LU86614A1 - STEELS FOR HOT-ROLLED STRIPS - Google Patents

Description

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3 2t st Inveettiora Patent Application: j, '- ί I. Request

METALLÜRGIC RESEARCH CENTER-CENTRÜM VOOR RESEARCH IM. OF

METALLURGIE, 'Ässociatlön ...... sans' but lücrstif-Vereni ^ ing '·' 2önder * ’winstoogmerk, 47 me ..... Montoyer> H540 Brussels, Belgium —..........

represented by EvT; ....... Freyllnger & E. Meyers, - Ing; Cons. en propr *

• iaä-rr- 46 -..... rue ....... du --Cimeti-ere r Luxembourg ......---------------- ---------------------------------------------- ί .L

acting in the capacity-jde.-mandatai.re s ___________________ ________________________________________ deposit ^ e thirty ..sep.teirihre .._. mil..new hundred ... eighty ... six .......... . t to ...... 3: .. 5 ..................... hours, at the Ministry of the Economy and the Middle Classes, in Luxembourg : 1. the present request for obtaining a patent of invention concerning:

Steels for hot rolled strips (-.

2. description er. French language - ..................................... of the invention in three copies: 3 . .................. 3 .............................. ..... drawing boards, in three copies: 4. the receipt of the taxes paid to the Office of Registration; in Luxembourg, on 30 September.3i s ~ s: 5. the delegation of power, dated. ................... the?.? ....... September ... 3 · ^ .: 6. the successor document (authorization) ; declare (s) assuming responsibility for this declaration. that Π es inventeuris: est ί sont):

Mr * .._ Pierre ....... HESSIAN .. * ...... 39. rue des Bavettes, B - .4050 ESNEUX ..

Mr. Jean-Claude HEM4AN, 98 rue des Brasseursj. B - 4570 BLEGNY-TREMBLEÜR ^ ^ ...........

Mr. Vincent LEROY, .....- 55 / -071, ..... Quai de- Rone.r '.. 4.00.0 .... Liêger - BelgAqr5 · claims; nt; for the above patent application the priority of one (or more) request ---------------------------------- ---------------------------------------------------------- ----------------------- depositedfs ”in (8) __________________________________________________________________________________ on (9l _____________________________________________________________________________________________________________________________________________________________________________________________________________ 5?” U $ ît NC f 10: ...... .... ____________________________________________... .... _______________________________ __________________ ...... .... ....

éht elect: fei-midie for him (she and if appointed, for his proxy, in Luxembourg ---------------------------— 46 me .... du ni me? Ή éye j. T.nxPTrihrrnrg ____________. ___________________ f 12

Sfiliieiteinti 1s delivery of a patent of invention for the object described and represented in ies appendices susmenîionr.é: -: with postponement of this delivery in Aix-nui T. ....... ------ --- mm ~.

____G Π. MINUTES · Deposit

The aforementioned request for a patent for invention has been filed with the Ministry of "Economy and Classes of Moyettcars. Serrice of Property > \ f - 1 i ¢ 1 t'r. i £ Master se! Ee-onoirne et oes L: .- set -, cff Μ;: - ν: ί · :. s. |.

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i ·; „” ’-L _ k {\ y -f / te head of service d / l | intellectual property.

vC \ Âr / 7 il2E_________yhz_____________ »BL-3953 ^ φ Descriptive memorandum filed in support of a patent application for:

Steels for hot rolled strips.

CENTRUM METALLURGICAL RESEARCH CENTER VOOR RESEARCH IN DE METALLURGIE /

Non-profit association -Vereniging zonder winstoogmerk, 47 rue de Montoyer, 1040 Bruxelles Belgique d * · * C 2373/8609.

METALLURGICAL RESEARCH CENTER -CENTRUM V00R RESEARCH IN DE METALLURGIE,

Non-profit association -Vereniging zonder winstoogmerk in BRUXELLES, (Belgium).

Steels for hot rolled strips.

The present invention relates to steels intended for the manufacture of hot rolled strips.

