LU87549A1 - METHOD FOR MANUFACTURING A THIN STEEL STRIP BY HOT ROLLING - Google Patents

Description

* Ο 7 R A Û GRAND-DUCHY OF LUXEMBOURG BL-4242 -Patent N ° O ....... / ........... 3 .......... fl. ....

June 30, 1989 'Monsieur le Muustre du ------------------------------------ de l'Économie et of the Middle Classes

Title issued Intellectual Property Service

_Green ..... ξΆν LUXEMBOURG

Invention Patent Application -------------------------------------------- ---------------_-------------------—---: - '(I) I. Request

Metallurgical Research Center- Centrum voor Research (^ in "de MëtaXltŒgïeV" "Ässöc sans '' BW ^ luctätlf-Véiiënigittg ^ '•' 2'an'd'er ...... wl ^ cisttö'öginHrlr; .. ..... tT-'ru ^ MonfiDyer ...... β-1Ό · 4 · () · ..... BRUXEiaasg --------------- r-

Represented ...... by · :: ...... Βτ ^ τ · ~ νΈ ^ ΊΛΈίβίΒ & ·· -Λ · ..... • ^ ng '"» ~ "COEts-Î .... ... e »------------ ΕνΙ · ......... 3.2 · 1 ··· Γ ····· Γ · 0Η% β ~ ά · · »·· Α3? '1 · θη · ..... BtF ·» ....... 1 ..... L-βΟΟΙ-SÏRASSEN ------------- -------------------------------------------- (3) deposit) this ..... thirty ........ june ..... millet ...... nine ....... cent ....... q.uatr. .e ....... twenty ..... nine___________________________________ (4) at ....... hours, at the Ministry of the Economy and the Middle Classes, in Luxembourg: 1. this request for the obtaining a patent for an invention relating to: "Method of manufacturing a thin steel strip (^ .......................... ......................... ................... ...... ..................... 1 2. description in language ...................... ........................................... of the invention in triplicate ; 3. .................. J ............................ .................... drawing boards, in triplicate; 4. receipt of the taxes paid to the Office of Registration istrement in Luxembourg on j ^ ___________; 5. the delegation of power, dated ......................................... ........ the ? ? June 1989__________.

6. the successor document (authorization); declares (s) assuming responsibility for this declaration, that the inventor (s) is (are): (6) ........ MES SIEN ...... Pierre / .. ...... 3.9, *, ....... rue ..... des Hayettes ...... B - 4050 ESNEUX (Belgium) HERMAN Jean-Claude, 98 rue des Brasseurs B - 4570 BLËGNY-TREMBLEU (BeïgTquë) .......................................... .................................................. .............................

claim) for the above patent application the priority of one (or more) application (s) for (7) patent of invention ....................... .......................... filed in (8) ... „Belgium _ ip. (q) U July 1988__________________________________________________________________________ ;. .................. '...............

under N ° (10) ..... 0.8.800.8.00 ................................. .................................................. .................................................. .................................................. .................................................. .

on behalf of (11) ........... Center for ...... Research ...... Metallurgical ................. ................

élitCélisent) domicile for him (her) and, if designated, for his representative, in Luxembourg__________________________________________________________ 321, route d'Arlon L - 8001 STRASSEN _ _________________________________________________ _ _______ ^) seeks (s) the grant of a patent for invention '' object described and represented in the abovementioned appendices, with postponement of this delivery to .................................. .................................................. .................................................. .................................................. ....month. (13)

LeldSpösaaK / agent ............................................... .................................................. .................................................. .................................................. .............. (14) / \ ^ Π. Deposit Minutes

The above dbrnaräcie patent invention has been filed with the Ministry of Economy and Middle Classes, Service of Property IntejketUSIS tN ^ ixembourg, dated: June 30, 1989 15 00 ^ Pr. the Minister of Economy and of the Middle Classes, ^ r "\ The head of the intellectual property department, A 68007_ _?, EXPLANATIONS RELATING TO THE ^ ÄBertfTOT FORM. '/ / 1 ~ Q | | if * (I) if applicable "Request for certificate of addition to the main patent, to request for main patent No .... ♦ jÇsTj.Au.” - (2) enter the name, first name, profession , ^ address of the applicant, when the applicant is a private individual or company name, legal form, address dupfège / pcial, when the applicant is a legal person - (3) enter the surname, first name, address of the authorized representative, industrial property, provided with a special power, if he / she is bound ^ 'represented by ........... acting in the capacity of agent "- (4) date of deposit in words - ( S) title of the invention - (6) enter the names, first names, addresses of the invdnteurrau the indication "(see) separate designation (will follow)", when (has chosen BL - 4242

