. (kgm / cm ² ). The numbers outside the brackets indicate the tensile strength (in kg / mm ² ).

As can be seen from Table III, when the heating ^{temperature exceeds 1220 °} C., the tensile strength increases, but the impact resistance value decreases. The final rolling temperature is more than 1000 ⁰ C addition, the ferrite grains and the pearlite structure can not be refined, whereby both the impact strength such as tensile strength are reduced. The temperature should be in the range 1100 to 1220 ⁰ C, the rolling temperature below 1000 ⁰ C.

Table IV

Relationships between post-rolling reheating and mechanical properties (example E); Thickness 25 mm

Heating temperature before rolling: 1250 ⁰ C rolling temperature: 900 C ⁰

Rewarming
temperature ( ⁰ C) tensile strenght Impact resistance after rolling (kg / mm ² ) (kg m / cm ⁸ ) like welded 61 1 400 59 1 500 58 2 600 57 4th 700 54 10 800 53 14th 900 51 15th

As can be seen from Table IV, the slab subjected to a heat treatment at a temperature of 1250 ° C according to the usual method, the ratio of the amount of precipitation to the solids-dissolved amount of niobium, tantalum or vanadium cannot be achieved. The impact strength values are therefore very low here.

80 ° / ₀ to obtain an elimination of 30 to the acid-insoluble niobium, tantalum or vanadium, it is necessary, therefore, the heated according to the conventional method and hot-rolled slab at a temperature of 550-800 ⁰ C, according to the invention, nachzuerwärmen.

Claims (2)

Patent claims: 1. Heat treatment process for obtaining a tough steel which contains less than 0.4 percent by weight carbon, less than 1.5 percent by weight manganese and one of the elements niobium, tantalum and vanadium in an amount of 0.02 to 0.2% by weight , characterized in that a slab of this steel is ^{heated at a temperature of 1100 to 1200 0} C for at least 15 minutes before hot rolling. 2. Heat treatment process according to claim 1, characterized in that the steel is at a temperature of 550 to 800 ° C for at least 15 minutes long after being hot-rolled. Considered publications:
Austrian patent specification No. 150 000. 1 sheet of drawings 809 518/473 3.68 © Bundesdruckerei Berlin