It is known that the hot rolled strip, or more simply hot strip, is obtained from a slab, cast in a mold or continuously, which is heated to a temperature suitable for hot rolling. This temperature can vary depending on the type of steel, but it is usually more than 1000 ° C. In a variant, this slab can be laminated directly, that is to say without intermediate reheating. The lamination of the slab essentially consists of an i to - 2 - * rough lamination, or roughing, which is followed by the finishing lamination. This last operation, carried out with the hot finishing train, has the effect of imparting to the strip the desired thickness for the subsequent treatments, in particular for cold rolling.

In the most common practice, the rolling of the steel strips at the finishing train is carried out in a temperature range where the steel is austenitic. In the case of steels intended for stamping and whose carbon content is between 0.01 and 0.1%, the final temperature of hot rolling in the finishing train must therefore be equal to or greater than 850 ° C.

It is also well known that the finish rolling comprises several successive passes during which the thickness of the strip is gradually reduced to the required final thickness. This reduction in thickness requires the application of a force called rolling force determined so as to cause the desired deformation. Furthermore, the temperature of the strip decreases continuously during rolling, while remaining in the austenitic range; it follows that the resistance to deformation of the steel increases and that, from one rolling pass to the next, the rolling force must be adjusted to operate the required deformation.

Knowledge of the growth law of resistance to deformation by rolling as a function of the decrease in strip temperature is the basis for good adjustment of the successive cages of the hot finishing train. It makes it possible in particular to determine the rolling force to be applied to each rolling pass, as a function of the temperature of the strip and of the thickness reduction to be carried out in this pass. From a blank of known thickness, it is thus possible to produce a hot strip of the desired thickness, with a very small dispersion of this thickness, both in the longitudinal direction and in the transverse direction of the strip. Current mathematical models for driving hot finishers | use this law of continuous growth of resistance to de-

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- 3 - The formation of steels in the austenitic phase as a function of temperature.

It may happen that the temperature of the strip decreases to the point that the allotropic transformation of austenite into ferrite occurs during rolling with the hot finishing train. In the curve of the resistance to deformation by rolling as a function of the temperature of the strip, this allotropic transformation is marked by a variation in the resistance to deformation by rolling to be overcome in order to effect a given reduction in thickness. This variation in resistance to deformation by rolling is explained by the fact that the ferritic structure does not have the same hardness as the austenitic structure.

The temperature at which this allotropic transformation occurs depends on the composition of the steel, and in particular on its carbon content, as shown by the well-known iron-carbon equilibrium diagram. The position of this variation in the curve of resistance to deformation by rolling as a function of the temperature of the strip therefore depends in particular on the carbon content of the steel.

In the case of steels having a very low carbon content, for example less than 0.01% and especially less than 0.005%, the iron-cabone diagram shows that the transformation temperature of austenite into ferrite is around 900 ° C. The completion of the finish rolling ^ entirely in the homogeneous austenitic range therefore requires an increase in the rolling temperatures, including the temperature of the initial slab, so that the end of rolling temperature is at least equal to 900 ° C. . Such rolling temperatures are however difficult to practice at present, because they imply very high temperatures for reheating the slabs. This condition goes against the current trend which advocates a lowering of this reheating temperature, on the one hand for reasons of energy savings and on the other hand for ... .......................

- 4 - continuous which constitutes the final product. Furthermore, such an increase in the end-of-rolling temperature would practically no longer make it possible to resort to direct rolling.

The Applicant has moreover observed that the carbon content of the steel also has a significant influence on the amplitude of the abovementioned variation in the resistance to deformation by rolling.

In the case of steels with medium carbon content (C: 0.01 - 0.1%), this variation results in the appearance of a substantially horizontal plateau or even a slight decrease in the resistance curve. to deformation by rolling as a function of the temperature of the strip.