Claim to the priority of the patent application filed in Belgium on July 11, 1988 No 08800800 Descriptive memorandum filed in support of an invention patent application for: "Method of manufacturing a thin strip of steel by hot rolling"

Metallurgical Research Center Centrum voor Research in de Metallurgie

Non-profit association Vereniging zonder winstoogmerk 47, rue Montoyer B-1040 BRUXELLES Belgium - 1 -

Method for manufacturing a thin strip of steel by hot rolling.

The present invention is in the field of hot rolling of steel. It relates to a process for manufacturing a thin strip of steel by hot rolling, intended to improve the formability of this type of strip.

5

It is known that a hot rolled strip is commonly obtained from a slab, which has been previously heated to an appropriate temperature. This slab may itself have been produced by rolling an ingot or by continuous casting to the desired dimensions, according to a technique used more and more widely. The reheating temperature of the slab can vary according to the type of steel, but it is usually higher than 1000 ° C., so that the steel has an entirely austenitic structure. 1 2 3 4 5 6

The heating of the slab is carried out in a suitable oven, for example a pushing oven located immediately upstream of the hot rolling mill. The slab can be reheated there from room temperature 4, if its previous cooling has been complete, or from an intermediate temperature if it is introduced into the reheating oven before being completely cooled. The latter technique, known as hot charging, is particularly interesting because of the energy savings it allows. This slab may also have been directly rolled from liquid steel; it can then have any thickness, including i - 2 - including a small thickness, that is to say less than 50 mm. Its temperature is then such that it does not require intermediate heating, but possibly equalization.

5 Hot rolling of the slab includes a roughing phase and a finishing phase. The purpose of roughing is to bring the slab to the intermediate thickness desired for the finish rolling. This roughing step can of course be omitted when the slab has a small thickness, obtained for example by direct rolling.

10 The finish rolling, carried out with the hot finishing train, is intended to give the steel strip the thickness required for subsequent operations, for example cold rolling, or that which is imposed by the user.

In current practice, the finishing rolling is carried out in a temperature range where the steel has an austenitic structure. This condition requires that the end of rolling temperature is not less than point A3 of the steel considered. By way of example, this temperature is around 850 ° C. for the usual steels whose carbon content is between 0.01% and 0.1%. Furthermore, the work hardening rate generally does not exceed 0.3 in the last reduction pass. It can be recalled that the rate of work hardening e is defined by the relation: f = l "T ^ -R '

25 1 R

in which R represents the reduction in thickness of the strip during the operation under consideration.

A steel strip rolled in the austenitic phase exhibits good isotropy characteristics as well as a satisfactory dimensional tolerance. It is however affected by an important drawback which is its poor formability, which results in values of the mean plastic anisotropy coefficient (r) generally less than 1, and most often between 0.8 and 1.

- 3 -

We are currently seeking to produce hot-rolled strips whose thickness is thinner than previously, in particular less than 1.5 mm, so that users can carry out in their installations the operations, including forming, which the 5 applications require. particular plans. For this purpose, it is necessary that the hot-rolled thin strip has a clearly improved formability, and in particular for stamping.

Two solutions can mainly be envisaged in an attempt to obtain such an improvement. Each, however, has significant drawbacks.

Firstly, one can seek to carry out the finishing rolling entirely in the austenitic range, as is done with the thicker strips. It is known that the finishing rolling comprises several successive reduction passes, in which the thickness of the strip is gradually reduced to the desired final thickness. The number of passes obviously increases when the final thickness to be produced decreases. Due to the inevitable drop in temperature of the strip during the various passes, maintaining the strip in the austenitic phase during the entire finishing rolling becomes very difficult, if not impossible, when the final thickness of the strip is less than 1.5 mm. The solution which would consist in increasing the reheating temperature of the slab, that is to say the temperature of the slab before hot rolling, is neither advantageous economically nor technically. Not only does it lead to an increase in energy consumption, but it also adversely affects the mechanical properties of the strip.

30

The fact of not being able to carry out the entire finishing rolling in the austenitic field leads to marked heterogeneity in the mechanical characteristics of the strip, as well as a great difficulty in respecting the dimensional tolerances of this same strip. The cause of these difficulties is that the rolling is carried out in part at temperatures at which the steel has a ferritic structure.

