LU82276A1 - PROCESS FOR OBTAINING IMPROVED QUALITY STEEL - Google Patents

Description

the ^ 3 | ΛήΛ c 1938/7903.

2.4 * 00

Claim of the priority of (s) fa ('·: ·· *. "/ - (:,) ccrîsspondant (s) deposited ^) cr ._ ^ <ul, _ ~ le-U. 01 * 0_ / under the number S'iS.OOl / | / ψ ~ METALLURGICAL RESEARCH CENTER -CENTRUM VOOR RESEARCH IN DE METALLURGIE,

Nonprofit organization -

Vereniging zonder winstoogmerk in BRUXELLES, (Belgium).

Process for obtaining improved quality steel.

; The present invention relates to a process for obtaining improved quality steel.

This process is applicable to thin and medium sheets and allows them to be given both high strength and sufficient ductility to be able to subject them, successfully, to a cold forming operation, and in better conditions than can be do this with standard sheets.

Such improvements are particularly advantageous in certain applications, in particular in the industry / of automobile construction, because they make it possible to achieve // / '2.- significant weight savings, while respecting the safety standards imposed.

The present invention specifically relates to a method for achieving such improvements.

These properties are obtained by subjecting, in the rolling hot, a sheet of determined composition, to an appropriate cooling operation, followed by a winding operation, when the temperature of the sheet has dropped below 600 ° C., and preferably at a value between 500 ° C and 550 ° C.

The term “appropriate cooling operation” should be understood to mean cooling, the speed of which is on the one hand, sufficient to avoid any formation of perlite and most often not less than 40 ° C./sec., And on the other hand, such that at the winding temperature, the metal has areas made up of ferrite or acicular ferrite and areas made up of carbon-rich austenite, which subsequently transform into bainite or martensite.

The process which is the subject of the present invention is therefore essentially characterized in that a steel sheet is subjected, in the hot rolling operation, to the said appropriate cooling operation, followed by a winding operation at a time. - »temperature below 600 ° C, sheet whose centesimal composition verifies the following relationships: 0.05% 4. C 4 0.25% 0.8% <Mn <2% 0.4% ç Cr 4 1.2 % 0.002% <<0.03% 0.05% ζ V 4 0.25 ° / o If £ 0.9%

Al of calming, the balance consisting of Fe, accompanied by ^ / its usual impurities. / 1 / * 13.-

Thanks to this process, the steel sheet which is subjected to it has a breaking load greater than 600 MPa, as well as a significant elongation, greater than 25%, a high consolidation rate (greater than 0.24) and a low elastic limit which corresponds, for the R / R ratio, to values not greater than

O | ^ JT

0.6. ‘

The properties obtained for the steel having been subjected to a above process are generally accentuated when the steel has the following composition: 0.07% 4 c 4 0.15% 1.2% 4 Mn 4 1.6 % 0.5% Z Si 4 0.9% 0.6% 4 Cr 4 1.0% 0.10% 4 v 4 0.20%.

It has also been found advantageous to incorporate molybdenum into said steel in a proportion not exceeding 0.6%, and preferably between 0.2 and 0.4%, the sum of the Mo + Cr contents remaining, however, included between 0.8% and 1.2%.

According to a particularly advantageous method of the process of the invention, the steel considered comprises Niobium in a proportion of 0.02% to 0.08%, and preferably between 0.04% and 0.06%. Thanks to the presence of Niobium in the above-mentioned proportions, higher strengths (Rr y 900 MPa) are obtained in steel with a significant elongation, greater than 13%, a high consolidation rate (n> 0 , 13) and an RQ 2 ^ Rr ratio always less than or equal to 0.6.

According to another particularly advantageous method of the process of the invention, when the steel sheet leaves the la- minoir, it has undergone a reduction of at least 40% from the time when its temperature has dropped below 1000 ° C. .

For comparison, and to show the advantageous properties obtained by steels having been treated according to Ley

W

4.- “Process of the present application, the comparative results obtained for steels of different compositions and subjected to different processes have been indicated on a table.

* A. Steels of composition C = 0.10%

Mn = 1.45%

If = 0.45% P = 0.01% S = 0.006%

Nb = 0.04%, * N = 0.006%

Pe = the balance, a steel with high elastic limit and wound at 600 ° C in the hot rolling.

B. Steels of composition C = 0.1%

Mn = 1.4%

Cr = 0.3%

If = 0.3% V = 0.15% N = 0.02%

Al = 0.05% S and P = usual, the balance being composed of Fe and its normal impurities, therefore a steel in accordance with that of the invention and wound at 600 ° C, after having been cooled in the hot of rolling, in accordance with the process of the invention.

The results below have been observed, which show the advantageous properties of steel B.

An * 0 * 2 if r Ä% steels * MPa MPa MPa A --- 406 522 24 B 310 --- 648 30

Claims (5)

5.- 1. Method for obtaining a steel of improved quality, in which a sheet of steel is subjected in the rolling hot to a cooling operation, characterized in that a cooling speed is ensured on the one hand sufficient lice to avoid any formation of perlite and most often not in-iërieure at 40 ° C / second, and on the other hand such that at the winding temperature, the metal has areas composed of ferrite or " of needle-like ferrite, and pads composed of carbon-rich austenite, which subsequently transform into bainite or mar-tensite, in that the sheet is then wound at a temperature below 600 ° C, the sheet to which the said process is applied consisting of a steel whose centesimal composition verifies the following relationships: 0.05% 4. C £ 0.25% 0.8% <Mn <2% 0.4% <Cr <1 , 2% 0.002% £ N2 ^ 0.03% 0.05% <V <0.25% If ^ 0.9% Al for calming,, the balance: Fe accompanied by its impurities usual. 2. Method according to claim 1, characterized in that it is applied to a steel whose composition verifies the following relationships: 0.07% ^ C £ 0.15% 1.2% Mn ^ 1.6% 0, 5% ^ if ^ 0.9% J 0.6% 4 Cr ^ 1.0% // 0.10% ^ V ^ 0.20%. / dd / * f: I 6 .- 3. Method according to either of Claims 1 and 2, characterized in that molybdenum is incorporated into said steel in a proportion not exceeding 0.6%, and preferably between 0.2 and 0.4%, the sum of the 4l Mo + Cr contents remaining however between 0.8% and 1.2%. 4. Method according to either of claims 1 to 3, characterized in that the steel also contains Nb, in a proportion of 0.02% to 0.08%, and preferably, - between 0 , 04% and 0.06%. 5. Method according to either of claims 1 to 4, characterized in that it is applied to a steel sheet which, at the outlet of the rolling mill, has undergone a reduction of at least 40% to from the moment its temperature has dropped below 1000 ° C. Drawings: - plates __ & .......... ppges dem ............. cover page .Λ ............ description pages , '..... λ ......... rules of claims ........... c-regé descriptif Luxembourg, ic Λ-0 mvολλ 60 Le π; c notary: / V V CÎiarles München