When the steel has a very low carbon content, for example less than 0.01% and especially 0.005%, this variation takes the form of a small drop in resistance to deformation by rolling. This effect is further accentuated if the steel contains elements such as titanium and / or niobium which fix the carbon in the form of carbides and therefore further reduce the content of free carbon. In these steels, the allotropic transformation takes place in an austenitic structure which contains practically no carbon; it is very rapid and gives rise to a ferri-tic structure which is all the more soft as its free carbon content is low. This results in a fall of up to 50 λ in the resistance to deformation by rolling to be overcome to ensure a given reduction. The uncertainties associated with the position and the amplitude of this variation make it impossible in practice to roll in the temperature range corresponding to the allotropic transformation of these steels. In particular, it is impossible to carry out, in this temperature range, the last rolling passes which give the strip the targeted thickness. If one also takes into account the heterogeneities of the temperature of the blanks, both in the longitudinal direction and in the transverse direction, it appears necessary to. roll these steels at very high temperatures, that is to say with

End of rolling temperatures equal to or greater than 920 ° C. Such

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High temperature rolling also comes up against the difficulties indicated above and becomes particularly difficult for thin hot strips.

When the allotropic transformation of austenite into ferrite is complete, the steel structure has become homogeneous again and the resistance to deformation by rolling increases again regularly as the temperature of the strip continues to decrease. Such increasing laws for rolling in the ferritic field have been observed by the applicant in the case of steels with medium carbon content (C: 0.01 - 0.1%) as well as in that of steels with very low content carbon (C û 0.01%).

However, it has not proved particularly advantageous to mine steels with a low carbon content in the ferritic field. In fact, the engagement temperatures for rolling such steels in the ferritic field are very low, that is to say below 850 ° C., which imposes waiting times between the roughing rolling mill and the rolling mill finisher and consequently reduces the productivity of the rolling mill train. In addition, the increase in the resistance to deformation by rolling of these steels when the temperature decreases below 800 ° C. is very significant and rapidly leads to prohibitive resistance to deformation by rolling.

On the other hand, it is advantageous to roll steels with very low carbon content (C ύ 0.01%) in the ferritic field. In fact, the allotropic transformation ends at a significantly higher temperature, close to 900 ° C., and the finishing rolling can therefore begin earlier. In addition, the ferritic structure formed in these steels is very soft due to the very low carbon content and requires only moderate rolling forces. Finally, the increase in resistance to deformation by rolling as the strip temperature decreases is lower than in the case of steels with medium carbon content. Such a rolling process in the ferritic field as well as steels with very low carbon content - 6 - suitable for this rolling are the subject of Luxembourg patent n ° 86.509 filed by the present applicant.

These steels, however interesting they may be, however still have the drawback of having a temperature range which cannot be used for finishing rolling, namely the range corresponding to the allotropic transformation of austenite into ferrite in the strip.

The present invention relates precisely to steels for which the diagram of the resistance to deformation by rolling as a function of temperature does not exhibit the abrupt and uncontrolled variation mentioned above in the temperature range corresponding to the finishing rolling of the strip to hot.

The invention is based on the observation of the unexpected influence exerted in this regard by an appropriate addition of an element such as boron, both to steels with a low carbon content and to those whose carbon content is very weak.

The steels which are the subject of the present invention are characterized in that they have the following weight composition: carbon: max. 0.1% nitrogen: max. 50 ppm aluminum: max. 0.07% phosphorus: max. 0.08% * sulfur: max. 0.025¾ manganese: 0.10 to 0.60% boron: max. 0.010% the rest being iron and unavoidable impurities.

According to a particularly advantageous variant of the invention, the carbon content of the steels is less than 0.01 £, and more preferably still less than 0.005%, and said steels also contain titanium and / or niobium in a content between 5 and 25 times the carbon content.

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c - 7 -

Thanks to the presence of boron, the steels according to the invention can be subjected to a finishing rolling under conventional conditions, without being exposed to the aforementioned risks of uncontrolled variations in the resistance to deformation in the various cages of the finishing train driving to unacceptable variations in the thickness of the hot strip.