A second solution could consist in carrying out all of the finishing rolling in the ferritic phase, that is to say at temperatures below the point A3 of the steel considered. This solution certainly improves the mechanical characteristics and compliance with the dimensional tolerances of the strip. However, when applied under conventional finish rolling conditions, it results in an unfavorable change in the crystalline texture of the steel; this results in a decrease in the value of the average plastic anisotropy coefficient r, which becomes significantly less than 1, and consequently a sharp reduction in the formability of the hot-rolled strip.

The object of the present invention is to provide a method of manufacturing a thin strip of steel by hot rolling, whereby the hot strip has not only excellent formability, but also very good mechanical properties and dimensional tolerances strictly observed.

20

It is based on the unexpected beneficial effect of an appropriate modification of the conditions of the finish rolling on the characteristics of a thin steel strip. By thin, it should be understood here that the final thickness of the hot rolled strip is less than 2 mm.

In accordance with the present invention, the method of manufacturing a thin strip of steel by hot rolling, in which a steel slab in the austenitic state is subjected, to a roughing rolling up to a intermediate thickness followed by a finishing rolling comprising several successive reduction passes to bring it gradually to the desired final thickness, is characterized in that before the hot rolling, the slab is brought to a reheating temperature, respectively equalization, between 1100 35 ° C and 800 ° C, in that one performs at least the last of said reduction passes with a work hardening rate e at least equal to 0.4 1 i - 5 - and in that the temperature at the end of the final rolling is between 750 ° C and 400 ° C.

Preferably, at least the last of said 5 reduction passes is carried out with a hardening rate € at least equal to 0.8.

XL is also preferable, according to the invention, that the reheating temperature, respectively equalization temperature, of the slab before hot rolling is between 1000 ° C and 850 ° C.

10

Always according to the invention, the temperature at the end of the final rolling is preferably between 700 ° C. and 500 ° C.

According to a particular variant of the invention, a speed 15 of work hardening is carried out at least equal to 60 s in at least the last of said reduction passes. It appeared that Obtaining a high coefficient was easier the higher the speed of work hardening 6.

In the context of the present invention, it has also proved to be advantageous to carry out the winding of the strip at a temperature between 700 ° C. and 350 ° C.

The conditions of hot rolling which have just been exposed lead to the formation, in the strip, of a microstructure giving it remarkable remarkable formability. This microstructure, of the dupLex type, consists of a core made up of recrystallized ferritic grains of normal size (ASTM <9), on either side that a layer of finer grains (ASTM 2 10) has developed. The cristaLLine texture of these layers has cristaL-Lographic orientations which lead to an improvement in the formability of the hot rolled thin strip.

The invention therefore also relates to a thin strip of hot rolled steel 35 having a dupLex microstructure composed of a core made of recrystallized ferritic grains of ASTM size. 9 -6 - and surface layers made up of finer grains, size ASTM ^ 10.

Advantageously, each of said surface layers has a thickness at least equal to 1/8, and preferably 1/4, of the thickness of the steel strip.

A hot rolled steel strip according to the present invention has an excellent formability, to which corresponds an average plastic anisotropy coefficient r at least equal to 1.1 and preferably at least equal to 1.4.

Other features and advantages of the present invention will become apparent in the examples which follow. It goes without saying that these 15 examples are in no way limiting and that they are only intended to illustrate the process and the steel strip which is the subject of the invention.

Examples 20

The tests covered seven steels with low and ultra-low carbon contents, the detailed composition of which is given in Table I.

They consisted of hot rolling with the finishing train, until a strip 1 mm thick was obtained. Various rolling conditions were varied, namely: "TR: temperature for reheating the slabs before rolling, (° C); - T £: equalization temperature before the final rolling - case of direct rolling, (° C); 30 -: finishing rolling temperature during the last reduction pass, (° C); TB: winding temperature after finishing rolling, (° C); - e: work hardening rate; • mm ^ - 6: work hardening speed (s).

-7 -

In each case, the average plastic anisotropy coefficient (r) was determined, which expresses the formability of the steel strip.

Table II summarizes the different rolling conditions experienced during these tests, as well as the results obtained. The sign (x) in the last column indicates, for comparison, the tests for which at least one of the rolling conditions (T_,

R

"TL, C, e) does not comply with the prescriptions of the present invention.