The favorable behavior of the steels according to the invention is described in detail below, with reference to the accompanying drawings, relating to preferred embodiments, and in which FIG. 1 shows the influence of an addition of boron on the curve of resistance to deformation by rolling as a function of temperature, in the case of a steel with low carbon content; FIG. 2 shows the influence of an addition of boron on the curve of resistance to deformation by rolling as a function of temperature, in the case of a steel with very low carbon content; and FIG. 3 presents l influence of addition of boron on the curve of resistance to deformation by rolling as a function of temperature, in the case of a steel with very low carbon content containing titanium or niobium.

The figures relate to steels whose compositions are given in the table below. They show the curves reflecting the evolution of the resistance to deformation by rolling (R.D.) expressed in arbitrary units, as a function of the decreasing temperature (T) of the hot strip. The curves presented in the figures are identified in the number assigned to the corresponding steel in this _ 3 table. In this table, the contents are expressed in 10% by weight.

Λ t - 8 -

Aci „r C Μη Si S P Al N Ti Nb B

n ° 1 45 160 8 13 12 39 2.6 2 38 202 6 9 13 34 2.7 - - 1.9 3 3 159 8 10 8 18 2.9 - - - 4 4 195 6 8 8 49 2, 3 - - 2.4 ^ 5 3 131 5 2 4 17 1.2 26 - - 6 2.5 147 13 14 16 16 2.3 - 35 - 7 3 180 9 8 10 30 2.0 35 - 2.2

FIG. 1 relates to steels 1 and 2, which have an average carbon content; steel n ° 1 does not contain boron, while steel n ° 2 contains 0,0019% by weight This diagram of figure 1 shows that the presence of boron has the effect of going up in a rather sensitive way the curve of resistance to deformation by. Twinning as a function of the temperature of the strip. This means that with steel n ° 2, the law which follows the resistance to deformation by rolling continues a little further than with steel n ° 1 and that the level of curve n ° 2 is located slightly higher and has a slightly steeper slope than that of curve n ° 1. Stopping or slowing the growth of the resistance to deformation by rolling is thus somewhat rejected towards a lower temperature. At the end of the plateau, that is to say when the allotropic transformation is completed, the two curves n ° 1 and n ° 2 meet and resume a common growth, in the ferritic domain, to quickly reach prohibitive levels of resistance to deformation by rolling.

FIG. 2 similarly translates the influence of boron on the position of the curve of resistance to deformation by rolling j as a function of the temperature of the strip in the case of steels n ° 3 i

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- 9 - and 4 with very low carbon content. It is obvious that the curve n ° 4 (boron steel) has a much more favorable appearance than the curve n ° 3 (boron-free steel) as regards the law of the resistance to deformation by rolling used for the conduct of the finisher train.

Finally, FIG. 3 clearly shows the clear influence of an addition of boron in the case of steels containing titanium (no. 5) or niobium (no. 6). The curves corresponding to these two steels show ’, between about 900 ° C and 850 ° C, a sudden and significant drop which makes it practically impossible to safely operate the rolling mill in this area. Curve 7, which shows the behavior of a steel of this category to which boron has been added, indicates that this sudden variation has disappeared and that the finishing train can be used reliably in this temperature range.

These examples show that an appropriate addition of boron to these types of hot strip steels makes it possible to eliminate or at least greatly reduce the variation in the resistance to deformation by rolling due to the allotropic transformation in the strip. Such an addition thus makes it possible to significantly lower the rolling temperatures while guaranteeing the dimensional qualities of the final product under conventional conditions of hot rolling at the finishing train.

Claims (2)

1. Steels for hot-rolled strips, characterized in that they have the following weight composition: carbon: max. 0.10% nitrogen: max. 50 ppm aluminum: max. 0.07% · * phosphorus: max. 0.08% sulfur: max. 0.025% manganese: 0.10 to 0.60? 0 boron: max. 0.0i0 K the remainder being iron and unavoidable impurities. 2. Steels according to claim 1, characterized in that their carbon content is less than 0.010 and preferably less than 0.005%, and in that they also contain titanium and / or niobium in a content of between 5 and 25 times the carbon content. "