These results clearly show that the method of the present invention leads to a marked increase in the coefficient (r), which is systematically greater than 1.1. This increase reflects a significant improvement in the formability, and especially the drawing, of a hot rolled steel strip in accordance with the present invention.

They also show that the process of the invention applies with the same advantage to low or ultra-low carbon steels, containing or not containing additions of titanium, niobium and / or boron.

- δ - TABLE I.

Chemical composition of steels (% by weight) 5 _

Steel C Mn Si S P N Al Nb Ti B

No_ 10 1 0.026 0.225 0.012 0.009 0.014 0.0065 0.034 - - - 2 0.038 0.202 0.006 0.009 0.013 0.0027 0.035 - - 0.0019 3 0.002 0.131 0.005 0.006 0.004 0.0022 0.017 - 0.026 - • 4 0.004 0.147 0.013 0.014 0.016 0.0023 0.016 0.035 - - 5 0.003 0.180 0.009 0.008 0.010 0.0020 0.030 0.020 0.032 - 15 6 0.004 0.195 0.006 0.008 0.008 0.0023 0.049 - - 0.0024 7 0.002 0.186 0.007 0.010 0.012 0.0021 0.038 - - - " a - 9 - * TABLE II.

(Laminating Conditions - Results.

Steel T or τ ° C c h T ° C T ° C r 5 „'R ** * E Ç A' l l b b Γ

No. S *

1 1200 - 0.22 205 910 700 0.88 X

1 1150 - 0.35 74 700 500 0.63 X

1,950 - 1.62 72,650,500 1.42

10 1,940 - 1.20 76 750 550 1.05 X

1,945 - 0.58 516,600 450 1.45 1,900 - 0.95 69,650 450 1.22

2 1000 - 0.26 518 895 650 0.93 X

2,900 - 0.45 416,670,500 1.25 15 2,950 - 1.60 84,685,500 1.50

2 1250 - 0.60 75 600 450 0.98 X

2,950 - 0.65 478 600 450 1.33 2 ~ 930 1.05 85 600 450 1.41

3 1200 0.68 206 750 620 0.74 X

20 3 910 0.83 418 600 450 1.53 3 880 - 1.28 97 650 500 1.58 4 950 - 0.88 612 600 450 1.64

4,950 - 0.22 513,600 450 1.07 X

5 - 950 1.65 94 650 500 1.62

25 5 920 - 1.43 78 780 620 1.06 X

6 900 - 1.72 94 600 450 1.62 6 1150 - 1.38 78 600 450 1.35 6 850 - 0.53 648 650 500 1.48 7 - 910 0.62 518 680 500 1, 44

30 7,920 - 0.22 412 650 500 1.08 X

7,930 - 1.27 88 600 450 1.57

Claims (8)

1. A method of manufacturing a thin strip of steel by hot rolling, in which a steel slab in the austenitic state is subjected to a rough rolling to an intermediate thickness followed by a rolling finishing comprising several successive reduction passes to gradually bring it to the desired final thickness, characterized in that before the hot rolling, the slab is brought to a reheating temperature, respectively equalization, between 1100 ° C and 800 ° C, and in that one carries out at least the last of said reduction passes 10 with a hardening rate € at least equal to 0.4 and in that the temperature at the end of finish rolling is between 750 ° C and 400 ° C. 2. Method according to claim 1, characterized in that said work hardening rate is at least equal to 0.8. 15 ‘ 3. Method according to either of claims 1 and 2, characterized in that the heating temperature, respectively equalization, of the steel slab before hot rolling is between 1000 ° C and 850 ° C. 20 4. Method according to either of Claims 1 to 3, characterized in that the said temperature for the end of finish rolling is between 700 ° C and 500 ° C. 5. Method according to either of claims 1 to 4, characterized in that one achieves a speed of work hardening at least -1 equal to 60 s in at least one of said reduction passes. 6. Thin strip of hot-rolled steel by the method of either of the preceding claims, characterized in that it comprises a core consisting of recrystallized ferritic grains of size ASTM 19 and the surface layers arranged on either side of said core, made up of finer grains, of size ASTM ^ 10. ή y »A - 11 -m. 7. Thin strip according to claim 6, characterized in that each of said surface layers has a thickness at least equal to 1/8 of the thickness of the strip. 8. Thin strip according to claim 7, characterized in that each of said surface layers has a thickness at least equal to 1/4 of the thickness of the strip.